THE X-RAY SPECTRAL VARIABILITY OF THE BL LACERTAE TYPE OBJECT PKS-2155-304

We present a detailed study of the hard X-ray properties of the BL Lacertae object PKS 2155-304 based on measurements made in 1988 and 1989 with the Large Area Counter (LAC) on board the Ginga satellite. The source exhibited a high degree of variability with a dynamic range of a factor 7 in the 2-6 keV band. The fastest amplitude variation was a factor 2 decline in the intensity in this band within 4 hours. The spectrum is characterized by a break which occurs at approximately 4 keV. Spectral fits to the data integrated in 6400 s time bins reveal that, in common with previous observations of BL Lacertae objects, the spectral slope is generally anticorrelated with intensity in the sense that the spectrum hardens as the intensity increases. However, the tracks of sequential points in the index-intensity plane are occasionally seen to differ during the rise and decay stages of individual flares. Furthermore, during one, or possibly two, flaring episodes the spectral index is observed to correlate with intensity variations. The X-ray properties of PKS 2155-304 are most readily interpreted in terms of direct synchrotron radiation originating within a relativistic jet, however, the X-ray emitting region is probably more complex than the geometries assumed in most standard jet models. Further development of time-dependent theoretical models will be necessary to explain some of the details of the present observations.