X-ray spectra of three BL Lac objects.

Abstract

We present the results of the analysis of twin EXOSAT observations of three BL Lac objects, namely PKS 0521-365, PKS 0548-322, and I Zw 186. The energy spectra between 0.02 and ~ 7 keV can generally be described by power laws with a low energy cut off due to absorption by hydrogen in our galaxy. However, while PKS 0521-365 has a spectrum with an energy slope of ~ 0.6 like many QSOs, the other two BL Lacs have steeper spectra. Furthermore the energy spectrum of PKS 0548-322 is better represented by a double power law with a steeper spectrum beyond a break occurring around 2-3 keV, and I Zw 186 shows a harder spectrum when the source is slightly brighter. At least in these last two cases, the X-ray spectral region proves to be of great interest because of the presence of features and of their variability with source intensity.