Einstein observatory X-ray spectra of BL Lac objects

Abstract

In this paper, we summarize the spectral X-ray data obtained with the Imaging Proportional Counter flown on the Einstein Observatory and interpret that data in the context of the overall electromagnetic distribution. The X-ray spectra in the IPC (0.2 – 4.0 keV) range are generally well represented by a model incorporating an intrinsic power law with a mean value of the power law index of 1.1, absorbed at low energies by gas in our Galaxy. For sufficiently bright objects, we have simultaneous Monitoring Proportional Counter data available, covering the range of 1.2 to 10 keV. The comparison of contemporaneous data from both instruments suggests a gradual steepening of the X-ray spectrum in the 1 – 3 keV range, by Δα of about 0.5. The absorption in the line of sight derived from the X-ray data is in close agreement with the galactic value measured using 21 cm radio techniques (Elvis, Lockman, and Wilkes 1988), which is in contrast to the result for quasars by Wilkes and Elvis (1987), where a soft excess is observed. We interpret the X-ray data in a context of the overall electromagnetic spectrum using a two-component synchrotron model (cf. Madejski and Schwartz 1988). We find that while in some cases the radio data imply a violation of the Compton limit, no such violation occurs for the optical region. We thus infer that while the radio emission is likely to be beamed (e. g. forming a jet-like structure), the infrared optical-UV-X-ray radiation can well be emitted isotropically. We briefly discuss the implication of this finding on the so-called “parent population” of BL Lac objects.