MULTIWAVELENGTH MONITORING OF THE BL LACERTAE OBJECT PKS 2155-304 .2.THE ROSAT OBSERVATIONS

Extensive ROSAT PSPC observations of the BL Lac object PKS 2155-304 ca rried out between November 12 and November 15, 1991 revealed the sourc e to be in a bright state with a soft X-ray flux of F(x) (0.1-2.4 keV) almost-equal-to 7 x 10(-10) erg cm-2 s-1. Rapid variations in the X-r ay flux of up to 30% within a day were apparent but at the sampling ra te of approximately 96 min all of the gross features in the X-ray ligh t curve of this source are time resolved. The soft X-ray flux shows a close correlation with the simultaneous UV flux recorded by IUE althou gh at the beginning and at the end of the PSPC observations there is s ome 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 respec t to the soft X-ray flux is less than or similar to 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 consis tent with a simple single power-law form with photon index GAMMA appro ximately -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 approximately 600 eV, but the presence of a narrow absorption trough with width less th an or similar to 40 eV or an absorption edge with moderate optical dep th (tau approximately 0.25) cannot be ruled out from the ROSAT data. T he optical to X-ray spectrum must have a break in the unaccessible ene rgy range between the IUE and ROSAT energy windows. The high energy pr operties of PKS 2155-304 are consistent with synchrotron emission from a relativistic jet. However, intensity variations without spectral ch anges are hard to accommodate in existing jet models. More refined, se lf-consistent time-dependent models are required to explain the detail s of the physical processes operating in this source.