Discovery of very high energy \gamma-ray emission from Centaurus A

Discovery of very high energy gamma-ray emission from Centaurus A with H.E.S.S

Abstract

We report the discovery of faint very high energy (VHE,  GeV) -ray emission from the radio galaxy Centaurus A in observations performed with the H.E.S.S. experiment, an imaging atmospheric Cherenkov telescope array consisting of four telescopes located in Namibia. Centaurus A has been observed for more than 120 h. A signal with a statistical significance of 5.0  is detected from the region including the radio core and the inner kpc jets. The integral flux above an energy threshold of 250 GeV is measured to be of the flux of the Crab Nebula (apparent luminosity: L(250 GeV) erg s, adopting a distance of 3.8 Mpc). The spectrum can be described by a power law with a photon index of . No significant flux variability is detected in the data set. However, the low flux only allows detection of variability on the timescale of days to flux increments above a factor of ( and , respectively). The discovery of VHE -ray emission from Centaurus A reveals particle acceleration in the source to  TeV energies and, together with M 87, establishes radio galaxies as a class of VHE emitters.

Subject headings:
galaxies: active — galaxies: individual ( (catalog Cen A)) — gamma rays: observations

1. Introduction

Centaurus A (Cen A) is the nearest active galaxy (for a review see Israel 1998). It is classified as a FR I radio galaxy; these are thought to be the parent population of BL Lac objects (Urry & Padovani, 1995). At radio wavelengths rich jet structures are visible, extending from the core and the inner pc and kpc jet to giant outer lobes with an angular extension of . The inner kpc jet has also been detected in X-rays, revealing a complex structure of bright knots and diffuse emission (Kraft et al., 2002). The angle of the jet axis to the line of sight is estimated to be 15-80 (see e.g. Horiuchi et al., 2006, and references therein). With a distance of 3.8 Mpc (Rejkuba 2004; corresponds to  kpc projected length) even the inner jet structures are resolvable with the angular resolution of current experiments in the very high energy (VHE;  GeV) regime181. The elliptical host NGC 5128 features a dark lane, a thin edge-on disk of dust and young stars, believed to be the remnant of a merger. Recent estimates for the mass of the central supermassive black hole give (Cappellari et al., 2008). The kpc-scale jet and the active nucleus are confirmed sources of strong non-thermal emission. In addition, more than 200 X-ray point sources with an integrated luminosity of L erg s are established to be associated with the host galaxy (Kraft et al., 2001). Recently, Croston et al. (2009) reported the detection of non-thermal X-ray emission from the shock of the southwest inner radio lobe from deep Chandra observations.

Cen A was detected at MeV to GeV energies by all instruments on board the Compton Gamma-Ray Observatory (CGRO) in the period 1991 – 1995, revealing a peak in the spectral energy distribution (SED) in representation at  MeV with a maximum flux of about  erg cm s (Steinle et al., 1998). Steinle et al. (1998) also reported variability of the Cen A source, especially pronounced at 10 MeV, while Sreekumar et al. (1999) found the EGRET flux was stable during the whole period of CGRO observations. A tentative detection of Cen A () at VHE during a giant X-ray outburst in the 1970’s was reported by Grindlay et al. (1975). Subsequent VHE observations made with the Mark III (Carramiñana et al., 1990), JANZOS (Allen et al., 1993), CANGAROO (Rowell et al., 1999; Kabuki et al., 2007), and H.E.S.S. (Aharonian et al., 2005) experiments resulted in upper limits.

Cen A has been proposed as a possible source of ultra-high energy cosmic rays (UHECR; E  eV) (Romero et al. 1996, but see also Lemoine 2008). Recently, the Pierre Auger Collaboration reported an anisotropy in the arrival direction of UHECR (Abraham et al., 2007, 2008). While a possible correlation with nearby active galactic nuclei is still under discussion, it has been pointed out that several of the events can possibly be associated with Cen A (e.g. Gorbunov et al., 2008; Moskalenko et al., 2008; Kachelriess et al., 2008).

Until now, the only firmly established extragalactic VHE -ray source with only weakly beamed emission is the giant radio galaxy M 87 (Aharonian et al., 2003, 2006a). M 87 showed strong flux outbursts in the VHE regime with timescales on the order of days (Aharonian et al., 2006a; Albert et al., 2008), pointing to a characteristic size of the emission region  cm, corresponding to Schwarzschild radii ( M, : relativistic Doppler factor). Recently, Aliu et al. (2009) and Acciari et al. (2009) reported VHE emission from the direction of the blazar 3C 66 A and the radio galaxy 3C 66 B (angular separation 6’). While Aliu et al. (2009) favor 3C 66 B as the origin of the VHE emission in their dataset, Acciari et al. (2009) exclude 3C 66 B as the origin of their detected emission with a significance of 4.3.

In this Letter the discovery of VHE emission from Cen A with the H.E.S.S. experiment is reported, and properties of the detected emission and their implications are discussed.

2. H.E.S.S. observations and results

Figure 1.— Smoothed excess sky map centered on the Cen A radio core (cross). Overlaid contours correspond to statistical significances of 3, 4, and 5, respectively. The inlay in the lower left corner shows the excess expected from a point source (derived from Monte Carlo simulations). The integration radius is 0.1225  and the map has been smoothed with a two-dimensional Gaussian of radius 0.02 to reduce the effect of statistical fluctuations. The cosmic-ray background in each bin is estimated using the template background method (Rowell, 2003).
Figure 2.— Optical image of Cen A (UK 48-inch Schmidt) overlaid with radio contours (black, VLA, Condon et al. 1996), VHE best fit position with 1  statistical errors (blue cross), and VHE extension upper limit (white dashed circle, 95% confidence level).
Figure 3.— Differential energy spectrum of Cen A as measured by H.E.S.S. The line is the best fit of a power law to the data (). The lower panel shows the residual of the fit.

The H.E.S.S. (High Energy Stereoscopic System) collaboration operates an array of four large imaging Cherenkov telescopes (IACT) for the detection of VHE -rays, located in the Southern Hemisphere in Namibia (Aharonian et al., 2006b). The H.E.S.S. observations of Cen A were performed between April 2004 and July 2008. A dead time corrected total live time of 115.0 h of good-quality data was recorded. The zenith angles of the observations range from 20 to 60 with a mean zenith angle of . The data were recorded with pointing offsets between 0.5 to 0.7 relative to the radio core position, to enable a simultaneous estimation of the background using events from the same field of view (reflected background) (Aharonian et al., 2006b). The data were analyzed with a standard Hillas-type analysis (Aharonian et al., 2006b) with an analysis energy threshold of  GeV for a zenith angle of 20.

Figure 1 shows the smoothed excess sky map of VHE -rays as measured with H.E.S.S. centered on the Cen A radio core position. A clear excess at the position of Cen A is visible. A point source analysis, using standard cuts as described in Aharonian et al. (2006b), was performed on the radio core position of Cen A, resulting in the detection of an excess with a statistical significance of (calculated following Eq. 17 of Li & Ma 1983; 330 excess events, , , ). A fit of the instrumental point spread function182 to the uncorrelated sky map results in a good fit (chance probability ) with a best fit position of , , well compatible with the radio core and the inner kpc jet region (Fig. 2). Assuming a Gaussian surface-brightness profile, we derive an upper limit of 0.2 on the extension (95% confidence level).

The differential photon spectrum of the source is shown in Fig. 3.183 A fit of a power-law function to the data is a statistically good description () with normalization  cm s TeV and photon index . The integral flux above 250 GeV, taken from the spectral fit, is  cm s, which corresponds to % of the flux of the Crab Nebula above the same threshold (Aharonian et al., 2006b), or an apparent luminosity of L(250  GeV) erg s (adopting a distance of 3.8 Mpc).

No significant variability has been found on time-scales of 28 min, nights and months (moon periods). From the error on the nightly flux (% Crab), we estimate the sensitivity of the dataset for flares with a duration of one day. For a detection in the Cen A dataset a flux increase during a single night by a factor would be needed (a factor for ). This can be compared to the VHE flux variation of factor detected from M 87 on time-scales of days.

The results have been cross-checked with independent analysis and calibration chains and good agreement was found.

3. Discussion

Figure 4.— Spectral energy distribution of Cen A. Shown are the VHE spectrum as measured by H.E.S.S. (red filled circles), previous upper limits and tentative detections in the VHE regime (purple markers; Grindlay et al. 1975: open diamond; Carramiñana et al. 1990: open cross; Allen et al. 1993: filled circle; Rowell et al. 1999: open triangle; Aharonian et al. 2005: open circle; Kabuki et al. 2007: filled squares), EGRET measurements in the GeV regime (Sreekumar et al. 1999: grey bow tie), and data from the NASA Extragalactic Database (NED) (grey filled circles).

Figure 4 shows the spectral energy distribution of Cen A ranging from X-rays to the VHE regime. The flux measured by H.E.S.S. is clearly below all previous upper limits in the VHE regime. Extrapolating the spectrum measured with EGRET in the GeV regime to VHE energies roughly matches the H.E.S.S. spectrum, though the softer end of the error range on the EGRET spectral index is preferred. The recently launched Fermi observatory should provide a more accurate spectrum in the MeV-GeV range soon.

Several authors have predicted VHE emission from Cen A, and more generally discussed VHE emission from radio galaxies. A first class of models proposed the immediate vicinity of the supermassive black hole as the region of VHE emission. Neronov & Aharonian (2007) and Rieger & Aharonian (2008) proposed a pulsar-type particle acceleration in the magnetosphere of the sub-Eddington accreting supermassive black hole, which has been applied successfully to the low-luminosity radio galaxy M 87. In this model the relation is expected (see Eq. (12) in Rieger & Aharonian, 2008). Since the mass of the central black hole and the VHE luminosity from Cen A are times lower than the ones in M 87, such a model could possibly be applied to the current data on Cen A.

It has also been proposed that, similar to the mechanism at work in other VHE blazars, VHE emission could be produced in the inner jet regions in radio galaxies (Bai & Lee, 2001; Chiaberge et al., 2001). Given the jet angle of 15-80 towards the observer (see e.g. Horiuchi et al., 2006, and references therein), the relativistic boosting in standard scenarios would be small. Ghisellini et al. (2005) proposed a two-flow type model (Sol et al., 1989), with a fast spine and a slower, mildly relativistic sheath propagating within the jet, which has been successfully applied to M 87 (Tavecchio & Ghisellini, 2008). Their model for the SED of Cen A (see Fig. 3 in Ghisellini et al., 2005), which was mainly constrained by the available CGRO data, cuts off at lower frequencies than VHE, but could possibly account for H.E.S.S. data by refining the parameters (see also in this context Marcowith et al., 1998). Lenain et al. (2008) modeled the VHE emission of Cen A with a multi-blob SSC model. In this model, VHE emission is expected to take place in the broadened jet formation zone, where even for a large jet angle, a few emission zones can move directly towards the observer and Doppler boost the emission. Their model prediction for the VHE emission from Cen A is well compatible with the H.E.S.S. data reported here (see Fig. 7 in Lenain et al., 2008).

More extended VHE emission may also be expected from Cen A. In this context, Stawarz et al. (2006) proposed that -rays emitted in the immediate vicinity of the active nucleus are partly absorbed by the starlight radiation in the host galaxy. The created pairs are quickly isotropized and radiate VHE -rays by inverse Compton scattering the starlight radiation. The small size of the resulting isotropic pair halo ( arcmin in diameter) is fully consistent with a point-like source for H.E.S.S., but could be resolved by the future CTA (Cherenkov Telescope Array)184 observatory. Stawarz et al.’s prediction for the VHE emission of Cen A within this model, resulting in a steady flux of a few erg cm s and a photon index of at the TeV energy range, agrees well with the H.E.S.S. spectrum (see Fig. 6 in Stawarz et al., 2006).

Furthermore, hadronic models have been invoked to predict VHE emission from radio galaxies. In this context, Reimer et al. (2004) proposed a synchrotron-proton blazar model for M 87 where the high energy component in the SED is interpreted in terms of synchrotron emission from either primary protons or secondary and created in the inner jet. At larger distances and in the peculiar case of Cen A, ultrarelativistic protons with  eV could be released within the kpc-scale jet, and would be then confined in the host galaxy by interactions with the interstellar magnetic field. Both the dust lane of Cen A and thermal gas of the merger remnant could act as efficient, dense targets of cold material for these protons, resulting in VHE -ray emission.

Kachelriess et al. (2008) considered possible UHECR emission from Cen A in the view of the CGRO observations and current limits provided by imaging atmospheric Cherenkov telescopes. Depending on the choice for the UHECR injection function in their model, some solutions proposed for the predicted VHE -ray spectrum are compatible with the H.E.S.S. results reported here (see Fig. 1 in Kachelriess et al., 2008).

It has also been argued that VHE emission could originate from the outer giant radio lobes (e.g. Hardcastle et al., 2008) lying at from the core. However, the VHE -ray excess presented here only matches the position of the core, the pc/kpc inner jets and the inner radio lobes. Only upper limits at VHE have been derived for the southern outer lobe (Kabuki et al., 2007).

Recently, Croston et al. (2009) reported the detection of non-thermal X-ray synchrotron emission from the shock of the southwest inner radio lobe. The edge of this lobe is located from the nucleus and reveals edge-brightened X-ray emission (Kraft et al., 2003). While the position is away from the best fit position of the VHE excess, it is well within the upper limit of the extension. Studying the spatial variation of the spectral index in X-rays across this shock region, Croston et al. (2009) constrained the high energy cut-off in the electron distribution to be . They investigated inverse Compton scattering the starlight radiation and the CMB from high energy particles in this lobe and predicted a VHE emission well compatible with the H.E.S.S. measurement reported here (see their Fig. 8) for both seed radiation fields. This study would suggest Cen A is analogous to a gigantic supernova remnant (SNR).

Besides the components of the AGN hosted in Cen A, one might wonder whether other sources in the host galaxy could be responsible for the VHE emission. For example, Kraft et al. (2001) detected more than 200 X-ray point sources in Cen A, and IACTs have detected about 60 VHE sources in our Galaxy, many of which are associated with SNRs and pulsar wind nebulae (PWNe). However, source ensembles, such as the sum of the contributions from SNRs/PWNe, would require an unrealistically large number of sources (assuming a typical luminosity of  erg s above 250 GeV per source).

Further information on the VHE excess position, more detailed spectral shape and variability studies are required to differentiate between the different models. However, if the VHE emission is due to a misaligned blazar-like process – such as leptonic or hadronic emission from the jet – the proximity of Cen A makes it a very good laboratory to further investigate emission processes and jet physics in blazars. If the VHE emission originates from another process – e.g. UHECR interacting with the interstellar medium, or a SNR type process at the shock – this would be very exciting, giving new insights into the physics in the VHE domain.

Cen A represents a rich potential for future VHE experiments. Our current data are at the edge of differentiating the possible emitting regions. With higher sensitivity (factor 10), better astrometric accuracy and angular resolution (e.g. and , respectively) (Hermann et al., 2007), CTA would allow the localization of the site of the VHE emission, and, possibly, reveal multiple VHE emitting sources within Cen A. More generally, the detection of VHE emission from Cen A together with the detection of M 87 and the galactic center poses the question of whether VHE emission (-ray brightness) might be a general feature of AGN. While the sensitivity of current generation experiments is probably too low to answer this question, one can hope that the CTA experiment will be able to detect a large enough sample of sources to shed some light on this issue.

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. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration, and NASA’s Astrophysics Data System.

Footnotes

  1. affiliation: Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany
  2. affiliation: Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin 2, Ireland
  3. affiliation: Yerevan Physics Institute, 2 Alikhanian Brothers St., 375036 Yerevan, Armenia
  4. affiliation: Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, D 91058 Erlangen, Germany
  5. affiliation: University of Durham, Department of Physics, South Road, Durham DH1 3LE, U.K.
  6. affiliation: supported by CAPES Foundation, Ministry of Education of Brazil
  7. affiliation: Centre d’Etude Spatiale des Rayonnements, CNRS/UPS, 9 av. du Colonel Roche, BP 4346, F-31029 Toulouse Cedex 4, France
  8. affiliation: Astroparticule et Cosmologie (APC), CNRS, Universite Paris 7 Denis Diderot, 10, rue Alice Domon et Leonie Duquet, F-75205 Paris Cedex 13, France
  9. affiliation: Landessternwarte, Universität Heidelberg, Königstuhl, D 69117 Heidelberg, Germany
  10. affiliation: Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, Amado, AZ85645, USA
  11. affiliation: Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany
  12. affiliation: Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, D 12489 Berlin, Germany
  13. affiliation: LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France
  14. affiliation: Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany
  15. affiliation: Centre d’Etude Spatiale des Rayonnements, CNRS/UPS, 9 av. du Colonel Roche, BP 4346, F-31029 Toulouse Cedex 4, France
  16. affiliation: IRFU/DSM/CEA, CE Saclay, F-91191 Gif-sur-Yvette, Cedex, France
  17. affiliation: Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, D 91058 Erlangen, Germany
  18. affiliation: IRFU/DSM/CEA, CE Saclay, F-91191 Gif-sur-Yvette, Cedex, France
  19. affiliation: Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany
  20. affiliation: Nicolaus Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warsaw, Poland
  21. affiliation: Unit for Space Physics, North-West University, Potchefstroom 2520, South Africa
  22. affiliation: Laboratoire d’Astrophysique de Grenoble, INSU/CNRS, Université Joseph Fourier, BP 53, F-38041 Grenoble Cedex 9, France
  23. affiliation: University of Durham, Department of Physics, South Road, Durham DH1 3LE, U.K.
  24. affiliation: LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, CNRS/IN2P3, 4 Place Jussieu, F-75252, Paris Cedex 5, France
  25. affiliation: Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany
  26. affiliation: University of Durham, Department of Physics, South Road, Durham DH1 3LE, U.K.
  27. affiliation: Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, F-91128 Palaiseau, France
  28. affiliation: Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany
  29. affiliation: Laboratoire d’Annecy-le-Vieux de Physique des Particules, CNRS/IN2P3, 9 Chemin de Bellevue - BP 110 F-74941 Annecy-le-Vieux Cedex, France
  30. affiliation: Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, D 12489 Berlin, Germany
  31. affiliation: University of Durham, Department of Physics, South Road, Durham DH1 3LE, U.K.
  32. affiliation: University of Namibia, Private Bag 13301, Windhoek, Namibia
  33. affiliation: Unit for Space Physics, North-West University, Potchefstroom 2520, South Africa
  34. affiliation: Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, F-91128 Palaiseau, France
  35. affiliation: Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany
  36. affiliation: University of Durham, Department of Physics, South Road, Durham DH1 3LE, U.K.
  37. affiliation: Astroparticule et Cosmologie (APC), CNRS, Universite Paris 7 Denis Diderot, 10, rue Alice Domon et Leonie Duquet, F-75205 Paris Cedex 13, France
  38. affiliation: Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany
  39. affiliation: Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin 2, Ireland
  40. affiliation: Laboratoire d’Annecy-le-Vieux de Physique des Particules, CNRS/IN2P3, 9 Chemin de Bellevue - BP 110 F-74941 Annecy-le-Vieux Cedex, France
  41. affiliation: Laboratoire d’Astrophysique de Grenoble, INSU/CNRS, Université Joseph Fourier, BP 53, F-38041 Grenoble Cedex 9, France
  42. affiliation: Nicolaus Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warsaw, Poland
  43. affiliation: Instytut Fizyki Ja̧drowej PAN, ul. Radzikowskiego 152, 31-342 Kraków, Poland
  44. affiliation: Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany
  45. affiliation: Landessternwarte, Universität Heidelberg, Königstuhl, D 69117 Heidelberg, Germany
  46. affiliation: Astroparticule et Cosmologie (APC), CNRS, Universite Paris 7 Denis Diderot, 10, rue Alice Domon et Leonie Duquet, F-75205 Paris Cedex 13, France
  47. affiliation: Laboratoire de Physique Théorique et Astroparticules, Université Montpellier 2, CNRS/IN2P3, CC 70, Place Eugène Bataillon, F-34095 Montpellier Cedex 5, France
  48. affiliation: Laboratoire de Physique Théorique et Astroparticules, Université Montpellier 2, CNRS/IN2P3, CC 70, Place Eugène Bataillon, F-34095 Montpellier Cedex 5, France
  49. affiliation: Laboratoire de Physique Théorique et Astroparticules, Université Montpellier 2, CNRS/IN2P3, CC 70, Place Eugène Bataillon, F-34095 Montpellier Cedex 5, France
  50. affiliation: Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany
  51. affiliation: Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, F-91128 Palaiseau, France
  52. affiliation: Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, D 12489 Berlin, Germany
  53. affiliation: Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin 2, Ireland
  54. affiliation: Laboratoire de Physique Théorique et Astroparticules, Université Montpellier 2, CNRS/IN2P3, CC 70, Place Eugène Bataillon, F-34095 Montpellier Cedex 5, France
  55. affiliation: Astroparticule et Cosmologie (APC), CNRS, Universite Paris 7 Denis Diderot, 10, rue Alice Domon et Leonie Duquet, F-75205 Paris Cedex 13, France
  56. affiliation: Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, F-91128 Palaiseau, France
  57. affiliation: IRFU/DSM/CEA, CE Saclay, F-91191 Gif-sur-Yvette, Cedex, France
  58. affiliation: Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, D 91058 Erlangen, Germany
  59. affiliation: IRFU/DSM/CEA, CE Saclay, F-91191 Gif-sur-Yvette, Cedex, France
  60. affiliation: Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, D 91058 Erlangen, Germany
  61. affiliation: Landessternwarte, Universität Heidelberg, Königstuhl, D 69117 Heidelberg, Germany
  62. affiliation: Landessternwarte, Universität Heidelberg, Königstuhl, D 69117 Heidelberg, Germany
  63. affiliation: Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, D 91058 Erlangen, Germany
  64. affiliation: Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, D 22761 Hamburg, Germany
  65. affiliation: Laboratoire d’Astrophysique de Grenoble, INSU/CNRS, Université Joseph Fourier, BP 53, F-38041 Grenoble Cedex 9, France
  66. affiliation: Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany
  67. affiliation: School of Physics & Astronomy, University of Leeds, Leeds LS2 9JT, UK
  68. affiliation: Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, D 72076 Tübingen, Germany
  69. affiliation: Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany
  70. affiliation: Unit for Space Physics, North-West University, Potchefstroom 2520, South Africa
  71. affiliation: Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany
  72. affiliation: Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, D 22761 Hamburg, Germany
  73. affiliation: LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, CNRS/IN2P3, 4 Place Jussieu, F-75252, Paris Cedex 5, France
  74. affiliation: Unit for Space Physics, North-West University, Potchefstroom 2520, South Africa
  75. affiliation: Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, D 91058 Erlangen, Germany
  76. affiliation: Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, D 91058 Erlangen, Germany
  77. affiliation: Toruń Centre for Astronomy, Nicolaus Copernicus University, ul. Gagarina 11, 87-100 Toruń, Poland
  78. affiliation: Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, D 91058 Erlangen, Germany
  79. affiliation: Landessternwarte, Universität Heidelberg, Königstuhl, D 69117 Heidelberg, Germany
  80. affiliation: Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, D 72076 Tübingen, Germany
  81. affiliation: Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, D 12489 Berlin, Germany
  82. affiliation: Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany
  83. affiliation: Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, F-91128 Palaiseau, France
  84. affiliation: University of Durham, Department of Physics, South Road, Durham DH1 3LE, U.K.
  85. affiliation: Nicolaus Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warsaw, Poland
  86. affiliation: Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, D 22761 Hamburg, Germany
  87. affiliation: IRFU/DSM/CEA, CE Saclay, F-91191 Gif-sur-Yvette, Cedex, France
  88. affiliation: Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany
  89. affiliation: Laboratoire d’Annecy-le-Vieux de Physique des Particules, CNRS/IN2P3, 9 Chemin de Bellevue - BP 110 F-74941 Annecy-le-Vieux Cedex, France
  90. affiliation: University of Durham, Department of Physics, South Road, Durham DH1 3LE, U.K.
  91. affiliation: LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France
  92. affiliation: Correspondence and requests for material should be sent to M. Raue (martin.raue@mpi-hd.mpg.de) and J.-P. Lenain (jean-philippe.lenain@obspm.fr).
  93. affiliation: Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, D 12489 Berlin, Germany
  94. affiliation: Astroparticule et Cosmologie (APC), CNRS, Universite Paris 7 Denis Diderot, 10, rue Alice Domon et Leonie Duquet, F-75205 Paris Cedex 13, France
  95. affiliation: LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France
  96. affiliation: LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, CNRS/IN2P3, 4 Place Jussieu, F-75252, Paris Cedex 5, France
  97. affiliation: Laboratoire de Physique Théorique et Astroparticules, Université Montpellier 2, CNRS/IN2P3, CC 70, Place Eugène Bataillon, F-34095 Montpellier Cedex 5, France
  98. affiliation: LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, CNRS/IN2P3, 4 Place Jussieu, F-75252, Paris Cedex 5, France
  99. affiliation: University of Durham, Department of Physics, South Road, Durham DH1 3LE, U.K.
  100. affiliation: LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France
  101. affiliation: Nicolaus Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warsaw, Poland
  102. affiliation: IRFU/DSM/CEA, CE Saclay, F-91191 Gif-sur-Yvette, Cedex, France
  103. affiliation: Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, F-91128 Palaiseau, France
  104. affiliation: LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, CNRS/IN2P3, 4 Place Jussieu, F-75252, Paris Cedex 5, France
  105. affiliation: Charles University, Faculty of Mathematics and Physics, Institute of Particle and Nuclear Physics, V Holešovičkách 2, 180 00
  106. affiliation: Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany
  107. affiliation: Instytut Fizyki Ja̧drowej PAN, ul. Radzikowskiego 152, 31-342 Kraków, Poland
  108. affiliation: University of Durham, Department of Physics, South Road, Durham DH1 3LE, U.K.
  109. affiliation: Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany
  110. affiliation: Centre d’Etude Spatiale des Rayonnements, CNRS/UPS, 9 av. du Colonel Roche, BP 4346, F-31029 Toulouse Cedex 4, France
  111. affiliation: Astroparticule et Cosmologie (APC), CNRS, Universite Paris 7 Denis Diderot, 10, rue Alice Domon et Leonie Duquet, F-75205 Paris Cedex 13, France
  112. affiliation: European Associated Laboratory for Gamma-Ray Astronomy, jointly supported by CNRS and MPG
  113. affiliation: University of Durham, Department of Physics, South Road, Durham DH1 3LE, U.K.
  114. affiliation: Obserwatorium Astronomiczne, Uniwersytet Jagielloński, Kraków, Poland
  115. affiliation: Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany
  116. affiliation: Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, D 12489 Berlin, Germany
  117. affiliation: Landessternwarte, Universität Heidelberg, Königstuhl, D 69117 Heidelberg, Germany
  118. affiliation: Laboratoire d’Astrophysique de Grenoble, INSU/CNRS, Université Joseph Fourier, BP 53, F-38041 Grenoble Cedex 9, France
  119. affiliation: Laboratoire d’Astrophysique de Grenoble, INSU/CNRS, Université Joseph Fourier, BP 53, F-38041 Grenoble Cedex 9, France
  120. affiliation: Astroparticule et Cosmologie (APC), CNRS, Universite Paris 7 Denis Diderot, 10, rue Alice Domon et Leonie Duquet, F-75205 Paris Cedex 13, France
  121. affiliation: Landessternwarte, Universität Heidelberg, Königstuhl, D 69117 Heidelberg, Germany
  122. affiliation: Astroparticule et Cosmologie (APC), CNRS, Universite Paris 7 Denis Diderot, 10, rue Alice Domon et Leonie Duquet, F-75205 Paris Cedex 13, France
  123. affiliation: Landessternwarte, Universität Heidelberg, Königstuhl, D 69117 Heidelberg, Germany
  124. affiliation: Unit for Space Physics, North-West University, Potchefstroom 2520, South Africa
  125. affiliation: Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany
  126. affiliation: European Associated Laboratory for Gamma-Ray Astronomy, jointly supported by CNRS and MPG
  127. affiliation: Correspondence and requests for material should be sent to M. Raue (martin.raue@mpi-hd.mpg.de) and J.-P. Lenain (jean-philippe.lenain@obspm.fr).
  128. affiliation: University of Durham, Department of Physics, South Road, Durham DH1 3LE, U.K.
  129. affiliation: Astroparticule et Cosmologie (APC), CNRS, Universite Paris 7 Denis Diderot, 10, rue Alice Domon et Leonie Duquet, F-75205 Paris Cedex 13, France
  130. affiliation: Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany
  131. affiliation: Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany
  132. affiliation: European Associated Laboratory for Gamma-Ray Astronomy, jointly supported by CNRS and MPG
  133. affiliation: Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, D 22761 Hamburg, Germany
  134. affiliation: Charles University, Faculty of Mathematics and Physics, Institute of Particle and Nuclear Physics, V Holešovičkách 2, 180 00
  135. affiliation: Laboratoire d’Annecy-le-Vieux de Physique des Particules, CNRS/IN2P3, 9 Chemin de Bellevue - BP 110 F-74941 Annecy-le-Vieux Cedex, France
  136. affiliation: School of Chemistry & Physics, University of Adelaide, Adelaide 5005, Australia
  137. affiliation: Nicolaus Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warsaw, Poland
  138. affiliation: University of Durham, Department of Physics, South Road, Durham DH1 3LE, U.K.
  139. affiliation: Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, D 44780 Bochum, Germany
  140. affiliation: Yerevan Physics Institute, 2 Alikhanian Brothers St., 375036 Yerevan, Armenia
  141. affiliation: Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, D 72076 Tübingen, Germany
  142. affiliation: Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, D 44780 Bochum, Germany
  143. affiliation: Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, D 91058 Erlangen, Germany
  144. affiliation: Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, D 44780 Bochum, Germany
  145. affiliation: Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, D 12489 Berlin, Germany
  146. affiliation: Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, D 72076 Tübingen, Germany
  147. affiliation: Landessternwarte, Universität Heidelberg, Königstuhl, D 69117 Heidelberg, Germany
  148. affiliation: Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, D 44780 Bochum, Germany
  149. affiliation: Nicolaus Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warsaw, Poland
  150. affiliation: School of Physics & Astronomy, University of Leeds, Leeds LS2 9JT, UK
  151. affiliation: LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France
  152. affiliation: University of Durham, Department of Physics, South Road, Durham DH1 3LE, U.K.
  153. affiliation: Obserwatorium Astronomiczne, Uniwersytet Jagielloński, Kraków, Poland
  154. affiliation: University of Namibia, Private Bag 13301, Windhoek, Namibia
  155. affiliation: Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, D 91058 Erlangen, Germany
  156. affiliation: Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, F-91128 Palaiseau, France
  157. affiliation: Obserwatorium Astronomiczne, Uniwersytet Jagielloński, Kraków, Poland
  158. affiliation: Laboratoire d’Astrophysique de Grenoble, INSU/CNRS, Université Joseph Fourier, BP 53, F-38041 Grenoble Cedex 9, France
  159. affiliation: Landessternwarte, Universität Heidelberg, Königstuhl, D 69117 Heidelberg, Germany
  160. affiliation: LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, CNRS/IN2P3, 4 Place Jussieu, F-75252, Paris Cedex 5, France
  161. affiliation: Astroparticule et Cosmologie (APC), CNRS, Universite Paris 7 Denis Diderot, 10, rue Alice Domon et Leonie Duquet, F-75205 Paris Cedex 13, France
  162. affiliation: Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany
  163. affiliation: Landessternwarte, Universität Heidelberg, Königstuhl, D 69117 Heidelberg, Germany
  164. affiliation: Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, D 22761 Hamburg, Germany
  165. affiliation: Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany
  166. affiliation: Laboratoire de Physique Théorique et Astroparticules, Université Montpellier 2, CNRS/IN2P3, CC 70, Place Eugène Bataillon, F-34095 Montpellier Cedex 5, France
  167. affiliation: Unit for Space Physics, North-West University, Potchefstroom 2520, South Africa
  168. affiliation: LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France
  169. affiliation: Laboratoire d’Annecy-le-Vieux de Physique des Particules, CNRS/IN2P3, 9 Chemin de Bellevue - BP 110 F-74941 Annecy-le-Vieux Cedex, France
  170. affiliation: LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, CNRS/IN2P3, 4 Place Jussieu, F-75252, Paris Cedex 5, France
  171. affiliation: Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands
  172. affiliation: IRFU/DSM/CEA, CE Saclay, F-91191 Gif-sur-Yvette, Cedex, France
  173. affiliation: Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany
  174. affiliation: Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany
  175. affiliation: Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, F-91128 Palaiseau, France
  176. affiliation: European Associated Laboratory for Gamma-Ray Astronomy, jointly supported by CNRS and MPG
  177. affiliation: Landessternwarte, Universität Heidelberg, Königstuhl, D 69117 Heidelberg, Germany
  178. affiliation: University of Durham, Department of Physics, South Road, Durham DH1 3LE, U.K.
  179. affiliation: Nicolaus Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warsaw, Poland
  180. affiliation: LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France
  181. H.E.S.S. angular resolution: per event, systematic error on position depending on the dataset (van Eldik et al., 2008).
  182. Derived from Monte Carlo simulations.
  183. To derive the energy spectrum, a looser cut on the distance to the source is used ( deg) to increase the number of photons (the standard cut is  deg).
  184. http://www.cta-observatory.org/

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