On Compton Scattering Scenarios for Blazar Flares

The synchrotron reflection scenario recently proposed to explain $\gamma$-rayflares observed from blazar jets is studied. Our analysis takes into accountthe angular distribution of the beamed radiation, the finite extent of thescattering region, and light travel-time effects. We compare energy densitiesand powers for synchrotron, SSC, reflected synchrotron (RSy), and externalCompton (EC) scattering processes. If the width of the scattering layer is muchlarger than $\Gamma R^\prime_B$, where $\Gamma$ and $ R^\prime_B$ denote thebulk Lorentz factor and comoving-frame radius of the plasma blob, respectively,then the ratio of the RSy and synchrotron energy densities $\sim 4 \Gamma^3n_{BLR} \sigma_{T} R^\prime_B$, where $n_{BLR}$ is the mean particle density inthe broad line region (BLR). Our results imply that Thomson-thick scatteringregions of narrow extent must be present for the synchrotron reflectionmechanism to operate effectively. This process seems unlikely to cause flaresin lineless BL Lac sources, where X-ray and TeV flares are common and the BLRis thought to be weak or absent. We sketch time profiles of flares for variousscenarios, including a model where the blob is energized by sweeping upsurrounding material.Comment: 12 pages, including 2 figures; uses epsf.sty, rotate.sty. Submitted to ApJ Letter