Author(s): Abramowski A., Acero F., Bazer-Bachi A. R., Becherini Y., Boisson C., Bolmont J., Borrel V., Brun F., Brun P., Boutelier T., Charbonnier A., Chounet L. -M., Coignet G., Degrange B., Djannati-Atai A., Dubois F., Dubus G., Espigat P., Farnier C., Fegan S., Feinstein F., Fiasson A., Fontaine G., Gallant Y., Gerard L., Giebels B., Marcowith A., Mehault J., Vasileiadis G., Vorobiov S.
(Article) Published:
Annual Review Of Astronomy And Astrophysics, vol. 520 p.A83 (2010)
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Ref HAL: in2p3-00496200_v1
Ref Arxiv: 1005.3702
DOI: 10.1051/0004-6361/201014484
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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 ..
Comments: 17 pages, 13 figures, Accepted for publication to Astronomy & Astrophysics