On Compton Scattering Scenarios for Blazar Flares

Abstract

The synchrotron reflection scenario recently proposed to explain $γ$-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, SSC, reflected synchrotron (RSy), and external Compton (EC) scattering processes. If the width of the scattering layer is much larger than $ΓR^\prime_B$, where $Γ$ and $ R^\prime_B$ denote the bulk Lorentz factor and comoving-frame radius of the plasma blob, respectively, then the ratio of the RSy and synchrotron energy densities $\sim 4 Γ^3 n_{BLR} σ_{T} R^\prime_B$, 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, including a model where the blob is energized by sweeping up surrounding material.