Bulk acceleration in relativistic jets and the spectral properties of blazars

Abstract

Models for the synchrotron and inverse Compton emission from relativistic jets with bulk acceleration are studied. The basic scheme explored is that the bulk velocity increases with increasing distance along the jet, while the emission spectrum shifts to lower frequencies. Thus synchrotron X-rays, emitted by the jet core, are weakly beamed, while the optical and radio emissions, produced further away, are more strongly beamed. We find that the different broad-band spectra of BL Lac objects selected by radio or X-ray flux can be accounted for as due to different orientation, radio-selected objects being viewed at small angles, X-ray-selected objects at large angles. Stringent conditions are imposed by this requirement on the jet properties. In particular, acceleration should occur slowly over a wide distance range, with small final bulk Lorentz factors {GAMMA} ~ 3-5. The space density of X-ray-selected BL Lac objects is expected to be 20-100 times that of radio selected BL Lac objects with the same X-ray luminosity. Synchrotron emission with steep spectrum is expected to be the main radiation mechanism in the X-ray band for objects seen at large angles. Inverse Compton emission, yielding flatter spectra and longer variability time scales, may be important in the X-ray band, especially for objects seen at small angles.