VHE gamma-ray emission of PKS 2155-304: spectral and temporal variability

A. Abramowski , Fabio Acero (1) , F. Aharonian , A. G. Akhperjanian , G. Anton , U. Barres de Almeida , A. R. Bazer-Bachi (2) , Y. Becherini (3) , B. Behera , W. Benbow , K. Bernlohr , A. Bochow , C. Boisson (4) , J. Bolmont (5) , V. Borrel (2) , J. Brucker , F. Brun (5) , P. Brun (6) , R. Buhler , T. Bulik , I. Busching , T. Boutelier (7) , P. M. Chadwick , A. Charbonnier (5) , R. C. G. Chaves , A. Cheesebrough , J. Conrad , L. -M. Chounet (8) , A. C. Clapson , G. Coignet (9) , L. Costamante , M. Dalton , M. K. Daniel , I. D. Davids , B. Degrange (8) , C. Deil , H. J. Dickinson , A. Djannati-Ataï (10) , W. Domainko , L. O'C. Drury , F. Dubois (9) , G. Dubus (7) , J. Dyks , M. Dyrda , K. Egberts , P. Eger , P. Espigat (3) , L. Fallon , C. Farnier (1) , S. Fegan (8) , F. Feinstein (1) , M. V. Fernandes , A. Fiasson (9) , A. Foerster , G. Fontaine (8) , M. Fussling , S. Gabici , Y. A. Gallant (1) , L. Gerard (3) , D. Gerbig , B. Giebels (8) , J. F. Glicenstein (6) , B. Glueck , P. Goret (6) , D. Goering , D. Hampf , M. Hauser , S. Heinz , G. Heinzelmann , G. Henri (7) , G. Hermann , J. A. Hinton , A. Hoffmann , W. Hofmann , P. Hofverberg , M. Holleran , S. Hoppe , D. Horns , A. Jacholkowska (5) , O. C. de Jager , C. Jahn , I. Jung , K. Katarzynski , U. Katz , S. Kaufmann , M. Kerschhaggl , D. Khangulyan , B. Khelifi (8) , D. Keogh , D. Klochkov , W. Kluzniak , T. Kneiske , Nu. Komin (6) , K. Kosack (6) , R. Kossakowski (9) , G. Lamanna (9) , J. -P. Lenain (4) , T. Lohse , C. -C. Lu , V. Marandon (3) , A. Marcowith (1) , Julien Masbou (9) , D. Maurin (5) , T. J. L. Mccomb , M. C. Medina (4) , J. Mehault (1) , R. Moderski , Emmanuel Moulin (6) , M. Naumann-Godo (8) , M. de Naurois (5) , D. Nedbal , D. Nekrassov , N. Nguyen , B. Nicholas , J. Niemiec , S. J. Nolan , S. Ohm , J-F. Olive (2) , E. de Ona Wilhelmi , B. Opitz , K. J. Orford , M. Ostrowski , M. Panter , M. Paz Arribas , G. Pedaletti , G. Pelletier (7) , P. -O. Petrucci (7) , S. Pita (3) , G. Puehlhofer , M. Punch (3) , A. Quirrenbach , B. C. Raubenheimer , M. Raue , S. M. Rayner , O. Reimer , M. Renaud (3) , R. de Los Reyes , F. Rieger , J. Ripken , L. Rob , S. Rosier-Lees (9) , G. Rowell , B. Rudak , C. B. Rulten , J. Ruppel , F. Ryde , V. Sahakian , A. Santangelo , R. Schlickeiser , F. M. Schoeck , A. Schoenwald , U. Schwanke , S. Schwarzburg , S. Schwemmer , A. Shalchi , I. Sushch , M. Sikora , J. L. Skilton , H. Sol (4) , L. Stawarz , R. Steenkamp , C. Stegmann , F. Stinzing , G. Superina (8) , A. Szostek (7) , P. H. Tam , J. -P. Tavernet (5) , R. Terrier (3) , O. Tibolla , M. Tluczykont , K. Valerius , C. van Eldik , G. Vasileiadis (1) , C. Venter , L. Venter (4) , J. P. Vialle (9) , A. Viana , P. Vincent (5) , M. Vivier (6) , H. J. Voelk , F. Volpe (8) , S. Vorobiov (1) , S. J. Wagner , M. Ward , A. A. Zdziarski , A. Zech (4) , H. -S. Zechlin

1 LPTA - Laboratoire de Physique Théorique et Astroparticules
2 CESR - Centre d'étude spatiale des rayonnements
3 APC (UMR_7164) - AstroParticule et Cosmologie
4 LUTH (UMR_8102) - Laboratoire Univers et Théories
5 LPNHE - Laboratoire de Physique Nucléaire et de Hautes Énergies
6 IRFU - Institut de Recherches sur les lois Fondamentales de l'Univers
7 LAOG - Laboratoire d'Astrophysique de Grenoble
8 LLR - Laboratoire Leprince-Ringuet
9 LAPP - Laboratoire d'Annecy de Physique des Particules
10 APC - AHE - APC - Astrophysique des Hautes Energies

Observations of very high energy gamma-rays from blazars provide information about acceleration mechanisms occurring in their innermost regions. Studies of variability in these objects allow a better understanding of the mechanisms at play. To investigate the spectral and temporal variability of VHE (>100 GeV) gamma-rays of the well-known high-frequency-peaked BL Lac object PKS 2155-304 with the H.E.S.S. imaging atmospheric Cherenkov telescopes over a wide range of flux states. Data collected from 2005 to 2007 are analyzed. Spectra are derived on time scales ranging from 3 years to 4 minutes. Light curve variability is studied through doubling timescales and structure functions, and is compared with red noise process simulations. The source is found to be in a low state from 2005 to 2007, except for a set of exceptional flares which occurred in July 2006. The quiescent state of the source is characterized by an associated mean flux level of 4.32 +/-0.09 x 10^-11 cm^-2 s^-1 above 200 GeV, or approximately 15% of the Crab Nebula, and a power law photon index of 3.53 +/-0.06. During the flares of July 2006, doubling timescales of ~2 min are found. The spectral index variation is examined over two orders of magnitude in flux, yielding different behaviour at low and high fluxes,which is a new phenomenon in VHE gamma-ray emitting blazars. The variability amplitude characterized by the fractional r.m.s. is strongly energy-dependent and is proportional to E^(0.19 +/- 0.01). The light curve r.m.s. correlates with the flux. This is the signature of a multiplicative process which can be accounted for as a red noise with a Fourier index of ~2. This unique data set shows evidence for a low level gamma-ray emission state from PKS 2155-304, which possibly has a different origin than the outbursts. The discovery of the light curve lognormal behaviour might be an indicator ..

Domaines: Phénomènes cosmiques de haute energie [astro-ph.HE]; Phénomènes cosmiques de haute energie [astro-ph.HE]

A. Abramowski, Fabio Acero, F. Aharonian, A. G. Akhperjanian, G. Anton, et al.. VHE gamma-ray emission of PKS 2155-304: spectral and temporal variability. Astronomy and Astrophysics - A&A, 2010, 520, pp.A83. ⟨10.1051/0004-6361/201014484⟩. ⟨in2p3-00496200⟩