The radiative deceleration of ultrarelativistic jets in active galactic nuclei

We report on detailed calculations of the Compton-scattering interaction between an ultrarelativistic jet that emanates from the vicinity of a massive black hole and the thermal radiation field produced by a surrounding accretion disk. We consider both thin- and thick-disk geometries and model the disk spectrum as a sum of blackbodies, taking into account the intensity enhancement due to internal refections in disks with central accretion funnels. We compute the Compton-drag deceleration of an initially hyperrelativistic jet for a variety of disk and outflow parameters. The resulting beamed spectrum of the upscattered disk photons is derived self-consistently by a Monte Carlo technique. Our calculations confirm Phinney's suggestion that the thermal radiation field produced by accretion in an active galactic nucleus could be very effective in decelerating ultrarelativistic jets that are accelerated by electromagnetic or hydromagnetic forces closer to the central black hole. We find that terminal Lorentz factors that are consistent with the values (γ is proportional to <~ 10) inferred in superluminal radio sources are readily produced in this model for plausible disk and jet parameters without having to invoke additional acceleration in the interaction zone. It is noted that BL Lac objects and other blazar sources may correspond to relativistic jets that have been decelerated by radiation in a limited luminosity range, with jets associated with brighter (fainter) disks remaining unobserved because they are too weakly (strongly) beamed. We also discuss the kinematic consequences of the transverse gradient in γ is proportional to that is predicted by our results. On the basis of these calculations, we propose a new interpretation of the hard X-ray component that is often detected in the spectra of BL Lac objects. We suggest that this emission be identified with the inverse Compton radiation that is produced in the course of the initial deceleration of the relativistic jets associated with these sources. We demonstrate that the observed properties of the hard X-ray component as well as the inferred values of γ if proportional to can be reproduced with this model without violating the constraint that the isotropic flux from the disk should remain lower than the beamed synchrotron flux from the jet. This model also accounts for the strong variability and the frequent occurrence of flat X-ray spectra among the most X-ray-bright BL Lac objects and leads to specific predictions for the cutoff energy of the high-energy emission in these sources.