Author(s): Aharonian F., G. Akhperjanian A., R. Bazer-Bachi A., Beilicke M., Chounet L.-M., Degrange B., Djannati-Ataï A., Dubus G., Espigat P., Feinstein F., Fontaine G., Gallant Y., Giebels B., Jacholkowska Agnieszka, Lemière A., Lemoine-Goumard M., Leroy N., Martineau-Huynh O., Marcowith A., Naurois De M., Ouchrif M., Punch M., Raux J., Rolland L., Tavernet J.-P., Terrier R., Théoret C.G., Tluczykont M., Vasileiadis G., Vincent P.
(Article) Published:
Astronomy & Astrophysics, vol. 442 p.895-907 (2005)
Links openAccess full text : 
Ref HAL: in2p3-00024349_v1
Ref Arxiv: astro-ph/0506593
DOI: 10.1051/0004-6361:20053353
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
89 citations
Abstract:
The High Energy Stereoscopic System (H.E.S.S.) has observed the high-frequency peaked BL Lac object PKS2155-304 in 2003 between October 19 and November 26 in Very High Energy (VHE) $\gamma$-rays (E $\geq$ 160 GeV for these observations). Observations were carried out simultaneously with the Proportional Counter Array (PCA) on board the Rossi X-ray Timing Explorer satellite (RXTE), the Robotic Optical Transient Search Experiment (ROTSE) and the Nancay decimetric radiotelescope (NRT). Intra-night variability is seen in the VHE band, the source being detected with a high significance on each night it was observed. Variability is also found in the X-ray and optical bands on kilosecond timescales, along with flux-dependent spectral changes in the X-rays. The average H.E.S.S. spectrum shows a very soft power law shape with a photon index of 3.37 $\pm$ 0.07${stat} \pm 0.10 {syst}. The energy outputs in the 2-10 keV and in the VHE gamma-ray range are found to be similar, with the X-rays and the optical fluxes at a level comparable to some of the lowest historical measurements, indicating that PKS2155-304 was in a low or quiescent state during the observations. Both a leptonic and a hadronic model are used to derive source parameters from these observations.
Comments: 16 pages, 13 figures, Accepted (June 22) for publication in A&A