Hard X-Ray Spectra of Broad-Line Radio Galaxies from the Rossi X-Ray Timing Explorer

摘要

We present the results of hard X-ray observations of four broad-line radio galaxies (BLRGs) with the Rossi X-Ray Timing Explorer. The original motivation behind the observations was to search for systematic differences between the BLRGs and their radio-quiet counterparts, the Seyfert galaxies. We do, indeed, find that the Fe Kα lines and Compton "reflection" components, which are hallmarks of the X-ray spectra of Seyfert galaxies, are weaker in BLRGs by about a factor of 2. This observational result is in agreement with the conclusions of other recent studies of these objects. We examine several possible explanations for this systematic difference, including beaming of the primary X-rays away from the accretion disk, a low iron abundance, a small solid angle subtended by the disk to the primary X-ray source, and dilution of the observed spectrum by beamed X-rays from the jet. We find that a small solid angle subtended by the disk to the primary X-ray source is a viable and appealing explanation, while all others suffer from drawbacks. We interpret this as an indication of a difference in the inner accretion disk structure between Seyfert galaxies and BLRGs, namely that the inner accretion disks of BLRGs have the form of an ion-supported torus or an advection-dominated accretion flow, which irradiates the geometrically thin outer disk.