Accretion-disk models of BL Lac objects

Abstract

The accretion-disk configuration has features that make it an attractive model for BL lac objects: jets along the disk axis and high efficiency of energy conversion with possible close to Eddington luminosities. On the other hand the characteristic power-law continuum spectrum of BL Lacs does not show a large UV bump, which is the typical signature of a thermal accretion-disk spectrum. Still, although the spectra of BL Lac objects are usually very smooth, high quality simultaneous data reveal deviations from smoothness. Using detailed calculations of accretion-disk spectra, including modifications due to electron-scattering, Comptonization, and optically thin regions (Wandel and Petrosian 1988), we fit the continuum UV and soft X-ray spectra of a few BL Lac objects, and determine the masses and accretion rates required. It turns out that the UV-soft X ray region can be well fitted by the spectrum of an accretion disk , as in quasars despite the difference in spectra, but it requires near Eddington accretion rates to produce the soft X-ray excess. For the BL Lac object with the best variability data, PKS 2155-304, we find that the dynamic and the viscous time scales in the disk regions that contribute most of the observed UV and soft X-rays are consistent with the respective time scales for variations in the continuum flux observed in these two wavebands. Finally, we compare the masses derived from fitting the continuum spectrum with the masses derived from the X-ray variability.