The High Energy Stereoscopic System ( H.E.S.S . )
has observed the high-frequency peaked BL Lac object PKS 2155 - 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 Nançay 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 .
A transient X-ray event with a 1500 s timescale is detected , making this the fastest X-ray flare seen in this object .
No correlation can be established between the X-ray and the \gamma -ray fluxes , or any of the other wavebands , over the small range of observed variability .
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 _ { sys } .
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 PKS 2155 - 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 .
These parameters are found to be sensitive to the model of Extragalactic Background Light ( EBL ) that attenuates the VHE signal at this source ’ s redshift ( z = 0.117 ) .