Variable X-ray spectra of BL Lac objects : HEAO 1 observations of PKS0548-322 and 2A 1219+305.

We present X-ray spectra for the BL Lac objects PKS 0548-322 and 2A 1219+305 measured with the HEAO 1 A-2 detectors during pointing maneuvers on 1978 September 30 and 1978 May 31, respectively. Both fit single power-law components with low-energy absorption. For 2A 1219+305, a thermal bremsstrahlung form gives an unacceptable fit. We find, from a comparison with other statistically poorer observations taken at 6 month intervals while the satellite was in its normal scanning mode, that the sources exhibit spectral variability. A summary of measurements of the five BL Lac objects detected with the A-2 experiment is presented, and we conclude that X-ray spectral changes in this class of source are common. Their general X-ray spectral characteristics distinguish BL Lac objects from other classes of X-ray-emitting active galactic nuclei. Analysis of their total spectra indicates that most of the energy is emitted in the 5-100 eV band.