X-RAY-INTENSITY AND SPECTRAL VARIATIONS IN THE BL LAC SOURCES H2155-304 AND PKS0548-322

The results from the X-ray observations of two BL Lacertae objects, H2155-304 and PKS0548-322, are presented. The observations were carried out with the HEAO-1 A2 Low Energy Detectors (LED's), the High Resolution Imager (HRI) and the Monitor Proportional Counter (MPC) aboard the Einstein Observatory. Correlated X-ray intensity variations of ~20 per cent in the 0.15-3.5 keV energy band and ~40 per cent in the 1.5-10 keV energy band are observed in H2155-304 over a period of about 8hr. A factor of 2 change in the intensity of PKS 0548-322 in the 3.5-10 keV energy interval is also detected over a time interval of 1 hr. Variations in the X-ray intensity over a time-scale of months and the hardening of the spectrum with increasing intensity are seen in the HEAO-1 A2 data on H2155-304. The shortest observed time-scale for the variability is used to derive an upper limit on the mass of the central accreting source in the two BL Lac objects studied here. © 1987 Oxford University Press. All rights reserved.