Very high energy gamma-ray follow-up observations of novae and dwarf novae with the MAGIC telescopes
In the last few years the Fermi-LAT instrument has detected GeV gamma-ray emission from several novae. Such GeV emission can be interpreted in terms of inverse Compton emission from electrons accelerated in the shock or in terms of emission from hadrons accelerated in the same conditions. The latter might reach much higher energies and could produce a second component in the gamma-ray spectrum at TeV energies. We perform follow-up observations of selected novae and dwarf novae in search of the second component in TeV energy gamma rays. This can shed light on the acceleration process of leptons and hadrons in nova explosions. We have performed observations with the MAGIC telescopes of 3 sources, a symbiotic nova YY Her, a dwarf nova ASASSN-13ax and a classical nova V339 Del, shortly after their outbursts. We did not detect TeV gamma-ray emission from any of the objects observed. The TeV upper limits from MAGIC observations and the GeV detection by Fermi constrain the acceleration parameters for electrons and hadrons.
A classical nova is a thermonuclear runaway leading to the explosive ejection of the envelope accreted onto a white dwarf (WD) in a binary system in which the companion is either filling or nearly filling its Roche surface . They are a type of cataclysmic variables, i.e. optically variable binary systems with a mass transfer from a companion star to WD. Novae are typically detected first in optical observations when the brightness of the object increases by 7-16 magnitudes before fading back to the original level. The energy spectra of novae often contain a thermal X-ray continuum. The symbiotic novae, like the classical novae, are also initiated by a thermonuclear explosion on the surface of the WD. However in the case of symbiotic novae, the WD is deep immersed in the wind of a late-type companion star (see e.g. ).
The diffusive shock acceleration at the blast wave of symbiotic novae was expected to accelerate particles up to energies of a few TeVs . In 2010 the first GeV -ray emission was detected by Fermi-LAT from the symbiotic nova V407 Cyg . In this contribution we present the results of the follow-up observations of three selected novae and dwarf novae: the symbiotic nova YY Her, the dwarf nova ASASSN-13ax and the classical nova V339 Del.
The -ray emission has been subsequently explained in terms of leptonic or hadronic models [4, 5]. Local radiation fields create a target for the IC scattering of the electrons. Protons accelerated in the same conditions can interact with matter producing gamma rays via proton-proton interactions. For instance,  attribute the GeV -ray emission to the IC process on the strong radiation field of the red giant. The same model predicts a second component in the TeV range due to proton-proton interactions with the wind of the red giant. Also  consider acceleration of leptons and hadrons in the nova shock. In that model the magnetic field, which determines the acceleration efficiency, is obtained assuming equipartition with the thermal energy density upstream of the shock. The GeV -ray emission is then a product of IC scattering of the nova light by the electrons.
In the last few years Fermi-LAT has discovered GeV -ray emission from a few more novae: V1324 Sco, V959 Mon, V339 Del, V1369 Cen, V745 Sco and the very recent Nova Sagittarii 2015 [8, 9, 5, 10]. Most of these sources are CN. Contrary to the symbiotic ones, the wind of the companion star is not strong, but they all show similar spectral properties. In CN the particle acceleration can occur e.g. on a bow shock between the nova ejecta and the interstellar medium or in weaker internal shocks due to inhomogeneity of the nova ejecta . In particular, orbital motion of the system can shape the nova ejecta into a faster polar wind and a denser material expanding on the equatorial plane .
So far no VHE -ray emission has been detected from any nova event. VERITAS observations of V407 Cyg starting 10 days after the nova explosion yielded a differential U.L. on the flux of at 1.6 TeV .
The three sources were first detected and observed by optical instruments. The results of the MAGIC observations were supported by the analysis of quasi-simultaneous Fermi-LAT observations.
2.1 MAGIC telescopes
The VHE gamma-ray observations were performed with the MAGIC telescopes. MAGIC is a system of two 17 m Cherenkov telescopes located on the Canary Island of La Palma at a height of 2200 m a.s.l. The telescopes can perform observations of gamma rays with energies as low as 50 GeV and up to tens of TeV. During Summer 2011 and 2012 MAGIC underwent a major upgrade. After the upgrade the sensitivity of the MAGIC telescopes in the best energy range (GeV) is of Crab Nebula flux in 50 h of observations . All the data used for this paper were taken after the upgrade. The data were analyzed using the standard analysis chain [14, 13]. The significance of a gamma-ray excess was computed according to Eq. 17 of . The upper limits on the flux were calculated following the approach of  using 95% C.L. and accounting for a possible 30% systematic uncertainty on the effective area of the instrument.
Fermi-LAT is a pair-conversion telescope launched in 2008 that detects photons with energies from MeV to GeV .
Thanks to a large FoV ( sr), the Fermi-LAT observatory, operated in scanning mode, provides coverage of the full sky every three hours enabling searches for transient sources and overlap with ground-based observatories.
The LAT data were analyzed in the energy range 100 MeV 300 GeV using an unbinned maximum likelihood method as implemented in the Fermi Science Tools v9r32p5, the P7REP_SOURCE_V15 LAT IRF, and associated standard Galactic and isotropic diffuse emission models111The P7REP data, IRFs, and diffuse models (gll_iem_v05.fit and iso_source_v05.txt) are available at http://fermi.gsfc.nasa.gov/ssc..
We selected events within a RoI of centered on the LAT best position reported by  for V339 Del and required a maximum Zd of in order to avoid contamination from Earth limb photons. Additionally, we applied a gtmktime filter (no.3) recommended for combined survey and pointed mode observations222http://fermi.gsfc.nasa.gov/ssc/data/analysis/documentation/Cicerone/
Cicerone_Likelihood/Exposure.html, selecting good quality data at times when either the rocking angle was less than or the edge of the analysis region did not exceed the maximum Zd at . Sources from the 2FGL catalogue located within the RoI were included in the model used to perform the fitting procedure. The U.L. were calculated at 95% C.L. using the Bayesian method provided with the Fermi Science Tools333http://fermi.gsfc.nasa.gov/ssc/data/analysis/scitools/python_tutorial.html.
3.1 YY Her
YY Her is a symbiotic nova system that undergoes a recurrent pattern of outbursts. MAGIC observations of YY Her occurred on MJD 56404 and could not be continued due to the Full Moon. No significant VHE gamma-ray emission was detected. We computed flux upper limits at 95% confidence level obtaining above 300 GeV. Also in Fermi-LAT no emission was detected over a longer interval from MJD 56392.5 to 56412.5. Upper limits at 95% confidence level were set as above 100 MeV. Differential upper limits obtained from the Fermi-LAT and MAGIC observations of YY Her are shown in Fig. 1.
ASASSN-13ax is a member of a different class of cataclysmic variables, the dwarf novae, which are known for significantly weaker optical outbursts (2-6 magnitudes) than classical novae.
Instead of undergoing a thermonuclear explosion on the surface of the WD, these outbursts are caused by the gravitational energy release from a partial collapse of the accretion disk surrounding the WD. The MAGIC observations were performed on two consecutive nights starting MJD 56478. In the absence of detectable VHE emission, upper limits at 95% confidence level were set as above 300 GeV. Fermi-LAT observations put a 95% C.L. U.L. on the flux of the source above 100 MeV at the level of in the time period MJD 56468.5 to 56488.5. Differential upper limits obtained from the Fermi-LAT and MAGIC observations of ASASSN-13ax are shown in Fig. 2.
3.3 V339 Del
V339 Del was a fast, classical nova detected by optical observations on observations on MJD 56520 (CBET #3628). The nova was exceptionally bright reaching a magnitude of Vmag (see top panel of Fig. 5), and it triggered follow-up observations at frequencies ranging from radio to VHE gamma-rays. Photometric measurements suggest a distance for V339 Del of kpc . The spectroscopic observations performed on MJD 56522.1 revealed emission wings extending to about km/s and a Balmer absorption component at a velocity of km/s . The pre-outburst optical images revealed the progenitor of nova V339 Del to be a blue star . Originally MAGIC observations of V339 Del were motivated by its extreme optical outburst. Soon after MAGIC started observations they were additionally supported by the detection of GeV emission by the Fermi-LAT from the direction of V339 Del. The MAGIC observations started already on the night of MJD 56520, however they were marred by bad weather conditions. The good quality data used for most of the analysis span 8 nights between MJD 56529 and 56537. The total effective time was 11.6 h. In addition to the nightly upper limits we performed a dedicated analysis of the poor quality (affected by calima, a dust layer originating from Sahara) night of MJD 56520. We applied an estimated energy and collection area corrections based on LIDAR measurements . No VHE gamma-ray signal was found from the direction of V339 Del.
We computed a night by night integral upper limit above 300 GeV (see bottom panel of Fig. 5. The differential upper limits for the whole good quality data set computed in bins of energy are shown in Fig. 4.
Nova V339 Del was the subject of a Fermi-LAT ToO observation  triggered by the optical discovery (CBET #3628); the ToO started on MJD 56520 and lasted for six days. The -ray emission from V339 Del was first detected by Fermi-LAT in 1-day bins on MJD 56522 . The emission peaked on MJD 56526 and entered a slow decay phase afterwards (Figure 5). For the light curves, the data were fitted using a power-law spectral model initially leaving the spectral index and the normalization free to vary. We then fixed the spectral index at the average value of 2.3 calculated over the most significant detections (TS9) to generate the plots shown in the middle panel of Figure 5. The LAT SED of V339 Del shown in Figure 4 was extracted in five logarithmically spaced energy bins from 100 MeV to 100 GeV. Similarly to the light curves, energy binned data shown in Figure 4 were fitted using a simple power-law and showing a 95% C.L. U.L. for TS9. In the period coincident with the MAGIC observations (MJD 56529 to 56539) the Fermi-LAT spectrum can be described by an effective power-law with a spectral index of and flux above 100 MeV of . The rather low statistical significance (TS=49) does not constrain the value of an exponential cut-off of the emission in this period. Note, however, that the most energetic photon, with GeV was recorded on MJD 56534, i.e. within the time period covered by MAGIC. The Fermi-LAT analysis for a broader time range, MJD 56526 to 56547, covering the whole decay phase of the Fermi-LAT light curve allowed us to obtain a more significant signal with a TS of 121. Nevertheless we obtain a similar value of flux above 100 MeV, , for this broader period. The spectrum in this case can be described, with a significance higher with respect to the simple power-law, by an exponentially cut-off power-law with a spectral index of and a cut-off energy of GeV.
The MAGIC telescopes performed observations of 3 objects: the symbiotic nova YY Her, the dwarf nova ASASSN-13ax and the classical nova V339 Del. No significant VHE gamma-ray emission was found from the direction of any of them. Out of these three objects, V339 Del is the only one detected at GeV energies. It has also extensive optical observations which shed some light on both the companion star and the photosphere of the nova. Therefore it has the highest potential for constraining the leptonic and hadronic processes in novae. MAGIC will continue follow-up observations of the promising novae candidates in the following years.
Acknowledgments.The MAGIC Collaboration would like to thank the Instituto de Astrofísica de Canarias for the excellent working conditions at the Observatorio del Roque de los Muchachos in La Palma. The financial support of the German BMBF and MPG, the Italian INFN and INAF, the Swiss National Fund SNF, the ERDF under the Spanish MINECO (FPA2012-39502), and the Japanese JSPS and MEXT is gratefully acknowledged. This work was also supported by the Centro de Excelencia Severo Ochoa SEV-2012-0234, CPAN CSD2007-00042, and MultiDark CSD2009-00064 projects of the Spanish Consolider-Ingenio 2010 programme, by grant 268740 of the Academy of Finland, by the Croatian Science Foundation (HrZZ) Project 09/176 and the University of Rijeka Project 126.96.36.199.02, by the DFG Collaborative Research Centers SFB823/C4 and SFB876/C3, and by the Polish MNiSzW grant 745/N-HESS-MAGIC/2010/0. JS is supported by Fundacja UŁ. The Fermi LAT Collaboration acknowledges generous ongoing support from a number of agencies and institutes that have supported both the development and the operation of the LAT as well as scientific data analysis. These include the National Aeronautics and Space Administration and the Department of Energy in the United States, the Commissariat à l’Energie Atomique and the Centre National de la Recherche Scientifique / Institut National de Physique Nucléaire et de Physique des Particules in France, the Agenzia Spaziale Italiana and the Istituto Nazionale di Fisica Nucleare in Italy, the Ministry of Education, Culture, Sports, Science and Technology (MEXT), High Energy Accelerator Research Organization (KEK) and Japan Aerospace Exploration Agency (JAXA) in Japan, and the K. A. Wallenberg Foundation, the Swedish Research Council and the Swedish National Space Board in Sweden. Additional support for science analysis during the operations phase is gratefully acknowledged from the Istituto Nazionale di Astrofisica in Italy and the Centre National d’Études Spatiales in France.
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