Studies of BL Lacertae Objects with the Einstein Observatory: The Soft X-Ray Spectra of OJ 287 and PKS 0735+178
Abstract
We present X-ray spectral data for the BL Lac objects OJ 287 and PKS 0735+l78, observed by the IPC aboard the Einstein Observatory. The X-ray spectra are well described by a power-law model with a low-energy cutoff consistent with photoelectric absorption within our own Galaxy. The best-fit values of the energy spectral index α in the 0.2-4.0 keV band are, respectively, 0.91 (+0.40, -0.38) and 0.76 (+0.47, -0.43) (90% confidence regions). The X-ray flux from OJ 287 is variable by a ratio of 3 from low to high state; PKS 0735+178 shows no indication of X-ray variability. We discuss our data analysis methods in some detail with an emphasis on the estimation of systematic uncertainties in the spectral parameters and flux. These two objects show distinct X-ray emission mechanisms. In OJ 287, where the X-ray spectrum lies roughly on the extrapolation of the infrared/optical/ultraviolet spectrum, we interpret the X-ray emission to be due to the synchrotron process from a volume common with either a beamed radio component (size of order 10^18^ cm) or a stationary optical component (size of order 10^16^ cm). In PKS 0735+178, where the X-ray spectrum lies below (and is flatter than) such an extrapolation, the X-ray emission is most likely due to the Compton process operating in one of the VLBI radio components as reported by Cotton et al. in 1980. The synchrotron self-Compton process, with modest kinematic Doppler factors (δ in the range 3-5) predicts the measured X-ray flux from PKS 0735+178, and lower than the measured flux in OJ 287. Using the superluminal expansion data for PKS 0735+178 reported by Baath in 1984 we derive the bulk Lorentz factor {GAMMA} for the emitting matter to be ~7, and the angle of motion to the line of sight φ ~ 13^deg^.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- July 1988
- DOI:
- 10.1086/166511
- Bibcode:
- 1988ApJ...330..776M
- Keywords:
-
- Bl Lacertae Objects;
- Heao 2;
- X Ray Spectra;
- Compton Effect;
- Light Curve;
- Proportional Counters;
- Synchrotron Radiation;
- Very Long Base Interferometry;
- Astrophysics;
- BL LACERTAE OBJECTS;
- GALAXIES: X-RAYS;
- RADIATION MECHANISMS;
- X-RAYS: SPECTRA