X-Ray Studies of BL Lacertae Objects

This thesis presents spectral X-ray data for BL Lac objects observed by the IPC and MPC aboard the Einstein Observatory and interprets that data in a context of their overall radiation spectra using synchrotron and synchrotron self-Compton models. The objects considered are: OJ 287, PKS 0735+178, I Zw 186, PKS 0548-322, Mkn 180, BL Lacertae, PKS 2155-304, H 0414-009 and H 0323+022. X-ray spectra of BL Lac objects are well described by a power law model with a low energy cutoff due to absorption within our own Galaxy. The best fit values of the energy spectral index (alpha) in the IPC (0.2 - 4.0 keV) band range from 0.73 to 2.35, with a mean of 1.2 and rms spread of 0.51. No single, universal index can fit the spectra of all objects. We find that for all objects except PKS 0735+178 the X-ray spectrum is an extrapolation of the infrared/optical/UV spectrum; in PKS 0735+178, the X-ray spectrum lies significantly below such an extrapolation. We interpret the overall electromagnetic distribution in those objects as arising due to the synchrotron process in at least two spatial regions, with sizes respectively (TURN)10('18) cm for the radio component and (TURN)10('16) cm for the optical component. For OJ 287, PKS 0735+178 and BL Lacertae we require relativistic beaming of the radio emission at an angle close to the line of sight with kinematic Doppler factors (delta) of (TURN)3 - 5, corresponding to bulk Lorentz factors (IR-PERP) in the same range. In other objects such beaming away from the line of sight is consistent with the data, but it is not required. We therefore do not require a hypothetical parent population of BL Lac objects (cf. Schwartz and Ku 1983). The electron and magnetic field energy densities in this radio component are near equipartition, and the electron lifetimes to synchrotron losses are of the order of the source crossing times. The optical component is severely dominated by the magnetic field, and beaming is not necessary. The calculated electron lifetimes to the synchrotron losses in this component are significantly shorter than the source crossing time, requiring continuous acceleration. In the objects where the X-ray spectrum lies on the extrapolation of the infrared-optical-ultraviolet spectrum, we interpret the X-ray emission to be due to the synchrotron process. In PKS 0735+178, the X-ray emission is most likely due to the Compton process operating in the radio component. In H 0323+022, the rapid (30 second) variability (Doxsey et al. 1983) suggests that a direct accretion process is responsible for the X-ray emission above 1 keV. We thus conclude that X-ray emission in BL Lac objects arises from diverse processes.