VHE gamma-ray emission of PKS 2155-304: spectral and temporal variability
Abstract
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 ..
Keywords
Cite
@article{arxiv.1005.3702,
title = {VHE gamma-ray emission of PKS 2155-304: spectral and temporal variability},
author = {HESS Collaboration and A. Abramowski and F. Acero and F. Aharonian and A. G. Akhperjanian and G. Anton and U. Barres de Almeida and A. R. Bazer-Bachi and Y. Becherini and B. Behera and W. Benbow and K. Bernlohr and A. Bochow and C. Boisson and J. Bolmont and V. Borrel and J. Brucker and F. Brun and P. Brun and R. Buhler and T. Bulik and I. Busching and T. Boutelier and P. M. Chadwick and A. Charbonnier and R. C. G. Chaves and A. Cheesebrough and J. Conrad and L. -M. Chounet and A. C. Clapson and G. Coignet and L. Costamante and M. Dalton and M. K. Daniel and I. D. Davids and B. Degrange and C. Deil and H. J. Dickinson and A. Djannati-Atai and W. Domainko and L. O'C. Drury and F. Dubois and G. Dubus and J. Dyks and M. Dyrda and K. Egberts and P. Eger and P. Espigat and L. Fallon and C. Farnier and S. Fegan and F. Feinstein and M. V. Fernandes and A. Fiasson and A. Foerster and G. Fontaine and M. Fussling and S. Gabici and Y. A. Gallant and L. Gerard and D. Gerbig and B. Giebels and J. F. Glicenstein and B. Glueck and P. Goret and D. Goering and D. Hampf and M. Hauser and S. Heinz and G. Heinzelmann and G. Henri and G. Hermann and J. A. Hinton and A. Hoffmann and W. Hofmann and P. Hofverberg and M. Holleran and S. Hoppe and D. Horns and A. Jacholkowska and O. C. de Jager and C. Jahn and I. Jung and K. Katarzynski and U. Katz and S. Kaufmann and M. Kerschhaggl and D. Khangulyan and B. Khelifi and D. Keogh and D. Klochkov and W. Kluzniak and T. Kneiske and Nu. Komin and K. Kosack and R. Kossakowski and G. Lamanna and J. -P. Lenain and T. Lohse and C. -C. Lu and V. Marandon and A. Marcowith and J. Masbou and D. Maurin and T. J. L. McComb and M. C. Medina and J. Mehault and R. Moderski and E. Moulin and M. Naumann-Godo and M. de Naurois and D. Nedbal and D. Nekrassov and N. Nguyen and B. Nicholas and J. Niemiec and S. J. Nolan and S. Ohm and J-F. Olive and E. de Ona Wilhelmi and B. Opitz and K. J. Orford and M. Ostrowski and M. Panter and M. Paz Arribas and G. Pedaletti and G. Pelletier and P. -O. Petrucci and S. Pita and G. Puehlhofer and M. Punch and A. Quirrenbach and B. C. Raubenheimer and M. Raue and S. M. Rayner and O. Reimer and M. Renaud and R. de los Reyes and F. Rieger and J. Ripken and L. Rob and S. Rosier-Lees and G. Rowell and B. Rudak and C. B. Rulten and J. Ruppel and F. Ryde and V. Sahakian and A. Santangelo and R. Schlickeiser and F. M. Schoeck and A. Schoenwald and U. Schwanke and S. Schwarzburg and S. Schwemmer and A. Shalchi and I. Sushch and M. Sikora and J. L. Skilton and H. Sol and L. Stawarz and R. Steenkamp and C. Stegmann and F. Stinzing and G. Superina and A. Szostek and P. H. Tam and J. -P. Tavernet and R. Terrier and O. Tibolla and M. Tluczykont and K. Valerius and C. van Eldik and G. Vasileiadis and C. Venter and L. Venter and J. P. Vialle and A. Viana and P. Vincent and M. Vivier and H. J. Voelk and F. Volpe and S. Vorobiov and S. J. Wagner and M. Ward and A. A. Zdziarski and A. Zech and H. -S. Zechlin},
journal= {arXiv preprint arXiv:1005.3702},
year = {2015}
}
Comments
17 pages, 13 figures, Accepted for publication to Astronomy & Astrophysics