X-ray spectrum of the BL Lacertae object PKS 2155-304.

The BL Lacertae object PKS 2155-304 was observed with Ginga for two days in May 1987. The 2-35 keV intensity showed a significant variability, the most rapid change being a 30 percent increase in 3 hr. The X-ray spectrum was described by a single power-law model with photon spectral index ranging between 2.7 and 2.9, but the power-law slope showed no clear correlation with the intensity level and stayed at a similar value for each day. The high-quality spectrum observed with Ginga up to 35 keV has revealed no significant hard tail. The 90 percent upper limit on the flux of the flat hard tail is 0.3 micro-Jy at 20 keV, less than half of the previously reported level. The present spectral results combined with the previously observed fast variability constrain the emission mechanism of PKS 2155-304; a simple homogeneous synchrotron self-Compton model cannot explain the whole emission spectrum, suggesting an inhomogeneous geometry in the emission region.