On Compton scattering scenarios for blazar flares

The synchrotron reflection scenario recently proposed to explain gamma-ray flares observed from blazar jets is studied. Our analysis takes into account the angular distribution of the beamed radiation, the finite extent of the scattering region, and light-travel-time effects. We compare energy densities and powers for synchrotron, synchrotron self-Compton, reflected synchrotron (RSy), and external Compton scattering processes. If the width of the scattering layer is much larger than Gamma R-B', where Gamma and R-B' denote the bulk Lorentz factor and the comoving frame radius of the plasma blob, respectively, then the ratio of the RSy and synchrotron energy densities is similar to 4 Gamma(3)n(BLR)sigma(T)R(B)' where n(BLR) is the mean particle density in the broad line region (BLR). Our results imply that Thomson-thick scattering regions of narrow extent must be present for the synchrotron reflection mechanism to operate effectively. This process seems unlikely to cause flares in lineless BL Lac sources, where X-ray and TeV flares are common and the BLR is thought to be weak or absent. We sketch time profiles of flares for Various scenarios.