1994A&A...288..433B

Extensive ROSAT PSPC observations of the BL Lac object PKS 2155-304 carried out between November 12 and November 15, 1991 revealed the source to be in a bright state with a soft X-ray flux of F_x(0.1-2.4keV)≃7x10^–10erg/cm²/s. Rapid variations in the X-ray flux of up to 30% within a day were apparent but at the sampling rate of ∼96 min all of the gross features in the X-ray light curve of this source are time resolved. The soft X-ray flux shows a close correlation with the simultaneous UV flux recorded by IUE although at the beginning and at the end of the PSPC observations there is some deviation from a strict linkage of the X-ray and UV light curves; detailed analysis has shown that the maximal lag of the UV with respect to the soft X-ray flux is ≲3 hours. The soft X-ray spectrum of PKS 2155-304 remained essentially unchanged over the full duration of the observations. Individual PSPC spectra are consistent with a simple single power-law form with photon index {GAMMA}~-2.65 plus galactic absorption. More complex spectral fits do not improve the quality of the fits significantly. We cannot confirm previous claims of absorption structures at energies ∼600eV, but the presence of a narrow absorption trough with width ≲40eV or an absorption edge with moderate optical depth (τ∼0.25 ) cannot be ruled out from the ROSAT data. The optical to X-ray spectrum must have a break in the unaccessible energy range between the IUE and ROSAT energy windows. The high energy properties of PKS 2155-304 are consistent with synchrotron emission from a relativistic jet. However, intensity variations without spectral changes are hard to accommodate in existing jet models. More refined, self-consistent time-dependent models are required to explain the details of the physical processes operating in this source.