X-Ray Variability of BL Lacertae Objects.

We have searched for X-ray emission from 63 optically active extragalactic objects using the A-1 detectors on the HEAO 1 spacecraft. Most of these sources are classified as BL Lacertae objects, and display one or more of the following properties: a high optical polarization, rapid optical variability, or a featureless (line free) optical spectrum. Fourteen objects are detected as sources of significant X-ray emission, eight of which were initially discovered with the A-1 detectors. Significant X-ray intensity variability is detected in all but three sources (and in these three objects only one observation is currently available, so that their X-ray behavior has yet to be determined). In somes cases (MRK 421, MRK 501, PKS 0548-322, and IZW 187) variations on monthly time scales are observed, while other sources (PKS 0521-365, PKS 0537-441, 1218+304, 3C 371, and PKS 2155-152) appear to vary on weekly (and in some cases, daily) time scales. In one source, PKS 2155-304, random factor of 2 variations in intensity are observed over time periods as short as 6 hours. A study of the X-ray variations observed in 3C 371 suggests that changes at these wavelengths result from either a succession (and partial superposition) of discrete events, or a variable injection rate. In those sources where X-ray emission is observed, the nonthermal optical flux extrapolates extremely well into the X-ray region, suggesting that emissions in these wavelength bands share a common origin. We show that both the variability and spectral form observed at X-ray wavelengths can be explained if (as suggested by the highly polarized optical flux) the emission is synchrotron radiation. It is necessary, however, that the synchrotron emitters move at relativistic velocities, in order to avoid excessive inverse-Compton losses that otherwise occur.