The H.E.S.S. Collaboration
Abstract

The shell-type supernova remnant RX J1713.7–3946 was observed during three years with the H.E.S.S. Cherenkov telescope system. The first observation campaign in 2003 yielded the first-ever resolved TeV gamma-ray image. Follow-up observations in 2004 and 2005 revealed the very-high-energy gamma-ray morphology with unprecedented precision and enabled spatially resolved spectral studies. Combining the data of three years, we obtain significantly increased statistics and energy coverage of the gamma-ray spectrum as compared to earlier H.E.S.S. results. We present the analysis of the data of different years separately for comparison and demonstrate that the telescope system operates stably over the course of three years. When combining the data sets, a gamma-ray image is obtained with a superb angular resolution of 0.06 degrees. The combined spectrum extends over three orders of magnitude, with significant gamma-ray emission approaching 100 TeV. For realistic scenarios of very-high-energy gamma-ray production, the measured gamma-ray energies imply efficient particle acceleration of primary particles, electrons or protons, to energies exceeding 100 TeV in the shell of RX J1713.7–3946.

Abstract

The shell-type supernova remnant RX J0852.0-4622 was detected in 2004 and re-observed between December 2004 and May 2005 with the High Energy Stereoscopic System (H.E.S.S.), an array of four Imaging Cherenkov Telescopes located in Namibia and dedicated to the observations of -rays above 100 GeV. The angular resolution of and the large field of view of H.E.S.S. ( diameter) are well adapted to studying the morphology of the object in very high energy gamma-rays, which exhibits a remarkably thin shell very similar to the features observed in the radio range and in X-rays. The spectral analysis of the source from 300 GeV to 20 TeV will be presented. Finally, the possible origins of the very high energy gamma-ray emission (Inverse Compton scattering by electrons or the decay of neutral pions produced by proton interactions) will be discussed, on the basis of morphological and spectral features obtained at different wavelengths.

Abstract

The shell-type supernova remnant (SNR) RCW 86 - possibly associated with the historical supernova SN 185 - was observed during the past three years with the High Energy Stereoscopic System (H.E.S.S.), an array of four atmospheric-Cherenkov telescopes located in Namibia. The multi-wavelength properties of RCW 86, e.g. weak radio emission and North-East X-ray emission almost entirely consisting of synchroton radiation, resemble those of two very-high energy (VHE; 100 GeV) -ray emitting SNRs RX J1713.7-3946 and RX J0852-4622. The H.E.S.S. observations reveal a new extended source of VHE -ray emission.The morphological and spectral properties of this new source will be presented.

Abstract

H.E.S.S. observations of the old-age (10 yr; diameter) composite supernova remnant (SNR) W 28 reveal very high energy (VHE) -ray emission situated at its northeastern and southern boundaries. The northeastern VHE source (HESS J1801233) is in an area where W 28 is interacting with a dense molecular cloud, containing OH masers, local radio and X-ray peaks. The southern VHE sources (HESS J1800240 with components labelled A, B and C) are found in a region occupied by several HII regions, including the ultracompact HII region W 28A2. Our analysis of NANTEN CO data reveals a dense molecular cloud enveloping this southern region, and our reanalysis of EGRET data reveals MeV/GeV emission centred on HESS J1801233 and the northeastern interaction region.

Abstract

The H.E.S.S. telescope array has observed the complex Monoceros Loop SNR/Rosette Nebula region which contains unidentified high energy EGRET sources and potential very-high-energy (VHE) -ray source. We announce the discovery of a new point-like VHE -ray sources, HESS J0632+057. It is located close to the rim of the Monoceros SNR and has no clear counterpart at other wavelengths. Data from the NANTEN telescope have been used to investigate hadronic interactions with nearby molecular clouds. We found no evidence for a clear association. The VHE -ray emission is possibly associated with the lower energy -ray source 3EG J0634+0521, a weak X-ray source 1RXS J063258.3+054857 and the Be-star MWC 148.

Abstract

The H.E.S.S. stereoscopic Cherenkov telescope system has observed the Crab nebula since December 2003 with the complete four-telescope array. The stable signal from this pulsar wind nebula (PWN) has been used to verify the performance and calibration of the instrument thanks to its high flux compared to the H.E.S.S sensitivity. These observations allow us also to study the radiation mechanisms of this PWN, in particular by focusing on the high energy part of its energy spectrum, where gamma-ray emission at energies above 30 TeV has been detected.

Abstract

Observations with H.E.S.S. revealed a new source of very high-energy (VHE) gamma-rays above 100 GeV – HESS J1825–137 – extending mainly to the south of the energetic pulsar PSR B1823–13. A detailed spectral and morphological analysis of HESS J1825–137 reveals for the first time in VHE gamma-ray astronomy a steepening of the energy spectrum with increasing distance from the pulsar. This behaviour can be understood by invoking radiative cooling of the IC-Compton gamma-ray emitting electrons during their propagation. In this scenario the vastly different sizes between the VHE gamma-ray emitting region and the X-ray PWN associated with PSR B1823–13 can be naturally explained by different cooling timescales for the radiating electron populations. If this scenario is correct, HESS J1825–137 can serve as a prototype for a whole class of asymmetric PWN in which the X-rays are extended over a much smaller angular scales than the gamma-rays and can help understanding recent detections of X-ray PWN in systems such as HESS J1640–465 and HESS J1813–178. The future GLAST satellite will probe lower electron energies shedding further light on cooling and diffusion processes in this source.

Abstract

Motivated by recent detections of pulsar wind nebulae in very-high-energy (VHE) gamma rays, a systematic search for VHE gamma-ray sources associated with energetic pulsars was performed, using data obtained with the H.E.S.S. (High Energy Stereoscopic System) instrument. The search for VHE gamma-ray sources near the pulsar PSR J1718-3825 revealed the new VHE gamma-ray source HESS J1718-385. We report on the results from the HESS data analysis of this source and on possible associations with the pulsar and at other wavelengths. We investigate the energy spectrum of HESS J1718-385 that shows a clear peak. This is only the second time a VHE gamma-ray spectral maximum from a cosmic source was observed, the first being the Vela X pulsar wind nebula.

Abstract

The source HESS J1809193 was discovered in 2006 in data of the Galactic Plane survey, followed by several re-observations. It shows a hard gamma-ray spectrum and the emission is clearly extended. Its vicinity to PSR J1809-1917, a high spin-down luminosity pulsar powerful enough to drive the observed gamma-ray emission, makes it a plausible candidate for a TeV Pulsar Wind Nebula (PWN). On the other hand, in this region of the sky a number of faint, radio-emitting supernova remnants can be found, making a firm conclusion on the source type difficult.

Here we present a detailed morphological study of recent H.E.S.S. data and compare the result with X-ray measurements taken with Chandra and radio data. The association with a PWN is likely, but contributions from supernova remnants cannot be ruled out.

Abstract

The H.E.S.S. 2004-2005 survey of the Galactic Plane at energies above 200 GeV had revealed a number of pulsar wind nebulae candidates, including the remarkable source HESS J1825-137. Spatially resolved spectral measurements of this source gave the first evidence of an energy-dependent morphology which was interpreted as being due to the cooling of relic electrons cumulated throughout pulsar’s history. Also for a few number of sources the asymmetry of the pulsar with respect to the nebula could be attributed to an asymmetric reverse shock from the associated supernova remnant due to inhomogeneities in the interstellar matter. Subsequently a class of large offset and relic nebulae emerged as an outstanding new type of VHE -ray source.
We discuss here the cases of such nebulae in the extended H.E.S.S. Galactic Plane survey data through an energetic criterion taking into account earlier epochs of pulsar injection as well as through investigation of CO data to search for inhomogeneities.

Abstract

The deeper and more extended survey of the central parts of the Galactic Plane by H.E.S.S. during 2005-2007 has revealed a number of new point-like, as well as, extended sources. Two point-like sources can be associated to two remarkable objects around “Crab-like” young and energetic pulsars in our Galaxy : G21.5-0.9 and Kes 75. The characteristics of each of the sources are presented and possible interpretations are briefly discussed.

Abstract

We present the results of a search for pulsed very-high-energy (VHE) -ray emission from young pulsars using data taken with the H.E.S.S. imaging atmospheric Cherenkov telescope system. Data on eleven pulsars, selected according to their spin-down luminosity relative to distance, are searched for -ray signals with periodicity at the respective pulsar spin period. Special analysis efforts were made to improve the sensitivity in the 100 GeV -ray energy domain in an attempt to reduce the gap between satellite and ground-based -ray instruments. No significant evidence for pulsed emission is found in any data set. Differential upper limits on pulsed energy flux are determined for all selected pulsars in the approximate -ray energy range between 100 GeV and  TeV, using different limit determination methods, testing a wide range of possible pulsar light curves and energy spectra. The limits derived here imply that the magnetospheric VHE -ray production efficiency in young pulsars is less than of the pulsar spin-down luminosity, requiring spectral turnovers for the high-energy emission of four established -ray pulsars, and constrain the inverse Compton radiation component predicted by several outer gap models.

Abstract

The binary system LS 5039 was serendipitously discovered with the High Energy Stereoscopic system (H.E.S.S.) during the scan of the inner galactic plane in 2004. Deeper observations were carried out in 2005, and brought clear evidence for TeV emission perodicity. This is the highest energy periodic source known so far. The observed flux modulation is attributed to a modulated absorption of the VHE gamma-ray emission of the compact object through pair creation on the stellar photosphere. Spectral modulation is also observed in this system; this might have several origins such as modulation of particle acceleration or reprocessing of high energy photons towards lower energy through cascading.

We will present detailed studies of the source variability (flux and spectral shape), the timescales compared to other wavelengths, and briefly review the implications for existing emission models.

Abstract

PSR B125963 / SS 2883 is a binary system consisting of a 48 ms radio pulsar orbiting a Be star with a period of 3.4 y in a highly eccentric orbit (e = 0.87). The system was first detected in TeV -rays by H.E.S.S. around the last periastron passage in March 2004. These observations established PSR B125963 / SS 2883 as the first variable galactic source in the very high energy (VHE) regime. A lightcurve for the system, covering mainly the post periastron part, could be deduced, clearly showing a variable flux in VHE photons. New data have been taken this year from April to June with the system approaching its next periastron (July 27, 2007). The status and outcome so far of the corresponding campaign will be discussed.

Abstract

Utilising the unprecedented TeV sky coverage of the H.E.S.S. galactic plane scan, we present the results of a search for Very High Energy gamma-ray sources coincident with the positions of known X-ray binaries. Although no significant detections were obtained, upper limits to the TeV flux from 18 X-ray binaries were derived.

Abstract

Since the discovery of TeV emission from the LS 5039/RX J1826.2-1450 binary system, microquasars are an established class of Very High Energy -raysources. Nonetheless, the current catalogue of -raybinaries remains somewhat limited, with only four examples known. We present the results of a systematic search for TeV emission from known X-ray binaries with similar properties to LS 5039/RX J1826.2-1450 using the H.E.S.S. atmospheric Cherenkov telescope array.

Abstract

Observations by the H.E.S.S. system of imaging atmospheric Cherenkov telescopes provide the most sensitive measurements of the Galactic Centre region in the energy range 150 GeV - 30 TeV. The vicinity of the kinetic centre of our galaxy harbours numerous objects which could potentially accelerate particles to very high energies (VHE,  GeV) and thus produce the -ray flux observed. Within statistical and systematic errors, the centroid of the point-like emission measured by H.E.S.S. was found [249] to be in good agreement with the position of the supermassive black hole Sgr A* and the recently discovered PWN candidate G359.95-0.04 [268]. Given a systematic pointing error of about 30”, a possible association with the SNR Sgr A East could not be ruled out with the 2004 H.E.S.S. data. In this contribution an update is given on the position of the H.E.S.S. Galactic Centre source using 2005/2006 data. The systematic pointing error is reduced to 6” per axis using guiding telescopes for pointing corrections, making it possible to exclude with high significance Sgr A East as the source of the VHE -rays.

Abstract

The rapidly varying non-thermal X-ray emission observed from Sgr A points to particle acceleration taking place close to the supermassive black hole. The TeV -ray source HESS J1745290 is coincident with Sgr A and may be closely related to the X-ray emission. Simultaneous X-ray and TeV observations are required to elucidate the relationship between these two objects. Here we report on joint H.E.S.S./Chandra observations in July 2005, during which an X-ray flare was detected. Despite a factor increase in the X-ray flux of Sgr A, no evidence is found for an increase in the TeV -ray flux. We find that an increase of the -ray flux of a factor 2 or greater can be excluded at a confidence level of 99%. This finding disfavours scenarios in which the bulk of the -ray emission observed is produced close to Sgr A.

Abstract

One interesting possibility is that the galactic center (GC) source HESS J1745-290 is associated with the galactic center source Sgr A*, the galactic center black hole, in which case we may expect variability as seen in IR and X-rays, with QPO frequencies predicted by Aschenbach et al. (2006). We will present the results of a search for such variable signatures using HESS observations of this source.

Abstract

The High Energy Stereoscopic System (H.E.S.S.), located in the Khomas Highlands of Namibia, is an array of four imaging atmospheric-Cherenkov telescopes designed to detect -rays in the very high energy (VHE; 100 GeV) domain. Its high sensitivity and large field-of-view (5) make it an ideal instrument to perform a survey within the Galactic plane for new VHE sources. Previous observations in 2004/2005 resulted in numerous detections of VHE gamma-ray emitters in the region l = 330 - 30 Galactic longitude. Recently the survey was extended, covering the regions l = 280 - 330 and l = 30 - 60, leading to the discovery of several previously unknown sources with high statistical significance. The current status of the survey will be presented.

Abstract

In the very-high-energy (VHE) gamma-ray wave band, pulsar wind nebulae (PWNe) represent to date the most populous class of Galactic sources. Nevertheless, the details of the energy conversion mechanisms in the vicinity of pulsars are not well understood, nor is it known which pulsars are able to drive PWNe and emit high-energy radiation. In this paper we present a systematic study of a connection between pulsars and VHE -ray sources based on a deep survey of the inner Galactic plane conducted with the High Energy Stereoscopic System (H.E.S.S.). We find clear evidence that pulsars with large spin-down energy flux are associated with VHE -ray sources. This implies that these pulsars emit on the order of 1% of their spin-down energy as TeV -rays.

Abstract

Scan-based observations of the Galactic plane and continuing re-observations of known very-high-energy (VHE) gamma-ray sources with the H.E.S.S. system of imaging atmospheric Cherenkov telescopes have revealed a wide variety of new VHE objects. While in many cases these objects can be associated with known sources in the X-ray, radio, or optical wavebands, a subset of them currently have no obvious cataloged lower-energy counterpart. An analysis of 8 such unidentified sources is presented here.

Abstract

The results from H.E.S.S. observations towards Westerlund 2 are presented. The detection of very-high-energy gamma-ray emission towards the young stellar cluster Westerlund 2 in the HII complex RCW49 by H.E.S.S. provides ample evidence that particle acceleration to extreme energies is associated with this region. A variety of possible emission scenarios is mentioned, ranging from high-energy gamma-ray production in the colliding wind zone of the massive Wolf-Rayet binary WR 20a, collective wind scenarios, diffusive shock acceleration at the boundaries of wind-blown bubbles in the stellar cluster, and outbreak phenomena from hot stellar winds into the interstellar medium. These scenarios are briefly compared to the characteristics of the associated new VHE gamma-ray source HESS J1023–575, and conclusions on the validity of the respective emission scenarios for high-energy gamma-ray production in the Westerlund 2 system are drawn.

Abstract

In view of the discovery of HESS J1023-575 (discussed in a separate presentation), we examine another very high energy (VHE) gamma-ray source possibly associated with massive star clusters. Particle acceleration in massive star forming regions can proceed at the interface of two interacting winds or result from a collective process; e.g. multiple shock acceleration or MHD turbulence. The gamma-ray emission can also take place at the edge of the superbubble blown by the winds and multiple supernova explosions. Non-thermal radiation from the shell structure then traces the interaction of energetic particles (ions and/or electrons) with the surrounding interstellar matter. In particular, HESS J1837-069 is spatially coincident with a recently discovered very massive star cluster. We discuss the VHE gamma-ray data resulting from H.E.S.S. observations on this or other possible such associations. We consider data in other wavelength domains, in particular in X-rays, and examine the available evidence that the VHE emission could originate in particles accelerated by the above-mentioned mechanisms in massive star clusters.

Abstract

We report here on a new VHE source, HESS J1908+063, disovered during the extended H.E.S.S. survey of the Galactic plane and which coincides with the recently reported MILAGRO unidentified source MGRO J1908+06. The position, extension and spectrum measurements of the HESS source are presented and compared to those of MGRO J1908+06. Possible counterparts at other wavelenghts are discussed. For the first time one of the low-lattitude MILAGRO sources is confirmed.

Abstract

The Gould belt, a well-known region of enhanced star formation in the solar neighbourhood, is observed to be an expanding disk with a diameter of about 1 kpc and a width of a few 100 pc. Most of the nearby OB stellar associations and molecular clouds are found to be aligned with the Gould belt. With the high star formation rate along the Gould belt, the local supernova rate during the last few million years is believed to be three to four times larger than the Galactic average. Under the assumption that supernova remnants are efficient accelerators of cosmic rays, the Gould belt and its environment should show an increased cosmic ray density with respect to the Galactic average. The cosmic rays are expected to interact with the dense molecular gas which results mainly in pi-meson production with subsequent decay in gamma-rays and neutrinos. We have searched for gamma-ray emission from various parts of the Gould belt with the HESS Cherenkov telescopes. Results will be presented at the conference.

Abstract

The high-frequency peaked BL Lac PKS2155304, the lighthouse of the Southern hemisphere sky at VHE gamma-ray energies, has been followed by the H.E.S.S. array of atmospheric Cherenkov telescopes since the first light of the project, first with a single telescope in 2002, then with two & three telescopes in 2003, and since 2004 with the full-sensitivity four-telescope array. In this mode, a number of multi-wavelength campaigns have been performed with observations from the Rossi X-ray Timing Explorer (RXTE), Rotse (Optical), Spitzer (IR), James Clark Maxwell Telescope (JCMT, sub-mm) and others in both quiescent and active states, based on both fixed campaigns and triggers from H.E.S.S. Here we present the results of this series of observations up to 2005 inclusive, together with the implications for source models of the spectral measurements and search for correlated variability with X-rays, Optical, and IR measurements. The exceptional flare activity of 2006 is covered in a separate paper at this conference.

Abstract

Since 2002 the VHE (100 GeV) -ray flux of the high-frequency peaked BL Lac PKS 2155304 has been monitored with the High Energy Stereoscopic System (HESS). An extreme -ray outburst was detected in the early hours of July 28, 2006 (MJD 53944). The average flux above 200 GeV observed during this outburst is 7 times the flux observed from the Crab Nebula above the same threshold. Peak fluxes are measured with one-minute time scale resolution at more than twice this average value. Variability is seen up to 600 s in the Fourier power spectrum, and well-resolved bursts varying on time scales of 200 seconds are observed. There are no strong indications for spectral variability within the data. Assuming the emission region has a size comparable to the Schwarzschild radius of a black hole, Doppler factors greater than 100 are required to accommodate the observed variability time scales.

Abstract

Using data derived from the H.E.S.S. telescope system and the LIDAR facility on site, a method of correcting for changing atmospheric quality based on reconstructed shower parameters is presented. The method was applied to data from the active galactic nucleus PKS 2155-304, taken during August and September 2004 when the quality of the atmosphere at the site was highly variable. Corrected and uncorrected fluxes are shown, and the method is discussed as a first step towards a more complete atmospheric calibration.

Abstract

The giant radio galaxy M 87 was observed at GeV/TeV -ray energies with the H.E.S.S. (High Energy Stereoscopic System) Cherenkov telescopes in the years 2003–2006. The observations confirm M 87 as the first extragalactic TeV -ray source not of the blazar type (first indications of a signal were reported by the HEGRA collaboration earlier). The TeV -ray flux from M 87 as measured with H.E.S.S. was found to be variable on time-scales of years and surprisingly also of days which strongly constrains the size of the emission region. The results (position, energy spectrum and light curve) as well as theoretical interpretations will be presented.

Abstract

Very-high-energy (VHE; 100 GeV) -ray observations of PKS 2005-489 and H 2356-309 were made with the High Energy Stereoscopic System (HESS) in 2005 and 2006. Previous 2004 data have been reanalysed to correct for the degradation of the optical efficiency of the HESS system. Both sources have been detected during all 3 years, at a level of 1-3% of the Crab flux. A total excess of 16 and 12, respectively, is accumulated. Significant flux variations are seen on a monthly basis for H 2356-309, and in 2006 for PKS 2005-489. The spectra confirm the previously reported values, in particular the hard spectrum of H 2356-309. Multiwavelength observations performed with XMM and RXTE in 2004 and 2005 reveal remarkable flux (10x) and spectral (=0.7) variations for PKS 2005-489. Despite a 10 flux increase above 1 keV, no flux variation is seen at VHE, implying in an SSC scenario a corresponding decrease of the energy density of the seed photons for inverse Compton (IC) scattering, not observed in the SED. A possible explanation is that a new component is emerging in the jet, whose electrons do not see the photons of the observed synchrotron peak. The SED of both objects shows the potential for significantly higher VHE fluxes.

Abstract

Very high energy (VHE; 100 GeV) -ray observations of PG 1553+113 were made with the High Energy Stereoscopic System (HESS) in 2005 and 2006. A strong signal, 10 standard deviations, is detected by HESS during the 2 years of observations (24.8 hours live time). The time-averaged energy spectrum, measured between 225 GeV to 1.3 TeV, is characterized by a very steep power law (photon index of ). The integral flux above 300 GeV is 3.4% of the Crab Nebula flux and shows no evidence for any variations, on any time scale. H+K (1.452.45m) spectroscopy of PG 1553+113 was performed in March 2006 with SINFONI, an integral field spectrometer of the ESO Very Large Telescope (VLT) in Chile. The redshift of PG 1553+113 is still unknown, as no absorption or emission lines were found.

Abstract

VHE observations of the distant (0.186) blazar 1ES 1101232 with H.E.S.S. are used to constrain the extragalactic background light (EBL) in the optical to near infrared band. As the EBL traces the galaxy formation history of the universe, galaxy evolution models can be tested with the data. In order to measure the EBL absorption effect on a blazar spectrum, we assume that usual constraints on the hardness of the intrinsic blazar spectrum are not violated. We present an update of the VHE spectrum obtained with H.E.S.S. and the multifrequency data that were taken simultaneously with the H.E.S.S. measurements. The data verify that the broadband characteristics of 1ES 1101-232 are similar to those of other, more nearby blazars, and strengthen the assumptions that were used to derive the EBL upper limit.

Abstract

Since the new generation of imaging atmospheric-Cherenkov telescopes came online with the commissioning of the four telescopes of the H.E.S.S. experiment in 2004, the number of known extragalactic -ray emitters in the very high energy (VHE) domain has more than doubled. All of the sources detected so far are active galactic nuclei and all but one belong to the class of BL Lac objects. The emission process for VHE -rays in this class of objects is not fully understood and a large sample of sources and multi-wavelength data is needed to discriminate between different models. Furthermore, VHE photons from these distant sources are attenuated via pair production with the extragalactic photon field in the optical to infrared wavelength band (extragalactic background light, EBL), which contains cosmological information on the star and galaxy formation history. With assumptions about the source physics, limits on this photon field can be derived. We report the detection of VHE gamma-rays from the BL Lac 1ES 0229+200 (z = 0.14) and 1ES 0347-121 (z = 0.1880) with the H.E.S.S. Cherenkov telescope system. 1ES 0347-121 is among the most distant source detected in VHE gamma-rays to date.

Abstract

Observations and monitoring of active galactic nuclei (AGN) are a key part of the scientific observation programme of the High Energy Stereoscopic System (H.E.S.S). The instrument was used to search for very high energy (VHE: 100 ) gamma rays coming from PKS 0548-322, a BL Lac object visible from the Southern Hemisphere. An excess of VHE gamma rays (6) from the object is detected. The broad-band spectral energy distribution (SED), including the VHE spectrum () is presented.

Abstract

Very high energy (VHE; 100 GeV) observations of a sample of selected active galactic nuclei (AGN) were performed between January 2005 and April 2007 with the High Energy Stereoscopic System (HESS), an array of imaging atmospheric-Cherenkov telescopes. Significant detections are reported elsewhere for many of these objects. Here, integral flux upper limits for twelve candidate very-high-energy (VHE; 100 GeV) gamma-ray emitters are presented. In addition, results from HESS observations of four known VHE-bright AGN are given although no significant signal is measured. For three of these AGN (1ES 1101232, 1ES 1218+304, and Mkn 501) simultaneous data were taken with the Suzaku X-ray satellite.

Abstract

Jets of Active Galactic Nuclei (AGN) are established emitters of very high energy (VHE; 100 GeV) -rays. VHE radiation is also expected to be emitted from the vicinity of super-massive black holes (SMBH), irrespective of their activity state. Accreting SMBH rotate and generate a dipolar magnetic field. In the magnetosphere of the spinning black hole, acceleration of particles can take place in the field gaps. VHE emission from these particles is feasible via leptonic or hadronic processes. Therefore quiescent systems, where the lack of a strong photon field allows the VHE emission to escape, are candidates for emission. The H.E.S.S.experiment has observed the passive SMBH in the nearby galaxy NGC 1399. No VHE -raysignal is observed from the galactic nucleus. Constraints set by the NGC 1399 observations are discussed in the context of different mechanisms for the production of VHE -rayemission.

Abstract

We conducted multiwavelength observations of the northern TeV blazars, Mkn501 and Mkn421, employing the ground-based -ray telescopes MAGIC and HESS, the Suzaku X-ray satellite and the KVA optical telescope. The observations for Mkn501 were performed in July 2006. The source showed one of the lowest fluxes both in very high energy (VHE) -ray and X-ray. No significant flux variability could be found in the VHE band while an overall increase of about 50% on a 1-day time scale could be seen in the light curve of the X-ray flux. A one-zone synchrotron self-Compton model can well describe our simultaneous spectral data of the VHE -ray and the X-ray emissions of Mkn501 in the quiescent state. The simultaneous observations of Mkn421 were carried out in April 2006. The source was clearly detected in all observations and showed a high state of activity both in VHE -ray and X-ray.

Abstract

On June 2, 2006, the Swift Burst Alert Telescope (BAT) triggered a bursting event in the 15-350 keV energy band. The burst position was being observed with the H.E.S.S. array of IACTs before the burst, throughout the duration of the burst, and after the burst. In particular, the burst position accidentally fell in the f.o.v. of the H.E.S.S. camera when the burst occurred. This is the first completely simultaneous observation of a soft gamma-ray bursting event with an IACT instrument. A search for VHE gamma-rays coincident with the burst event as well as that during the afterglow period was performed. No signal was found during the period covered by the H.E.S.S. observation. The Swift X-ray Telescope, which started observation 83 seconds after the BAT trigger, detected an X-ray counterpart of the event. No optical/infrared counterpart was found. Due to the very soft BAT spectrum (photon index ) of the burst compared to other Swift GRBs and its proximity to the galactic center, the burst might have been caused by a galactic X-ray burster (e.g. a low-mass X-ray binary). However, the possibility of it being a cosmological GRB cannot be ruled out. Since the nature of the event is still unclear, we discuss the implications according to the two different bursting scenarios.

Abstract

Gamma-ray bursts (GRBs) are among the potential very-high-energy (VHE) gamma-ray and cosmic-ray sources. Particles are accelerated to highly-relativistic speeds. This might give rise to emission of VHE gamma-ray and/or cosmic-ray particles with ultra-high energy  eV. Despite its generally fast-fading behavior seen in many longer wavebands, the time evolution of any VHE radiation is still not clear. In order to probe the largely unexplored VHE spectra of GRBs, a GRB observing program has been set up by the H.E.S.S. collaboration. With the high sensitivity of the H.E.S.S. array and given favorable observational conditions, VHE flux levels predicted by GRB models are within reach. Extra-galactic background light (EBL) absorption is considered in cases where redshifts of the GRBs are reported. We present the H.E.S.S. VHE gamma-ray observations of and results from some of the reported GRB positions during the past few years, including recent observations in early 2007.

Abstract

Dwarf Spheroidal galaxies are amongst the best targets to search for a Dark Matter (DM) annihilation signal. The annihilation of WIMPs in the center of Sagittarius dwarf spheroidal (Sgr dSph) galaxy would produce high energy -rays in the final state. Observations carried out with the H.E.S.S. array of Imaging Atmospheric Cherenkov telescopes are presented. A careful modelling of the Dark Matter halo profile of Sgr dwarf was performed using latest measurements on its structural parameters. Constraints on the velocity-weighted cross section of Dark Matter particles are derived in the framework of Supersymmetric and Kaluza-Klein models.

Abstract

With the H.E.S.S. system of four Cherenkov telescopes a signal of very high energy -radiation from the direction of the Galactic center has been detected. The interpretation of the signal due to dark matter annihilation is discussed and limits on the annihilation cross sections and density profiles are given.

Abstract

A recently proposed novel technique for the detection of cosmic rays with arrays of Imaging Atmospheric Cherenkov Telescopes is applied to data from the High Energy Stereoscopic System (H.E.S.S.). The method relies on the ground based detection of Cherenkov light emitted from the primary particle prior to its first interaction in the atmosphere. The charge of the primary particle (Z) can be estimated from the intensity of this light, since it is proportional to Z. Using H.E.S.S. data, an energy spectrum for cosmic-ray iron nuclei in the energy range 13–200 TeV is derived. The reconstructed spectrum is consistent with previous direct measurements and is the most precise measurement so far in this energy range.

Abstract

Due to energy losses in the interstellar medium, cosmic ray electrons at TeV energies carry information on local (within a few hundred parsecs) accelerators. However, measurements of the spectrum of the cosmic ray electrons beyond 1 TeV are extremely difficult due to the rapidly declining flux and the much more numerous background of nucleonic cosmic rays. The very large collection area of Cherenkov telescope arrays makes them promising instruments with which to measure these high energy electrons. While Cherenkov telescopes solve the problem of low fluxes of cosmic ray electrons in the TeV range, they still have to deal with the problem of distinguishing electrons from the nucleonic background. Here we report on first results towards a measurement of the cosmic ray electron spectrum with the High Energy Stereoscopic System (H.E.S.S.). The improved background supression that is needed for such a measurement is achieved by an event classification with the “Random Forest” algorithm based on decision trees.

Abstract

Clusters of galaxies, the largest gravitationally bound objects in the universe, are expected to contain a significant population of hadronic and leptonic cosmic rays. Potential sources for these particles are merger and accretion shocks, starburst driven galactic winds and radio galaxies. Furthermore, since galaxy clusters confine cosmic ray protons up to energies of at least 1 PeV for a time longer than the Hubble time they act as storehouses and accumulate all the hadronic particles which are accelerated within them. Consequently clusters of galaxies are potential sources of VHE ( 100 GeV) gamma rays. Motivated by these considerations, promising galaxy clusters are observed with the H.E.S.S. experiment as part of an ongoing campaign. Here, upper limits for the VHE gamma ray emission for the Abell 496 and Coma cluster systems are reported.

Abstract

Starburst galaxies are characterized by extremely high star-formation rates and, as a consequence, very high supernova rates. These rates, as well as the gas density, are orders of magnitude higher than in the Milky Way. Starburst galaxies contain both a high cosmic-ray flux and high density of target material for proton-proton and inverse-Compton interactions. These objects are therefore viable candidates for observable levels of VHE -ray emission. Nearby starburst galaxies, such as NGC 253 and M83, allow a study of general processes during galaxy formation and evolution of high redshift galaxies. These two galaxies were observed with H.E.S.S. stereoscopic array of atmospheric-Cherenkov telescopes. Upper limits from these observations are presented here.

30th International Cosmic Ray Conference The H.E.S.S. Collaboration
F. Aharonian, A.G. Akhperjanian , U. Barres de Almeida , A.R. Bazer-Bachi , B. Behera , M. Beilicke , W. Benbow , K. Bernlöhr , C. Boisson , O. Bolz , V. Borrel , I. Braun , E. Brion , A.M. Brown , R. Bühler , T. Bulik , I. Büsching , T. Boutelier , S. Carrigan , P.M. Chadwick , L.-M. Chounet , A.C. Clapson , G. Coignet , R. Cornils , L. Costamante , M. Dalton , B. Degrange , H.J. Dickinson , A. Djannati-Ataï , W. Domainko , L.O’C. Drury , F. Dubois , G. Dubus , J. Dyks , K. Egberts , D. Emmanoulopoulos , P. Espigat , C. Farnier , F. Feinstein , A. Fiasson , A. Förster , G. Fontaine , M. Füßling , Y.A. Gallant , B. Giebels , J.F. Glicenstein , B. Glück , P. Goret , C. Hadjichristidis , D. Hauser , M. Hauser , G. Heinzelmann , G. Henri , G. Hermann , J.A. Hinton , A. Hoffmann , W. Hofmann , M. Holleran , S. Hoppe , D. Horns , A. Jacholkowska , O.C. de Jager , I. Jung , K. Katarzyński , E. Kendziorra , M. Kerschhaggl, B. Khélifi , D. Keogh , Nu. Komin , K. Kosack , G. Lamanna , I.J. Latham , M. Lemoine-Goumard , J.-P. Lenain , T. Lohse , J.M. Martin , O. Martineau-Huynh , A. Marcowith , C. Masterson , D. Maurin , T.J.L. McComb , R. Moderski , E. Moulin , M. Naumann-Godo , M. de Naurois , D. Nedbal , D. Nekrassov , S.J. Nolan , S. Ohm , J-P. Olive , E. de Oña Wilhelmi, K.J. Orford , J.L. Osborne , M. Ostrowski , M. Panter , G. Pedaletti , G. Pelletier , P.-O. Petrucci , S. Pita , G. Pühlhofer , M. Punch , B.C. Raubenheimer , M. Raue , S.M. Rayner , M. Renaud , J. Ripken , L. Rob , S. Rosier-Lees , G. Rowell , B. Rudak , J. Ruppel , V. Sahakian , A. Santangelo , R. Schlickeiser , F.M. Schöck , R. Schröder , U. Schwanke , S. Schwarzburg , S. Schwemmer , A. Shalchi , H. Sol , D. Spangler , Ł. Stawarz , R. Steenkamp , C. Stegmann , G. Superina , P.H. Tam , J.-P. Tavernet , R. Terrier , C. van Eldik , G. Vasileiadis , C. Venter , J.P. Vialle , P. Vincent , M. Vivier , H.J. Völk , F. Volpe, S.J. Wagner , M. Ward , A.A. Zdziarski , A. Zech
Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany, Yerevan Physics Institute, 2 Alikhanian Brothers St., 375036 Yerevan, Armenia, Centre d’Etude Spatiale des Rayonnements, CNRS/UPS, 9 av. du Colonel Roche, BP 4346, F-31029 Toulouse Cedex 4, France, Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, D 22761 Hamburg, Germany, Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, D 12489 Berlin, Germany, LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France, DAPNIA/DSM/CEA, CE Saclay, F-91191 Gif-sur-Yvette, Cedex, France, University of Durham, Department of Physics, South Road, Durham DH1 3LE, U.K., Unit for Space Physics, North-West University, Potchefstroom 2520, South Africa, Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, F-91128 Palaiseau, France, Laboratoire d’Annecy-le-Vieux de Physique des Particules, CNRS/IN2P3, 9 Chemin de Bellevue - BP 110 F-74941 Annecy-le-Vieux Cedex, France, Astroparticule et Cosmologie (APC), CNRS, Universite Paris 7 Denis Diderot, 10, rue Alice Domon et Leonie Duquet, F-75205 Paris Cedex 13, France thanks: UMR 7164 (CNRS, Université Paris VII, CEA, Observatoire de Paris), Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin 2, Ireland, Landessternwarte, Universität Heidelberg, Königstuhl, D 69117 Heidelberg, Germany, Laboratoire de Physique Théorique et Astroparticules, CNRS/IN2P3, Université Montpellier II, CC 70, Place Eugène Bataillon, F-34095 Montpellier Cedex 5, France, Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, D 91058 Erlangen, Germany, Laboratoire d’Astrophysique de Grenoble, INSU/CNRS, Université Joseph Fourier, BP 53, F-38041 Grenoble Cedex 9, France, Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, D 72076 Tübingen, Germany, LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, CNRS/IN2P3, 4 Place Jussieu, F-75252, Paris Cedex 5, France, Institute of Particle and Nuclear Physics, Charles University, V Holesovickach 2, 180 00 Prague 8, Czech Republic, Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, D 44780 Bochum, Germany, University of Namibia, Private Bag 13301, Windhoek, Namibia, Obserwatorium Astronomiczne, Uniwersytet Jagielloński, Kraków, Poland, Nicolaus Copernicus Astronomical Center, Warsaw, Poland, School of Physics & Astronomy, University of Leeds, Leeds LS2 9JT, UK, School of Chemistry & Physics, University of Adelaide, Adelaide 5005, Australia, Toruń Centre for Astronomy, Nicolaus Copernicus University, Toruń, Poland, European Associated Laboratory for Gamma-Ray Astronomy, jointly supported by CNRS and MPG



Abstract:

30th International Cosmic Ray Conference Primary particle acceleration above 100 TeV in the shell-type Supernova Remnant RX J1713.7–3946 with deep H.E.S.S. observations
D. Berge, F. Aharonian, W. Hofmann, M. Lemoine-Goumard, O. Reimer, G. Rowell, H.J. Völk, for the H.E.S.S. Collaboration
CERN PH Department, CH-1211 Geneva 23, Switzerland
Max-Planck-Institut für Kernphysik, P.O. Box 103980, D-69029 Heidelberg, Germany
Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin 2, Ireland
Laboratoire Leprince-Ringuet, IN2P3/CNRS, Ecole Polytechnique, F-91128 Palaiseau,
France
Stanford University, HEPL KIPAC, CA 94305-4085, USA
School of Chemistry & Physics, University of Adelaide, Adelaide 5005, Australia
berge@cern.ch

Abstract:

Introduction

Figure 1: Upper panel: H.E.S.S. gamma-ray excess images from the region around RX J1713.73946 are shown for three years. Lower panel: 1D distributions generated from the non-smoothed, acceptance-corrected gamma-ray excess images.

The energy spectrum of cosmic rays measured at Earth exhibits a power-law dependence over a broad energy range. Starting at a few GeV it continues to energies of at least . The power-law index of the spectrum changes at two characteristics energies: in the region around – the knee region – the spectrum steepens, and at energies beyond it hardens again. Up to the knee, cosmic rays are believed to be of Galactic origin, accelerated in shell-type supernova remnants (SNRs) – expanding shock waves initiated by supernova explosions [11]. However, the experimental confirmation of an SNR origin of Galactic cosmic rays is difficult due to the propagation effects of charged particles in the interstellar medium. The most promising way of proving the existence of high-energy particles in SNR shells is the detection of very-high-energy (VHE) gamma rays (), produced in interactions of cosmic rays close to their acceleration site [9].

Recently the VHE gamma-ray instrument H.E.S.S. has detected two shell-type SNRs, RX J1713.73946 [1, 4] and RX J0852.0–4622 [2, 6]. The two objects show an extended morphology and exhibit a shell structure, as expected from the notion of particle acceleration in the expanding shock fronts. While it is difficult to attribute the measured VHE gamma rays unambiguously to nucleonic cosmic rays (rather than to cosmic electrons), the measured spectral shapes favour indeed in both cases a nucleonic cosmic-ray origin of the gamma rays [4, 6].

Apart from the first unambiguous proof of multi-TeV particle acceleration in SNRs, the question of the highest observed energies remains an important one. Only the detection of gamma rays with energies of 100 TeV provides experimental proof of acceleration of primary particles, protons or electrons, to the knee region (1 PeV). Here we present a combined analysis of H.E.S.S. data of RX J1713.73946 of three years, from 2003 to 2005. A comparison of the three data sets demonstrates the expected steady emission of the source as well as the stability of the system. Special emphasis is then devoted to the high-energy end of the combined spectrum.

H.E.S.S. observations

The High Energy Stereoscopic System (H.E.S.S.) consists of four identical Cherenkov telescopes that are operated in the Khomas Highland of Namibia. Its large field of view of make H.E.S.S. the currently best suited experiment in the field for the study of extended VHE gamma-ray sources such as young Galactic SNRs.

The H.E.S.S. observation campaign of RX J1713.73946 started in 2003. The data were recorded during the commissioning phase of the telescope system, with 2 out of the 4 telescopes operational. The data set revealed extended gamma-ray emission resembling a shell structure. It was actually the first ever resolved image of an astronomical source obtained with VHE gamma rays. In 2004, observations were conducted with the full telescope array. The H.E.S.S. data enabled analysis of the gamma-ray morphology and the spectrum of the remnant with unprecedented precision. A very good correlation was found between the X-ray and the gamma-ray image. The differential spectrum showed deviations from a pure power law at high energies. The 2005 observation campaign was aiming at extending the energy coverage of the spectrum to as high energies as possible. Therefore the observations were preferentially pursued at zenith angles larger than in the two years before to make use of the drastically increased effective collection area of the experiment at high energies. The analysis of these data are presented in the following (a more detailed discussion of this analysis can be found in [7]).

Analysis results

Figure 2: Combined H.E.S.S. image of the SNR RX J1713.73946 from the 2004 and 2005 data. A simulated point source (PSF) is also shown.
Figure 3: Left: Comparison of H.E.S.S. energy-flux spectra of three years. The black curve is the best fit of a power law with exponential cutoff to the combined data, as shown on the right, where the combined H.E.S.S. -ray spectrum of RX J1713.73946 is shown. The data are well described by the fit function, which is continued as dashed line beyond the fit range for illustration. The arrow is a model-independent upper limit, determined in the energy range from 113 to 300 TeV.

The analysis techniques used here are presented in detail elsewhere ([3, 8]). The gamma-ray morphology measured in three years is seen in the upper panel of Fig. 1. The images are readily comparable. Very similar angular resolutions are achieved for all years. Good agreement is achieved, as can also be seen from the 1D distributions shown in the lower panel, where also the statistical errors are plotted. Shown from left to right are a slice along a thick box (cf. Fig. 1, upper panel), an azimuthal profile of the shell region, and a radial profile. All the distributions are generated from the non-smoothed, acceptance-corrected excess images. Clearly, there is no sign of disagreement or variability, the H.E.S.S. data of three years are well compatible with each other.

The combined H.E.S.S. image is shown in Fig. 2. Data of 2004 and 2005 are used for this Gaussian smoothed (), acceptance-corrected gamma-ray excess image. In order to obtain optimum angular resolution, a special high-resolution analysis is applied here. Besides choosing only well reconstructed events, the cut on the minimum event multiplicity is raised to three telescopes, disregarding the 2003 data. Moreover, an advanced reconstruction method is chosen, algorithm 3 of [10]. The image corresponds to 62.7 hours of observation time. 6702 gamma-ray excess events are measured with a statistical significance of . An angular resolution of () is achieved. The image confirms nicely the published H.E.S.S. measurements [1, 4], with 20% better angular resolution and increased statistics. The shell of RX J1713.73946, somewhat thick and asymmetric, is clearly visible and almost closed. The gamma-ray brightest parts are located in the north and west of the SNR.

The gamma-ray spectra measured with H.E.S.S. in three consecutive years are compared to each other in Fig. 3 (left). In order to compare the data, a correction for the variation of optical efficiency of the telescopes over the years must be applied [388]. After that correction, very good agreement is found. The measured spectral shape remains unchanged over time. The absolute flux levels are well within the systematic uncertainty of 20%. As expected for an object like RX J1713.73946, no flux variation is seen on yearly timescales. Clearly, the performance of the telescope system is under good control.

The combined data of three years are shown in Fig. 3 (right). This energy spectrum of the whole SNR region corresponds to  hours of H.E.S.S. observations. The combined spectrum extends over almost three decades in energy beyond 30 TeV, and is compatible with previous H.E.S.S. measurements. Taking all events with energies above 30 TeV, the cumulative significance is . Different spectral models can be fit to the data. A pure power law is clearly ruled out, alternative spectral models like a power law with exponential cutoff, a broken power law, or a power law with energy-dependent index, all provide significantly better descriptions of the data, but none of these alternative models is favoured over the other.

Summary

The complete H.E.S.S. data set of the SNR RX J1713.73946 recorded from 2003 to 2005 is presented here. Very good agreement is found for both the gamma-ray morphology and the differential energy spectra over the years. The combined analysis confirms the earlier findings nicely: the gamma-ray image reveals a thick, almost circular shell with significant brightness variations. The spectrum follows a hard power law with significant deviations at higher energies (beyond a few TeV).

In the combined image using  hours of H.E.S.S. observations an unprecedented angular resolution of is achieved. The high-energy end of the combined spectrum approaches 100 TeV with significant emission beyond 30 TeV. Given the systematic uncertainties in the spectral determination at these highest energies and comparable statistical uncertainties despite the long exposure time, this measurement is presumably close to what can be studied with the current generation of imaging atmospheric Cherenkov telescopes.

From the largest measured gamma-ray energies one can estimate the corresponding energy of the primary particles. In case of -decay gamma rays, energies of 30 TeV imply that primary protons are accelerated to in the shell of RX J1713.73946. On the other hand, if the gamma rays are due to Inverse Compton scattering of VHE electrons, the electron energies at the current epoch can be estimated in the Thompson regime as . At these large energies Klein–Nishina effects start to be important and reduce the maximum energy slightly such that is a realistic estimate.

RX J1713.73946 remains an exceptional SNR in respect of its VHE gamma-ray observability, being at present the remnant with the widest possible coverage along the electromagnetic spectrum. The H.E.S.S. measurement of significant gamma-ray emission beyond 30 TeV without indication of a termination of the high-energy spectrum provides proof of particle acceleration in the shell of RX J1713.73946 beyond  eV, up to energies which start to approach the region of the cosmic-ray knee.

References

  • [1] Aharonian et al. (H.E.S.S. Collaboration). Nature, 432:75–77, November 2004.
  • [2] Aharonian et al. (H.E.S.S. Collaboration). A&A, 437:L7, 2005.
  • [3] Aharonian et al. (H.E.S.S. Collaboration). A&A, 430:865, February 2005.
  • [4] Aharonian et al. (H.E.S.S. Collaboration). A&A, 449:223–242, 2006.
  • [5] Aharonian et al. (H.E.S.S. Collaboration). A&A, 457:899–915, 2006.
  • [6] Aharonian et al. (H.E.S.S. Collaboration). ApJ, 661:236–249, May 2007.
  • [7] Aharonian et al. (H.E.S.S. Collaboration). A&A, 464:235–243, March 2007.
  • [8] D. Berge, S. Funk, and J. Hinton. A&A, 466:1219–1229, May 2007.
  • [9] L. O. Drury, F. A. Aharonian, and H. J. Voelk. A& A, 287:959–971, July 1994.
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  • [11] A. M. Hillas. Journal of Physics G Nuclear Physics, 31:R95, May 2005.

30th International Cosmic Ray Conference H.E.S.S. observations of the supernova remnant RX J0852.0-4622: shell-type morphology and spectrum of a widely extended VHE gamma-ray source
M. Lemoine-Goumard, F. Aharonian, B. Degrange, L. Drury, U. Schwanke, for the H.E.S.S. Collaboration
CENBG, Université Bordeaux I, CNRS-IN2P3, Le Haut-Vigneau, 33175 Gradignan, France
Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin, Ireland
Max-Planck-Institut für Kernphysik, P.O. Box 103980, Heidelberg, Germany
LLR, Ecole Polytechnique, CNRS-IN2P3, 91128 Palaiseau, France
Institut für Physik, Humboldt-Universität zu Berlin, D 12489 Berlin, Germany
lemoine@cenbg.in2p3.fr

Abstract:

Introduction

Shell-type supernova remnants (SNR) are widely believed to be the prime candidates for accelerating cosmic rays up to  eV, but until recently, this statement was only supported by indirect evidence, namely non-thermal X-ray emission interpreted as synchrotron radiation from very high energy electrons from a few objects. A more direct proof is provided by the detection of very high energy -rays, produced in nucleonic interactions with ambient matter or by inverse Compton scattering of accelerated electrons off ambient photons.
Here, we present recent data on RX J0852.04622 obtained with H.E.S.S. in 2004 and 2005.

The H.E.S.S. detector and the analysis technique

H.E.S.S. is an array of four 13 m diameter imaging Cherenkov telescopes located in the Khomas Highlands in Namibia, 1800 m above sea level [661]. Each telescope has a tesselated mirror with an area of 107 m [30] and is equipped with a camera comprising 960 photomultipliers [38] covering a field of view of 5 diameter. Due to the powerful rejection of hadronic showers provided by stereoscopy, the complete system (operational since December 2003) can detect point sources at flux levels of about 1% of the Crab nebula flux near zenith with a significance of 5  in 25 hours of observation. This high sensitivity, the angular resolution of a few arc minutes and the large field of view make H.E.S.S. ideally suited for the study of the -ray morphology of extended sources. During the observations, an array level hardware trigger required each shower to be observed by at least two telescopes within a coincidence window of 60 ns [20]. The data were recorded in runs of typical 28 minute duration in the so-called “wobble mode”, where the source is offset from the center of the field of view, and were calibrated as described in detail in [15]. In a first stage, a standard image cleaning was applied to shower images to remove the pollution due to the night sky background. The results presented in this paper were obtained using a 3D-modeling of the light-emitting region of an electromagnetic air shower, a method referred to as “the 3D-model analysis” [23], and were also cross-checked with the standard H.E.S.S. stereoscopic analysis based on the Hillas parameters of showers images [667]. The excess skymap was produced with a background subtraction called the “Weighting Method” [22]. In this method, the signal and the background are estimated simultaneously in the same portion of the sky. In each sky bin (treated independently), the signal and the background are estimated from those events originating from this bin exclusively; this is done by means of a likelihood fit in which each event is characterized by a discriminating parameter whose distribution is fairly different for -rays and hadrons. In the case of the 3D-Model, this discriminating parameter is the 3D-width of the electromagnetic shower.

H.E.S.S. results

RX J0852.04622 is a shell-type SNR discovered in the ROSAT all-sky survey. Its X-ray emission is mostly non-thermal [16]. Indeed, up to now no thermal X-rays were detected from this source, which could imply a limit on the density of the material in the remnant , where is the filling factor of a sphere taken as the emitting volume in the region chosen [536]. The X-ray non-thermal spectrum of the whole remnant in the 2-10 keV energy band is well described by a power law with a spectral index of and a flux  [14]. In the TeV range, the announcement of a signal from the North-Western part of the remnant by CANGAROO was rapidly followed by the publication of a complete -ray map by H.E.S.S. obtained from a short period of observation (3.2 hours) [13]. The study of this source is really complex due to several points: its extension (it is the largest extended source ever detected by a Cherenkov telescope), its location at the South-Eastern corner of the Vela remnant and the uncertainty on its distance and age. Indeed, RX J0852.04622 could be as close as Vela ( 250 pc) and possibly in interaction with Vela, or as far as the Vela Molecular Ridge ( 1 kpc). Figure 1 presents the -ray image of RX J0852.04622 obtained with the 3D-Model from a long observation in 2005 (corresponding to 20 hours live time). The morphology appearing from this skymap reveals a very thin shell of radius and thickness smaller than . Another interesting feature is the remarkably circular shape of this shell, even if the Southern part shows a more diffuse emission. Keeping all events inside a radius of around the center of the remnant, the cumulative significance is about 19 and the cumulative excess is events. The overall -ray morphology seems to be similar to the one seen in the X-ray band, especially in the Northern part of the remnant where a brightening is seen in both wavebands. The correlation coefficient between the -ray counts and the X-ray counts in bins of 0.2 0.2 is found to be equal to 0.60 and comprised between 0.54 and 0.67 at 95 confidence level. The differential energy spectrum (Fig. 2) extends from 300 GeV up to 20 TeV. The spectral parameters were obtained from a maximum likelihood fit of a power law hypothesis dN/dE =   to the data, resulting in an integral flux above 1 TeV of () and a spectral index of 2.24 . An indication of curvature at high energy can be noticed.

Figure 1: Excess skymap of RX J0852.04622 smoothed with a Gaussian of 0.06 standard deviation, obtained with the 3D-Model. The white lines are the contours of the X-ray data from the ROSAT All Sky Survey for energies higher than 1.3 keV (smoothed with a Gaussian of 0.06 standard deviation to enable direct comparison of the two images).
Figure 2: Differential energy spectrum of RX J0852.04622, for the whole region of the SNR. The shaded area gives the 1 confidence region for the spectral shape under the assumption of a power law. The spectrum ranges from 300 GeV to 20 TeV.

Emission processes

One of the key issues is the interpretation of the -ray signal in terms of an electronic or a hadronic scenario. Despite the large uncertainty on the distance and age of the remnant, the multi-wavelength data already give some strong constraints. In a leptonic scenario, where -rays are produced by Inverse Compton scattering of high energy electrons off ambient photons, the ratio of the X-ray flux and the -ray flux determines the magnetic field to be close to . This value is completely independent of the distance and only assumes a filling factor (fraction of the Inverse Compton emitting electrons containing the magnetic field responsible for the synchrotron emission) of 1; this low magnetic field seems hardly compatible with the amplification suggested by the thin filaments resolved by Chandra [17]. In the nearby case ( pc), the limit on the width of the shell obtained by the morphological analysis of the H.E.S.S. data is  pc, which leads to an escape time by diffusion and by convection lower than both the age of the remnant and the synchrotron cooling time for energies higher than  TeV. Therefore, one would expect to see a variation of the width of the shell with the energy, which is not observed by H.E.S.S. [14] and disfavours the electronic scenario at this distance. In a hadronic scenario, in which we assume that the -ray flux is entirely due to proton-proton interactions, one can estimate the total energy in accelerated protons in the range  TeV required to produce the -ray luminosity observed by H.E.S.S.:

In this energy range, the characteristic cooling time of protons through the production channel is approximately independent of the energy and can be estimated to be: . Assuming that the proton spectrum continues down to  GeV with the same spectral slope as that of the photon spectrum, the total energy injected into protons is estimated to be:

Therefore, for densities compatible with the absence of thermal X-rays, the only way to explain the entire -ray flux by proton-proton interactions in a homogeneous medium is to assume that RX J0852.04622 is a nearby supernova remnant (D  pc). Indeed, for larger distances and a typical energy of the supernova explosion of  erg, the acceleration efficiency would be excessive. Nevertheless, a distance of 1 kpc should also be considered if one assumes that RX J0852.04622 is the result of a core collapse supernova which exploded inside a bubble created by the wind of a massive progenitor star [18]. According to stellar wind theory, the size of the bubble evolves according to the formula:  pc. For a density of 1 , the radius of this bubble would be equal to 45 pc. In the case of a close supernova remnant, its size would be significantly lower than the size of the bubble and the hypothesis of a homogeneous medium would be satisfactory. In the opposite, for larger distances ( kpc), the presence of the Vela Molecular Ridge can produce a sudden increase of the density leading to a smaller bubble ( pc for a density of ), which would make the proton-proton interactions efficient at the outer shock.

Summary

We have firmly established that the shell-type supernova remnant RX J0852.04622 is a TeV emitter and for the first time we have resolved its morphology in the -ray range, which is highly correlated with the emission observed in X-rays. Its overall -ray energy spectrum extends over two orders of magnitude, providing the direct proof that particles of  TeV are accelerated at the shock.
The question of the nature of the particles producing the -ray signal observed by H.E.S.S. was also addressed. In the case of a close remnant, the results of the morphological study combined with our spectral modeling highly disfavour the leptonic scenario which is unable to reproduce the thin shell observed by H.E.S.S. and the thin filaments resolved by Chandra. In the case of a medium distance, the explosion energy needed to explain the -ray flux observed by H.E.S.S., taking into account the limit on the density implied by the absence of thermal X-rays, would highly disfavour the hadronic process. At larger distances, both the leptonic and the hadronic scenario are possible, at the expense, for the leptonic process, of a low magnetic field of . Such a small magnetic field exceeds typical interstellar values only slightly and is difficult to reconcile with the theory of magnetic field amplification at the region of the shock.
However, at present, no firm conclusions can be drawn from the spectral shape. The results which should hopefully be obtained by GLAST or H.E.S.S. II at lower energies will therefore have a great interest for the domain.

Acknowledgements

The support of the Namibian authorities and of the University of Namibia in facilitating the construction and operation of H.E.S.S. is gratefully acknowledged, as is the support by the German Ministry for Education and Research (BMBF), the Max Planck Society, the French Ministry for Research, the CNRS-IN2P3 and the Astroparticle Interdisciplinary Programme of the CNRS, the U.K. Science and Technology Facilities Council (STFC), the IPNP of the Charles University, the Polish Ministry of Science and Higher Education, the South African Department of Science and Technology and National Research Foundation, and by the University of Namibia. We appreciate the excellent work of the technical support staff in Berlin, Durham, Hamburg, Heidelberg, Palaiseau, Paris, Saclay, and in Namibia in the construction and operation of the equipment.

References

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30th International Cosmic Ray Conference H.E.S.S. observations of the supernova remnant RCW 86
S. Hoppe & M. Lemoine-Goumard for the H.E.S.S. Collaboration
Max-Planck-Institut für Kernphysik, P.O. Box 103980, Heidelberg, Germany
CENBG, Université Bordeaux I, CNRS-IN2P3, Chemin du Solarium, 33175 Gradignan, France
hoppe@mpi-hd.mpg.de, lemoine@cenbg.in2p3.fr

Abstract:

Introduction

Shell-type supernova remnants are widely believed to be the prime candidates for accelerating cosmic rays up to  eV. The most promising way of proving the existence of high energy particles accelerated in SNR shells is the detection of VHE -rays produced in nucleonic interactions with ambient matter. Recently, the H.E.S.S. instrument has detected VHE -ray emission from two shell-type SNRs, RX J1713.7-3946 [318] and RX J0852.0-4622 [26]. They both show an extended morphology highly correlated with the structures seen in X-rays. Although a hadronic origin is highly probable in the above cases, a leptonic origin can not be ruled out.
Another young shell-type SNR is RCW 86 (also known as G315.4-2.3 and MSH14-63). It has a complete shell in radio [34], optical [37] and X-rays [35], with a nearly circular shape of ’ diameter. It received substantial attention because of its possible association with SN 185, the first historical galactic supernova. However, strong evidence for this connection is still missing: using optical observations, Rosado et al. [36] found an apparent kinematic distance of 2.8 kpc and an age of  10 000 years, whereas recent observations of the North-East part of the remnant with the Chandra and XMM-Newton satellites strengthen the case that the event recorded by the Chinese was a supernova and that RCW 86 is its remnant [39]. These observations also reveal that RCW 86 has properties resembling the already established TeV emitting SNRs mentioned above: weak radio emission and X-ray emission (almost) entirely consisting of synchrotron radiation, which could be due to the expansion of the shock in a wind blown bubble. The South-Western rim seems to be completely different, with hard X-ray emission, observed by ROSAT [31], mainly coming from stellar ejecta possibly interacting with circumstellar layers ejected before the SN explosion. The relatively large size of the remnant - about 40’ in diameter - and the observation of non-thermal X-rays make it a promising target for -ray observations, aiming at increasing the currently modest number of remnants where the shells are resolved in VHE -rays. Hints for -ray emission from RCW 86 were seen with the CANGAROO-II instrument [41], but no firm detection was claimed. Here, we present recent data on RCW 86 obtained with the full H.E.S.S. array in 2005 and 2006 .

The H.E.S.S. detector and the analysis technique

H.E.S.S. is an array of four imaging Cherenkov telescopes located 1800 m above sea level in the Khomas Highlands in Namibia [661]. Each telescope has a tesselated mirror with an area of 107 m [30] and is equipped with a camera comprising 960 photomultipliers [38] covering a field of view of 5 diameter. Due to the effective rejection of hadronic showers provided by its stereoscopy, the complete system (operational since December 2003) can detect point sources at flux levels of about 1% of the Crab nebula flux near zenith with a statistical significance of 5  in 25 hours of observation [512]. This high sensitivity, the angular resolution of a few arc minutes and the large field of view make H.E.S.S. ideally suited for morphology studies of extended VHE -ray sources.
The data on RCW 86 were recorded in runs of typically 28 minutes duration in the so-called “wobble mode”, where the source is slightly offset from the center of the field of view. As a cross-check, the obtained data were analysed using two independent analysis chains, which share only the raw data. The first one is based on the combination of a semi-analytical shower model and a parametrisation based on the moment method of Hillas to yield the combined likelihood of the event being initiated by a -ray [32]. We will call this method the “Combined Model analysis” in the following. The second analysis method is the standard stereoscopic analysis based on the Hillas parameters of the shower images [307].

H.E.S.S. results

Figure 1: VHE -ray emission from RCW 86, as seen with H.E.S.S.. The top image shows the excess skymap obtained with the Combined Model analysis where shower images are matched against image templates, whereas the bottom image results from the classical, slightly less sensitive Hillas analysis technique. White contours correspond to 3, 4, 5, 6 sigma significance, obtained by counting gamma rays within 0.11 from a given location.

RCW 86 was observed for about 30 hours with the H.E.S.S. instrument with a mean zenith angle of . Within a circular region of 27’ radius (defined a priori so that it encompasses the whole remnant) around the centre of the SNR ( = 144243, = 29’), a clear VHE -ray signal with more than standard deviations is detected with both analysis methods described above. The exact morphology of the gamma-ray emission is still under study: whereas one type of data analysis shows hints of a 3/4 shell resembling the shape of the X-ray emission (Fig. 1 top, Fig 2), this morphology is not quite as evident with the other analysis technique (Fig. 1 bottom), and more data may be required to fully settle this issue. The differential energy spectrum of RCW 86, , was extracted from a circular region of diameter 22’ around the position = 144212, = 24’ which is – different from the region for which the detection significance was determined – adjusted to the H.E.S.S. data to include 90 % of the -ray excess. It is well described by a power-law with a spectral index of and a flux normalisation at 1 TeV of . The integral flux in the energy range 1 - 10 TeV is 8% of the integrated flux of the Crab nebula within the same range. However, at this level of data statistics, a power-law with index and an exponential cut-off at TeV is also a good fit to the data.

Figure 2: Significance contours of gamma-ray emission (from the Combined Model analysis; 3, 4, 5, 6 sigma) superimposed onto the XMM X-ray image of the remnant [39].

Discussion

There are two basic mechanisms for -ray production in young supernova remnants, inverse Compton scattering of high energy electrons off ambient photons (leptonic scenario) and mesons produced in inelastic interactions of accelerated protons with ambient gas decaying into -rays (hadronic scenario). The measured -ray flux spectrum from RCW 86 translates into an energy flux between 2 and 10 TeV of . In a leptonic scenario, the ratio of this energy flux and the X-ray flux between 2 and 10 keV (, see Winkler [42]) determines the magnetic field to be close to . This value, completely independent of the distance and age of the SNR, is compatible with the calculation made by J. Vink et al. [39] based on the thin filaments resolved by Chandra for a distance of 2.5 kpc in which he also deduces a high speed of the blast wave (). However, it is more than ten times lower than the value proposed by H. J. Voelk and his colleagues [40] for the same distance using a much lower velocity of the shock of as suggested by optical data in the Southern region of the SNR [36].
In a hadronic scenario, one can estimate the total energy in accelerated protons in the range  TeV required to produce the -ray luminosity observed by H.E.S.S. using the relation:

(1)

in which is the characteristic cooling time of protons through the production channel. The correspnding can be calculated using:

Finally, the total energy injected in protons is calculated by extrapolating the proton spectrum with the same spectral shape as the photon spectrum down to 1 GeV. Therefore, this estimation is highly dependent on the shape of the -ray spectrum. Assuming that the proton spectrum is a power-law with index , one would obtain a total energy injected into protons of . For densities of , the only way to explain the entire -ray flux by proton-proton interactions in a homogeneous medium is to assume that RCW 86 is a close supernova remnant ( kpc). Indeed, for larger distances and a typical energy of the supernova explosion of  erg, the acceleration efficiency would be excessive. For an exponential cut-off power-law with and TeV, the total energy injected into protons would be erg which would make the hadronic scenario possible even at larger distances. However, the observation of TeV gamma-rays from the remnant up to more than 10 TeV favors somewhat the scenario of a young – and therefore close-by – remnant with high expansion speed, easing the acceleration of high-energy particles.

Summary

H.E.S.S. observations have led to the discovery of the shell-type SNR RCW 86 in VHE -rays . The -ray signal is extended but the exact morphology of the emission region is still under study. The flux from the remnant is 8% of the flux from the Crab nebula, with a similar spectral index of 2.5, but the spectrum is also well described by a power law with index 1.9 and a cutoff around 5 TeV. The question of the nature of the particles producing the -ray signal observed by H.E.S.S. was also addressed. However, at present, no firm conclusions can be drawn from the spectral shape.

Acknowledgements

The support of the Namibian authorities and of the University of Namibia in facilitating the construction and operation of H.E.S.S. is gratefully acknowledged, as is the support by the German Ministry for Education and Research (BMBF), the Max Planck Society, the French Ministry for Research, the CNRS-IN2P3 and the Astroparticle Interdisciplinary Programme of the CNRS, the U.K. Science and Technology Facilities Council (STFC), the IPNP of the Charles University, the Polish Ministry of Science and Higher Education, the South African Department of Science and Technology and National Research Foundation, and by the University of Namibia. We appreciate the excellent work of the technical support staff in Berlin, Durham, Hamburg, Heidelberg, Palaiseau, Paris, Saclay, and in Namibia in the construction and operation of the equipment.

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30th International Cosmic Ray Conference Discovery of very high energy gamma-ray emission in the W 28 (G6.40.1) region, and multiwavelength comparisons
G. Rowell, E. Brion, O. Reimer, Y. Moriguchi, Y. Fukui, A. Djannati-Ataï, S. Funk
School of Chemistry & Physics, University of Adelaide, Adelaide 5005, Australia
DAPNIA/DSM/CEA, CE Saclay, F-91191 Gif-sur-Yvette, Cedex, France
Stanford University, HEPL & KIPAC, Stanford, CA 94305-4085, USA
Department of Astrophysics, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan
APC, 11 Place Marcelin Berthelot, F-75231 Paris Cedex 05, France
for the H.E.S.S. Collaboration www.mpi-hd.mpg.de/hfm/HESS
growell@physics.adelaide.edu.au

Abstract:

Introduction & H.E.S.S. Results

The study of shell-type SNRs at -ray energies is motivated by the idea that they are the dominant sites of hadronic Galactic cosmic-ray (CR) acceleration to energies approaching the knee ( eV) and beyond, e.g. [76]. CRs are then accelerated via the diffusive shock acceleration (DSA) process (eg. [45, 73]). Gamma-ray production from the interaction of these CRs with ambient matter and/or electromagnetic fields is a tracer of such particle acceleration, and establishing the hadronic or electronic nature of the parent CRs in any -ray source is a key issue. Already, two shell-type SNRs, RX J1713.73946 and RX J0852.04622, exhibit shell-like morphology in VHE -rays [58, 60, 61] to 20 TeV and above. Although a hadronic origin of the VHE -ray emission is highly likely in the above cases, an electronic origin is not ruled out.

W 28 (G6.40.1) is a composite or mixed-morphology SNR, with dimensions 50x45 and an estimated distance between 1.8 and 3.3 kpc (eg. [46, 75]). It is an old-age SNR (age 3.5 to 15 yr [53]), thought to have entered its radiative phase of evolution [75]. The shell-like radio emission [69, 65] peaks at the northern and northeastern boundaries where interaction with a molecular cloud [44] is established [77, 78]. The X-ray emission, which overall is well-explained by a thermal model, peaks in the SNR centre but has local enhancements in the northeastern SNR/molecular cloud interaction region [67]. Additional SNRs in the vicinity of W 28 have also been identified: G6.670.42 and G7.060.12 [63]. The pulsar PSR J180123 with spin-down luminosity erg s and distance  kpc [70], is at the northern radio edge.

Given its interaction with a molecular cloud, W 28 is an ideal target for VHE observations. This interaction is traced by the high concentration of 1720 MHz OH masers [48, 47, 71], and also the location of very high-density ( cm) shocked gas [78, 77]. Previous observations of the W 28 region at VHE energies by the CANGAROO-I telescope revealed no evidence for such emission [55] from this and nearby regions.

The High Energy Stereoscopic System (H.E.S.S.: see [66] for details and performance) has observed the W 28 region over the 2004, 2005 and 2006 seasons. After quality selection, a total of 42 hr observations were available for analysis. Data were analysed using the moment-based Hillas analysis procedure employing hard cuts (image size 200 p.e.), the same used in the analysis of the inner Galactic Plane Scan datasets [57, 59]. An energy threshold of  GeV results from this analysis. The VHE -ray image in Fig. 1 shows that two source of VHE -ray emission are located at the northeastern and southern boundaries of W 28. The VHE sources are labelled HESS J1801233 and HESS J1801240 where the latter can be further subdivided into three components A, B, and C. The excess significances of both sources exceed 8 after integrating events within their fitted, arcminute-scale sizes. Similar results were also obtained using an alternative analysis [49].

Figure 1: H.E.S.S. VHE -ray excess counts, corrected for exposure and Gaussian smoothed (with 4.2 std. dev.). Solid green contours represent excess significance levels of 4, 5, and 6, for an integrating radius =0.1. The VHE sources HESS J1801233 and a complex of sources HESS J1800-240 (A, B & C) are indicated. The thin-dashed circle depicts the approximate radio boundary of the SNR W 28 [65, 50]. Additional objects indicated are: HII regions (black stars); W 28A2, G6.10.6 6.2250.569; The 68% and 95% location contours (thick-dashed yellow lines) of the  MeV EGRET source GRO J18012320; the pulsar PSR J180123 (white triangle). The inset depicts a pointlike source under identical analysis and smoothing as for the main image.

W 28: The Multiwavelength View

We have revisited EGRET MeV/GeV data, including data from the CGRO observation cycles (OC) 1 to 6, which slightly expands on the dataset of the 3rd EGRET catalogue (using OCs 1 to 4; [51], revealing the source 3EG J18002338. We find a pointlike  MeV source in the W 28 region, labelled GRO J18012320 in Fig 1. The 68% and 95% location contours of GRO J18012320 match well the location of HESS J1801233. However we cannot rule out a connection to HESS J1800240 due to the degree-scale EGRET PSF.

Fig. 2 presents CO (=1–0) observations from the NANTEN [43] Galactic Plane survey [54] covering the line-of-sight velocity ranges = 0 to 10 km s and 10 to 20 km s. These ranges represent distances 0 to 2.5 kpc and 2.5 to 4 kpc respectively and encompass the distance estimates for W 28. We cannot rule out however, distances 4 kpc for the 10 km s cloud components. It is clear that molecular clouds coincide well with the VHE sources. The northeastern cloud 10 km s component near HESS J1801233, is already well-studied [77, 78]. Contributions from the 10 km s cloud components are also likely. The molecular cloud to the south of W 28 coincides well with HESS J1800240 and its three VHE components. The 10 km s component of this cloud coincides well with HESS J1800240B, and may represent the dense molecular matter surrounding the ultra-compact HII region W 28A2. This cloud also extends to 20 km s and thus, similar to HESS J1801233, the total VHE emission in HESS J1800240 may result from several molecular cloud components in projection.

Figure 2: Left: NANTEN CO(J=1-0) image (linear scale in K km s) for =0 to 10 km s with VHE -ray significance contours overlaid (green) — levels 4,5,6 and other features as in Fig. 1. Right: NANTEN CO(J=1-0) image for =10 to 20 km s (linear scale and same maximum as for left panel).

Figure 3: Left: VLA 90cm radio image [50] in Jy beam. The VHE significance contours (green) from Fig. 1 are overlaid along with the HII regions (white stars) and the additional SNRs and SNR candidates (with yellow circles indicating their location and approximate dimensions) discussed in text. Right: ROSAT PSPC image — 0.5 to 2.4 keV (smoothed counts per bin [67]). Overlaid are contours (cyan — 10 linear levels up to 5 W m sr) from the MSX 8.28 ¯m image. Other contours and objects are as for the left panel. The X-ray Ear representing a peak at the northeastern edge is indicated.

Fig. 3 compares the radio (left panel — VLA 90 cm [50]), infrared and X-ray views (right panel MSX 8.28 ¯m and ROSAT PSPC 0.5 to 2.4 keV [67]) with the VHE results. HESS J1801233 overlaps the northeastern shell of the SNR, coinciding with a strong peak in the 90 cm continuum emission. Additional SNRs G6.670.42 and G7.060.12 [63, 52] are indicated. The non-thermal radio arc G5.710.08 is a SNR candidate [50], and is possibly a counterpart to HESS J1801240C. The distances to G6.670.42 and G5.710.08 are presently unknown. The unusual, ultracompact HII region W 28A2, is positioned within of the centroid of HESS J1800240B. W 28A2, at a distance 2 kpc, exhibits energetic bipolar molecular outflows [72, 68, 56] and may therefore be an energy source for particle acceleration in the region. The other HII regions G6.10.6 [74] and 6.2250.569 [64] are also associated with radio emission.

The X-ray morphology (Fig. 3 right panel) shows the central concentration of X-ray emission. A local X-ray peak or Ear is seen at the northeastern W 28 boundary. The HII regions, W 28A2 and G6.10.6 are prominent in the MSX 8.28 ¯m image (Fig. 3 right panel), indicating that a high concentration of heated dust still surrounds these very young stellar objects.

Discussion and Conclusions

H.E.S.S. and NANTEN observations reveal VHE emission in the W 28 region spatially coincident with molecular clouds. The VHE emission and molecular clouds are found at the northeastern boundary, and south of W 28 respectively. The SNR W 28 may be a source of power for the VHE sources, although there are additional potential particle accelerators in the region such as other SNR/SNR-candidates, HII regions and open clusters. Further details concerning these results and discussion are presented in [62].

Acknowledgements

The support of the Namibian authorities and of the University of Namibia
in facilitating the construction and operation of H.E.S.S. is gratefully
acknowledged, as is the support by the German Ministry for Education
and Research (BMBF), the Max Planck Society, the French Ministry
for Research, the CNRS-IN2P3 and the Astroparticle Interdisciplinary
Programme of the CNRS, the U.K. Particle Physics and Astronomy
Research Council (PPARC), the IPNP of the Charles University,
the Polish Ministry of Science and Higher Education, the South
African Department of Science and Technology and National Research
Foundation, and by the University of Namibia. We appreciate the
excellent work of the technical support staff in Berlin, Durham,
Hamburg, Heidelberg, Palaiseau, Paris, Saclay, and in Namibia in the
construction and operation of the equipment. The NANTEN project is
financially supported from JSPS (Japan Society for the Promotion of
Science) Core-to-Core Program, MEXT Grant-in-Aid for Scientific
Research on Priority Areas, and SORST-JST (Solution Oriented
Research for Science and Technology: Japan Science and Technology
Agency). We also thank Crystal Brogan for the VLA 90 cm image.

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30th International Cosmic Ray Conference The Monoceros very-high-energy gamma-ray source
A. Fiasson, J. A. Hinton, Y. Gallant, A. Marcowith, O. Reimer, G. Rowell, for the H.E.S.S. Collaboration
Laboratoire de Physique Théorique et Astroparticules, IN2P2/CNRS, Université Montpellier II, CC 70, Place Eugène Bataillon, F-34095 Montpellier Cedex 5, France
School of Physics & Astronomy, University of Leeds, Leeds LS2 9JT, UK
Stanford University, HEPL & KIPAC, Stanford, CA 94305-4085, USA
School of Chemistery & Physics, University of Adelaide, Adelaide 5005, Australia
Armand.Fiasson@lpta.in2p3.fr, J.A.Hinton@leeds.ac.uk

Abstract:

Introduction

Shell type supernova remnants (SNRs) are believed to be particle accelerator to energy up to a few hundred TeV. Observations of very high energy -ray (VHE; E 100 GeV) from these objects (Aharonian et al. 2006) confirm the presence of particles with energy higher than 10 TeV in these regions. The presence of molecular clouds in the vicinity of SNRs could reveal the nature of such particles as they would interact and produce VHE rays. The Monoceros SNR (G205.5+0.5), situated at 1.6 kpc (Graham et al. 1982), apparently interacting with the Rosette Nebula (a young stellar cluster/ molecular cloud complex situated at 1.4 0.1 kpc(Heinsberger et al. 2000)) is a candidate.
In the case of interaction of accelerated particles with interstellar medium producing neutral pions which decays in two rays, we expect a correlation between -ray emission and matter concentration. We used NANTEN data to trace target material. The NANTEN 4m diameter sub-mm telescope at Las Campanas observatory, Chile, has been conducting a CO (J=10) survey of the galactic plane since 1996, including the Monoceros region (Mizuno & Fukui 2004).

H.E.S.S. observations and results

The H.E.S.S. experiment is an array of four Cherenkov telescope installed in Namibia which detects rays with energy in the 100 GeV to 50 TeV range. A more complete description of the H.E.S.S. experiment is given in Aharonian et al. 2004. The Monoceros loop region has been observed between March 2004 and March 2006 (Aharonian et al. 2007). The dataset includes 13.5 hours of data after quality selection and dead-time correction and was taken at zenith angles ranging between 29 and 59. It corresponds to a mean energy threshold of 400 GeV with standard cuts used in spectral analysis and 750 GeV with hard cuts used for the source search and position fitting.
We made a search for a point-like source on this dataset using a source size of 0.11 and a ring of radius 0.5 for background estimation. We found an excess corresponding to a statistical significance of 7.1. Fig. 1 shows the NANTEN CO map with 4 and 6 levels of statistical confidence contours for the VHE -ray excess (yellow contours). As we made a blind search for a point-like source, the probability we get an excess at this position is increased by the number of positions in the field of view, here 10. This leads to a post-trials statistical significance of 5.3. The excess is confirmed by an independent analysis based on a fit of camera images to a shower model (Model Analysis, see de Naurois 2006), which yields to a significance of 7.3 (5.6 post-trials).

Figure 1: CO (J=10) emission from the Monoceros SNR / Rosette Nebula region. The gray-scale corresponds to velocity integrated (0-30 km.s) emission from the NANTEN Galactic Plane Survey (white areas mean highest flux). The 4 and 6 levels for the statistical significance of a point-like VHE -ray source are shown as yellow contours. Extended cyan contours are radio observations at 8.35 GHz of the Rosette Nebula. The white dashed circle is the Green catalog nominal position and size of the Monoceros SNR. The dotted green contours are 95% and 99% confidence level for the position of the EGRET source 3EG J0634+0521. And last, the position of the binary pulsar SAX J0635.2+053 is marked as a red square and the position of Be-stars with pink stars.
Figure 2: Statistical significance map of the H.E.S.S. VHE -ray source. The rms size limit is shown as a dotted circle. Dotted green contours are 95% and 99% confidence level for the position of the EGRET source 3EG J0634+0521. The unidentifed X-ray source 1RXS J063258.3+054857 is marked with a triangle and the Be-star MCW 148 with a star.

The fitted position of this new source HESS J0632+057, is 63258.3, +548’20” (RA/Dec. J2000) with 28” statistical errors on each axis (fig. 2). We estimated systematics errors at 20” on each axis. The fig. 3 represents the distribution of signal in function of the angular distance around the fitted position. The distribution is fully compatible with the point spread function (red curve). We derived an upper limit on the size of the source of 2’ at 95% confidence assuming a Gaussian profile for the source.
The reconstructed energy spectrum of the excess is consistent with a power-law of index and differential flux at 1 TeV cmsTeV. The first errors are statistical errors and the second are estimated systematic errors. Fig.4 represents the VHE -ray reconstructed flux together with that for the EGRET sources 3EG0634+0521 and the upper limit derived by the HEGRA telescope array for the EGRET source position (converted to differential flux assuming the spectral shape observed by H.E.S.S.). There is no evidence of flux variability in our dataset but the sparse sampling of data together with the weakness of the source do not permit to constrain strongly intrinsic variability of the source.

Figure 3: Distribution of -ray candidates events as function of squared angular distance from the bes fit position of HESS J0632+057. The red line is the point spread function corresponding to this dataset obtained with Monte-Carlo simulations.
Figure 4: Reconstructed VHE -ray spectrum of HESS J0632+057 compared to the EGRET source 3EG J0634+0521. The upper limit obtained using the HEGRA instrument for the EGRET source position is shown.

Possible associations

The region where lies HESS J0632+057 is a complex region and although there is no clear counterpart, it may be associated with various objects known at other wavelengths.

3eg J0634+0521

In the same region lies also an EGRET source, 3EG J0634+0521 (Hartman et al., 1999). Considering that the source is flagged as confused and possibly extended, our measurement, which lies between 95% and 99% confidence region, is compatible with its position. Furthermore, the reported third EGRET catalogue flux above 100 MeV is consistent with an extrapolation of the H.E.S.S. spectrum. A global fit of the two spectra gives a photon index of 2.41 0.06 (fig.4).

The Monoceros Loop SNR

The possible association of spectra in the GeV and TeV band is an argument in favor of an hadronic interpretation of the VHE -ray emission. In this case, the Monoceros loop SNR is a good candidate for acceleration of particles. This remnant, which has an age of 310 years, is rather old in comparison to known shell type SNRs emitting VHE rays (2000 years). However, cosmic rays acceleration may occur even at later evolutionary phase (late Sedov or Radiative, see Yamazaki et al. 2006). Given the point-like nature of the VHE -ray emission, to explain VHE rays as a product of accelerated cosmic rays interacting with interstellar medium requires the presence of a dense molecular cloud coincident with the emission. An unresolved molecular cloud listed in CO survey at 115 GHz (Oliver et al. 1996) lies rather close to HESS J0632+057. The distance estimate for this cloud (1.6 kpc) is consistent with that for the Monoceros SNR. NANTEN survey shows that the intensity peak of this cloud is significantly shifted to the east of the H.E.S.S. source (fig. 1). There is no evidence of other dense clouds along the line of sight in the NANTEN data.

1 Rxs j063258.3+054857

1 RXS J063258.3+054857 is a faint ROSAT source which is potential counterpart of HESS J0632+057, given the uncertainty of the position of the two objects. Given the number of sources in the field of view, the chance probability of coincidence of the two source is 0.1%. X rays are useful to discriminate between scenario of VHE -ray emission. If -rays are due to inverse Compton scattering from a population of accelerated electrons, X rays are expected to come from synchrotron emission of the same population. In this case, the weakness of this source (10erg cm s) compared to the TeV flux (10erg cm s) required a very low magnetic field (3G), unless a strong radiation source exists in the neighbourhood of the emission region. Important absorption of the X-ray emission may also explain weakness of the ROSAT source. In the case of a hadronic scenario, production of pions leads to secondary electrons which produce a weaker X-ray source, probably compatible with the measured ROSAT flux.

Mwc 148

A massive emission-line Be-star lies within the H.E.S.S. error circle. Given the fact that there are only three stars of this type in the field of view, the chance probability of the association is 10. Stars of this spectral type have winds with typical velocities and mass loss rates of 1000 km.s and 10M. Stellar winds may induce internal or external shocks where particles can be accelerated, but no association of VHE -ray emission with similar stars have been already detected and seems unlikely. Another hypothesis is that this star is a part of a binary system with a compact companion not already detected. Further observations are required to constrain this scenario.

Acknowledgments

The support of the Namibian authorities and of the University of Namibia in facilitating the construction and operation of H.E.S.S. is gratefully acknowledged, as is the support by the German Ministry for Education and Research, the CNRS-IN2P3 and the Astroparticle Interdisciplinary Programme of the CNRS, the U.K. Particle Physics and Astronomy Research Coucil (PPARC), the IPNP of the Charles University, the South African Department of Science and Technology and National Research Foundation, and by the University of Namibia. We appreciate the excellent work of the technical support staff in Berlin, Durham, Hamburg, Heidelberg, Palaiseau, Paris, Saclay, and in Namibia in the construction and operation of the equipment. The NANTEN project is financially supported from JSPS (Japan Society for the Promotion of Science) Core-to-Core Program, MEXT Grant-in-Aid for Scientific Research on Priority Areas, and SORST-JST (Solution Oriented Research for Science and Technology: Japan science and Technology Agency). We would also like to thank Stan Owocki and James Urquhart for very useful discussions.

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30th International Cosmic Ray Conference Crab nebula spectrum as seen by H.E.S.S.
B. Khélifi, C. Masterson, S. Pita, E. Oña-Wilhelmi for the H.E.S.S. collaboration
Laboratoire Leprince-Ringuet, Ecole Polytechnique/IN2P3/CNRS, Palaiseau, France
Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin 2, Ireland
AstroParticule et Cosmologie, Paris VII/IN2P3/CNRS, Paris, France
khelifi@llr.in2p3.fr

Abstract:

Introduction

The Crab nebula was discovered at very high energies (VHE; 100 GeV) in 1989 [88] and the emission has been confirmed by a number of other experiments (e.g. [89, 90, 643]). This pulsar wind nebula (PWN) has a high flux relative to other known VHE sources and its emission is expected to be stable. As a result, the Crab nebula is commonly used as a standard ‘calibration candle’ for the ground-based gamma-ray detectors, and a particular attention is paid here to the control of the analysis chain accuracy. Indeed, the detector ageing results from a decrease of the overall optical efficiency (a combination of mirrors, light-cones, and photomultipliers degradation) and from ageing of electronics components of cameras. The detector response is measured, calibrated [640] and used for the data analysis [641].

Important questions on the origin of the non-thermal emission of the Crab nebula remain. It is commonly admitted that its spectral energy distribution (SED) can be well-reproduced with a mechanism based on a synchrotron self-Compton (SSC) emission of high energy electrons/positrons (e.g. [94]) even if a contribution from proton radiation is not excluded at high energies (e.g. [95]). However, the acceleration mechanisms of these leptons and hadrons are still under investigation (Cf. [96] for a recent review). Thus, multi-wavelength observations are still necessary to understand the underlying physics, in particular observations of VHE gamma-rays above 30 TeV.

H.E.S.S. observations and data analysis

The Crab nebula has been observed with the complete array for 58.4 hours from December 2003 to December 2006. After data-quality selection based on good weather conditions and good detector operation, an exposure of 29.4 hours live-time is obtained. The periods of the Crab observations suffer sometimes of poor weather conditions in Namibia. All observations were taken in wobble mode whereby the source is alternately offset by a fixed distance within the field of view, alternating between 28 minutes runs in positive and negative declination (or right ascension) directions.

In table 1 we present, for each observation period considered, the live-time (in hours), mean zenith angle (in degrees), mean position (in degrees) of the Crab pulsar position relative to the centre of the field of view and mean optical efficiency (in percent) of the detection system.

Year 2004 2005 2006 All
Live-time [h] 20.6 5.4 3.4 29.4
Zenith Angle [deg] 52.2 47.7 49.2 51.1
Offset [deg] 0.65 0.58 0.70 0.65
OptEff [%] 8.3 7.8 7.0 8.1
Table 1: Summary of the Crab observations. The row descriptions are given in the text.

The data are processed with the HAP (H.E.S.S. Analysis Package) software as follows. In order to reject the overwhelming background of night-sky diffuse light and hadronic showers, a two-level image cleaning is performed to remove pixels containing only background noise. After image cleaning, the Hillas parameters [307] are computed. For comparison, two methods are used to reconstruct the characteristics of the atmospheric showers, i.e. the impact parameter (), the shower maximum () and the shower direction. The first method [641], called hereafter Hillas, is based on a geometrical reconstruction of the shower characteristics from the Hillas parameters (tracks of the projected direction of the shower in the field of view). The second one, called Model3D [98], uses a model of the atmospheric shower as a ‘Cherenkov ellipsoid’ and its parameters are adjusted to the camera images. Cuts are applied to the parameters derived by these methods to improve the signal to (hadronic) noise ratio. For the Model3D analysis, the standard cuts of the Hillas analysis are applied together with cuts on the ‘Cherenkov ellipsoid’ size. The remaining background is estimated from regions at same distance from the field of view centre as the Crab pulsar position for the observations (cf. fig. 9 of [641]).

The energy of each event is estimated from , and the images charges within the Hillas ellipses (). Look-up tables given the image charges as a function of energy (), and ( are derived from gamma-ray simulations made with Kaskade [99] for different fixed energies, zenith angles, offsets and optical efficiencies. Given the measured , and , inverting the tables provides an estimation of the event energy. To determine the energy spectrum, the instrument response functions (effective areas and energy resolutions) are derived from the same gamma-ray simulations, and a forward-folding algorithm developed by the CAT collaboration [100] is used. A likelihood fit is used to adjust different spectral shape hypotheses. A test of the hypotheses with a likelihood ratio is made to determine the spectrum shape that best adjusts to the data.

H.E.S.S. results

The main results of the analysis of the Crab observations are given in table 2. For each method of shower reconstruction and for each year, the number of gamma-rays above the analysis energy threshold, the significance and the integral flux above 1 TeV are listed. A strong signal is detected and, independently of the year and the analysis method, the integral flux is basically constant, illustrating the good correction for the effects of the detector ageing.

Figure 1: Distribution of the run-wise integral fluxes above 1 TeV for the Hillas analysis.

The run-wise fluxes are also computed and their distribution is given in fig. 1 for the Hillas analysis. It follows a Gaussian distribution (black line) with a /dof of . The best-fit parameters are and in units of . The flux derived is thus compatible with a steady flux with Gaussian fluctuations of .

Method Year Excess Significance
[] [] []
Hillas 2004 5788 122
2005 1674 70
2006 1069 57
All 8531 151
Model3D 2004 5208 130
2005 1612 74
2006 1008 59
All 7828 161
Table 2: Results of the observations. The column descriptions are given in the text.
Hillas Model3D
Table 3: Summary of spectrum fits. The row descriptions are given in the text.

For both analyses, the energy spectrum is computed for two different spectral hypotheses: a pure power-law () and a power-law with an exponential cut-off (). The fit results are listed in table 3. The parameter is in units of , in TeV. is the ratio between the maximum likelihood of the fit over the fit and its distribution follows asymptotically a law with one degree of freedom. From this parameter and independently of the analysis method used, it can clearly be seen that the fitted spectrum shape is not compatible with a pure power-law with a probability less than . The use of a ‘parabolic’ spectrum shape () fits the data equally well as a power-law with an exponential cut-off. Note that the fit results are compatible between the different analyses.

Figure 2: Comparison of the Crab spectrum fits between this analysis and that published in [641]. The lines are the best-fit shapes.

Figure 2 shows the Crab spectrum derived with these two analyses carried out with the HAP software, together with the H.E.S.S. spectrum published in [641]. In the following, the results of the fit for the Hillas analysis are used and the flux measurements for each energy bin (differential flux) are given in table 4. Here, the measurements on high energy bins above 30 TeV should be emphasised in which a signal is detected at the level of . A signal is detected significantly at the highest energies which allows the spectrum curvature to be measured more accurately . Figure 3 shows the comparison of the best-fit parameters and between these analyses and the results from [641]. The parameters are quite compatible between these and the exponential cut-off energy, , is compatible with 15 TeV.

Figure 3: Comparison of the best-fit parameters between this analysis and those published in [641].
Mean Energy Significance
[] []
0.39 16.7
0.62 73.4
0.97 78.4
1.54 67.7
2.43 55.9
3.84 41.3
6.06 30.2
9.54 22.4
15.0 12.4
23.5 7.9
36.7 4.1
57.0 3.8
Table 4: Flux measurements for each energy bin for the Hillas analysis.

Conclusions

Analysis of Crab data carried out with the new software framework HAP yields results which are consistent with those published previously by H.E.S.S. in [641]. The measured Crab flux is compatible with a steady flux between December 2003 and December 2006, indicating that all effects of the detector ageing are correctly taken into account. The integral flux above 1 TeV is . Its energy spectrum is not compatible with a pure power-law shape and is well-represented by a power-law with an exponential cut-off ().

Comparing the results of different analyses presented here, one finds that the differences of flux and spectrum index estimated are well within the systematics detailed in [641].

A clear signal is detected above 30 TeV which allows the curved nature of the Crab nebula spectrum to be clearly confirmed. This measured spectrum seems to be still compatible with a SSC scenario in the Klein-Nishina regime as described in [94]. An adjustment of the fit parameters of this radiation model on our data is still necessary to confirm this scenario.

Acknowledgements

The support of the Namibian authorities and of the University of Namibia in facilitating the construction and operation of H.E.S.S. is gratefully acknowledged, as is the support by the German Ministry for Education and Research (BMBF), the Max Planck Society, the French Ministry for Research, the CNRS-IN2P3 and the Astroparticle Interdisciplinary Programme of the CNRS, the U.K. Science and Technology Facilities Council (STFC), the IPNP of the Charles University, the Polish Ministry of Science and Higher Education, the South African Department of Science and Technology and National Research Foundation, and by the University of Namibia. We appreciate the excellent work of the technical support staff in Berlin, Durham, Hamburg, Heidelberg, Palaiseau, Paris, Saclay, and in Namibia in the construction and operation of the equipment.

References

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30th International Cosmic Ray Conference Energy Dependent Morphology in the PWN candidate HESS J1825–137
S. Funk, J. A. Hinton,O. C. deJager for the H.E.S.S. collaboration
Kavli Institute for Particle Astrophysics and Cosmology, SLAC, Menlo Park, CA-94025, USA
School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
Unit for Space Physics, North-West University, Potchefstroom 2520, South Africa
Stefan.Funk@slac.stanford.edu

Abstract:

Introduction

The pulsar PSR B1823–13 and its surrounding X-ray pulsar wind nebula (PWN) G18.0–0.2 is a system that has been studied by H.E.S.S. in very high-energy gamma-rays above 200 GeV in unprecedented detail [101]. PWNe seem to constitute a significant fraction of the population of identified Galactic VHE gamma-ray sources detected by H.E.S.S. [102] and as also suggested by a statistical assessment of the correlation between Galactic VHE -ray sources and energetic pulsars (see Carrigan et al., these proceedings). The gamma-ray emission in these objects is typically thought to be generated by Inverse Compton scattering of relativistic electrons accelerated in the termination shock of the PWN.

Considering the population of VHE gamma-ray PWNe, HESS J1825–137 is probably thus far the best example of the emerging class of so-called offset Pulsar Wind nebulae in which an extended VHE gamma-ray emission surrounding an energetic pulsar is offset into one direction of the pulsar. This offset is generally thought to arise from dense molecular material in one direction of the pulsar that prevents an symmetric expansion of the PWN (see e.g. [103] for a hydro-dynamical simulation and discussion of this effect).

As one of the best studied objects in VHE gamma-rays with an observation time of nearly 70 hours, HESS J1825–137 has been used as a template for the association of asymmetric PWN in VHE -rays and X-rays [102, 104]. In HESS J1825–137 the claimed association between the VHE -ray source and the X-ray PWN rests on the following properties of the source:

  • Same morphology (i.e. asymmetric extension to the south) in both bands but X-ray nebula much smaller ( 5”) than -ray ( 0.5) emission region

  • Spectral steepening of the VHE gamma-ray source away from the pulsar (i.e. decrease of gamma-ray extension with increasing energy). Interestingly the maximum of the VHE -ray emission is not coincident with the pulsar position but is shifted to the south-west.

The vastly different sizes of the emission region in the two wavebands prevents at first glance a direct identification as a counterpart, since the morphology can not be matched between X-rays and gamma-rays. As will be explained in the following, the different sizes can be explained in a time-dependent leptonic model by different cooling timescales of the X-ray and of the VHE gamma-ray emitting regions. Caution should however be used, if such an association serves as a template for other unidentified H.E.S.S. VHE gamma-ray sources with an energetic pulsar in the vicinity, in cases in which no X-ray PWN has been detected so far.

Observational data

CO-Observations performed in the composite survey [142] show a dense molecular cloud in the distance band between 3.5 and 4 kpc to the north of PSR B1823–13 (located at kpc) [106]. This cloud seems to support the picture of an offset PWN and could explain why the X-ray and VHE emission is shifted to the south of the pulsar. Given the relatively high gamma-ray flux and the rather large distance of the system of 4 kpc (in comparison to the Crab), the required gamma-ray luminosity is comparable to the Crab luminosity. The spin-down luminosity of the pulsar is, however, two orders of magnitude lower than the Crab spin-down luminosity. Assuming the distance of  kpc is correct this shows that the efficiency of converting spin-down power to gamma-ray luminosity must be much higher than in the Crab Nebula, not unexpected, given the large magnetic field in the Crab Nebula. Detailed time-dependent modelling of the source shows indeed that (especially below TeV) the energy injection into the system must have been about an order of magnitude higher in the past. Potentially the spin-down power of the pulsar was significantly higher in the early stage of the pulsar evolution. For the lower energy end of the H.E.S.S. spectrum and for modest magnetic fields of a few G as suggested by the large VHE gamma-ray flux, the electron lifetimes become comparable to the pulsar age and therefore “relic” electrons released in the early history of the pulsar can survive until today and provide the required luminosity. It should be noted that to this date no sensitive X-ray observation of the region coinciding with the peak of the VHE gamma-ray emission has been performed and a low surface-brightness extension to the south of the X-ray PWN found by Gaensler et al. [107] remains an interesting possiblility that should eventually be tested.

Figure 1: Three-colour image showing the gamma-ray emission in different energy bands (red: 0.2-0.8 TeV, green 0.8-2.5 TeV and blue: above 2.5 TeV). The different gamma-ray energy bands show a shrinking with increasing energy away from the pulsar PSR B1823–13.

Energy dependent morphology

Given the large data set with nearly 20,000 -ray excess events, a spatially resolved spectral analysis of HESS J1825–137 could be performed. For the the first time VHE -ray astronomy an energy dependent morphology (see Figure 1) was established [101] in which the size of the emission region decreases with increasing energy. This shrinking size with increasing energy is equivalent to the statement of a steepening of the spectral index away from the pulsar. The spectrum in HESS J1825–137 changes from a rather hard photon index close to the pulsar to a softer value of at a distance of away from the pulsar. Figure 2 shows the surface brightness as a function of the distance from the pulsar for different energy bands. Two clear trends are apparent in this figure: a) the peak of the surface brightness shifts to lower energies (as already suggested by the steepening of the energy spectrum away from the pulsar) b) at low energies the surface brightness is nearly independent of the distance whereas at the higher energies the surface brightness drops rapidly with increasing distance from the pulsar. The right panel of Figure 2 shows the derived radius corresponding to the 50% containment of the surface brightness. This radius drops with increasing energy as already apparent in Figure 1.

The steepening of the energy spectrum away from a central pulsar is a property commonly observed in X-ray studies of PWNe other than the Crab. For most of these system the total change in the photon index is close to similar to what is seen in HESS J1825–137. It should be noted that the results shown here represent the first unambiguous detection of a spectral steepening at a fixed electron energy (since the synchrotron emission seen in X-rays depends on the magnetic field) in a PWN system. Spectral variation with distance from the pulsar could result from (1) energy loss of particles during propagation, with radiative cooling of electrons as the main loss mechanism, from (2) energy dependent diffusion or convection speeds, and from (3) variation of the shape of the injection spectrum with age of the pulsar. Concerning (1): Loss mechanisms include amongst others adiabatic expansion, ionisation loss, bremsstrahlung, synchrotron losses and inverse Compton losses. Only synchrotron and IC losses can result in a electron lifetime that decreases with increasing energy. A source decreasing source size with increasing energy is therefore generally seen as indicative of electrons as the radiating particles.

Figure 2: Left: Surface brightness as a function of distance from the pulsar for different energy bands (derived from Figure 4 in Aharonian et al [101]. The surface brightness is defined as the differential gamma-ray flux at a given energy scaled by the area of the extraction region and normalised by the flux for that energy at the pulsar position r = 0. Right: Distance from the pulsar at which the surface brightness drops to 50% of the flux at the pulsar position. The error bars are derived by fitting the falling points of the left plot, varying the fit parameters within the errors and recalculating the 50% containment radius.

For continuous injection and short radiative lifetimes of the electrons (in comparison to the age of the source), the spectral index of the electrons changes by one unit as a result of the cooling, yielding in a change of 0.5 in the the photon index. This matches roughly what is seen in HESS J1825–137 when comparing the inner and the outer nebula. The lower energy gamma-rays (i.e. below TeV) correspond to mostly un-cooled low energy electrons (i.e. the spectral index consistent with the injection spectral index). At these low energies the electron lifetime becomes comparable to the age of the source and the size is rather independent of the energy. At higher energies the cooling break takes effect and the source size shrinks with increasing energy as expected from electron cooling. The XMM-Newton X-ray emitting electrons typically have much higher energies ( TeV) than the -ray emitting electrons ( TeV), assuming a typical magnetic field of 5 G. The synchrotron cooling lifetime of X-ray emitting electrons is therefore expected to be much smaller, resulting in a smaller spatial extension in X-rays.

For systems like HESS J1825–137 a detailed study in X-rays trying to detect the low surface brightness nebula in the soft X-ray band would be very beneficial, is however very hard to achieve given the absorption of soft X-rays. The upcoming GLAST-satellite will observe this object in a thus far rather unexplored energy regime especially above  GeV, where the angular resolution of the instrument becomes comparable to the angular resolution of the ground-based instrument. In this energy range GLAST will probe even lower energy electrons and it will be interesting to compare the sizes of the GLAST and the H.E.S.S. emission region. The H.E.S.S. results have shown that a wealth of detail exists in gamma-rays at an angular scale of . Future instruments like CTA or AGIS might improve this angular resolution even further.

Acknowledgements

”The support of the Namibian authorities and of the University of Namibia in facilitating the construction and operation of H.E.S.S. is gratefully acknowledged, as is the support by the German Ministry for Education and Research (BMBF), the Max Planck Society, the French Ministry for Research, the CNRS-IN2P3 and the Astroparticle Interdisciplinary Programme of the CNRS, the U.K. Science and Technology Facilities Council (STFC), the IPNP of the Charles University, the Polish Ministry of Science and Higher Education, the South African Department of Science and Technology and National Research Foundation, and by the University of Namibia. We appreciate the excellent work of the technical support staff in Berlin, Durham, Hamburg, Heidelberg, Palaiseau, Paris, Saclay, and in Namibia in the construction and operation of the equipment.”

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30th International Cosmic Ray Conference Discovery of the candidate pulsar wind nebula HESS J1718-385 in very-high-energy gamma-rays
S. Carrigan, Y.A. Gallant, J.A. Hinton, Nu. Komin, K. Kosack and C. Stegmann for the H.E.S.S. collaboration
Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany
Laboratoire de Physique Théorique et Astroparticules, IN2P3/CNRS, Université Montpellier II, CC 70, Place Eugène Bataillon, F-34095 Montpellier Cedex 5, France
Landessternwarte, Universität Heidelberg, Königstuhl, D 69117 Heidelberg, Germany
Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, D 91058 Erlangen, Germany
svenja.carrigan@mpi-hd.mpg.de

Abstract:

Introduction

It has long been known that pulsars can drive powerful winds of highly relativistic particles. Confinement of these winds leads to the formation of strong shocks, which may accelerate particles to PeV energies.

The best studied example of a pulsar wind nebula (PWN) is the Crab nebula, which exhibits strong non-thermal emission across most of the electromagnetic spectrum from radio to 50 TeV -rays [117]. More recently, VHE -ray emission has been detected from the Vela X PWN [111], which is an order of magnitude older (11 kyr) than the Crab nebula, and its nebula is significantly offset from the pulsar position, both in X-rays and VHE -rays. Offset nebulae in both X-rays and VHE -rays have also been observed in the Kookaburra Complex [109] and for the PWN associated with the -ray source HESS J1825137 [114, 110]. The latter source appears much brighter and more extended in VHE -rays than in keV X-rays. This suggests that searches at TeV energies are a powerful tool for detecting PWNe.

Motivated by these detections, a systematic search for VHE -ray sources associated with high spin-down energy loss rate pulsars was performed, using data obtained with the H.E.S.S. instrument. The VHE -ray data set used in the search includes all data used in the H.E.S.S. Galactic plane survey [343], an extension of the survey to , dedicated observations of Galactic targets and re-observations of H.E.S.S. survey sources. It spans Galactic longitudes and Galactic latitudes , a region covered with high sensitivity in the survey. These data are being searched for VHE emission from pulsars from the Parkes Multibeam Pulsar Survey [118]. The search for a possible -ray excess is done in a circular region with radius (as in [343]) around each pulsar position, sufficient to encompass a large fraction of a possible PWN. The statistical significance of the resulting associations of the VHE -ray source with the pulsar is evaluated by repeating the procedure for randomly generated pulsar samples, modelled after the above-mentioned parent population.

In this search, it is found that pulsars with high spin-down energy loss rates are on a statistical basis accompanied by VHE emission. The search for VHE -ray emission near the pulsar PSR J17183825 revealed the new VHE -ray source HESS J1718385. This paper deals with the results from the HESS data analysis of HESS J1718385 and with its possible associations with PSR J17183825 and other objects seen in radio and X-ray wavelengths.

H.E.S.S. Observations and Analysis

The data on HESS J1718385 are composed primarily from dedicated observations of the supernova remnant RX J1713.73946 [283], which is located at about 1.6 south-west of HESS J1718385. After passing the H.E.S.S. standard data quality criteria based on hardware and weather conditions, the data set for HESS J1718385 has a total live time of 82 hours. The standard H.E.S.S. analysis scheme [342] is applied to the data, including optical efficiency corrections. In this analysis, hard cuts are applied, which include a rather tight cut on the shower image brightness of 200 photo-electrons and are suitable for extended, hard-spectrum sources such as PWN. These cuts also improve the angular resolution and therefore suppress contamination from the nearby RX J1713.73946. To produce a sky map, the background at each test position in the sky is derived from a ring surrounding this position with a mean radius of 1 and a width scaled to provide a background area that is about 7 times larger than the area of the on-source region.

For spectral studies, only observations in which the camera centre is offset by less than 2 from the best-fit source position are used to reduce systematic effects due to reconstructed -ray directions falling close to edge of the field of view. The remaining live time of the data sample is 73 hours. The spectral significance is calculated by counting events within a circle of radius 0.2 from the best-fit position, chosen to enclose the whole emission region while reducing systematic effects arising from morphology assumptions. The proximity of the strong source makes it necessary to choose the background data from off-source observations (matched to the zenith angle and offset distribution of the on-source data) instead of from areas in the same field of view. For a more detailed description of methods for background estimation, see [115].

Results

The detection significance from the search for VHE -ray emission within 0.22 of the location of PSR J17183825 is . A very conservative estimate of the number of trials involved ([343]) leads to a corrected significance of .

Figure 1: An image of the VHE -ray excess counts of HESS J1718385, smoothed with a Gaussian of width 0.06. The colour scale is set such that the blue/red transition occurs at approximately the 3 significance level. The black contours are the 4, 5 and 6 significance contours. The position of the pulsar PSR J17183825 is marked with a green triangle and the Galactic plane is shown as a white dotted line. The best-fit position for the -ray source is marked with a black star and the fit ellipse with a dashed line.

Figure 1 shows the smoothed excess count map of the 1  1 region around HESS J1718385. A two-dimensional Gaussian brightness profile, folded with the H.E.S.S. point-spread function, is fit to the distribution before smoothing. Its parameters are the width in two dimensions and the orientation angle, defined counter-clockwise from North. The intrinsic widths (with the effect of the point-spread function removed) for the fit are and and the orientation angle is 33. The best-fit position for the centre of the excess is RA = , Dec =  (epoch J2000). H.E.S.S. has a systematic pointing error of .

For the spectral analysis, a statistical significance of (with 343 excess counts) is derived. Figure 2 shows the measured spectral energy distribution for HESS J1718385 (in representation).

Figure 2: The energy spectrum of HESS J1718385, which is fit by a curved profile (solid line). Alternatively, the fit of an exponentially cut-off power law is shown (dashed line, refer to the text for details on both fits). The first point in the spectrum lacks statistics due to lower exposure at small zenith angles and is plotted as an upper limit with at a confidence level of .

The spectrum is fit by a curved profile (shown as the solid line):

(1)

The peak energy is  TeV, the differential flux normalisation  TeV cm s and . This fit has a of . The integral flux between  TeV is about 2 % of the flux of the Crab nebula in the same energy range [342].

Alternatively, fitting the spectrum by an exponentially cut-off power law (