The Beaming Pattern and Spectrum of Radiation from Inverse Compton Scattering in Blazars

By including Klein-Nishina effects, we generalize previous calculations ofthe beaming pattern of photons produced by inverse Compton scattering. For anisotropic distribution of soft photons upscattered by nonthermal electrons witha power-law density distribution embedded in a plasma moving with relativisticbulk speed we show that the observed radiation intensity is proportional toD^(3+p), where D is the Doppler boosting factor, and p the electrondistribution index. This agrees with previous computations performed in theThomson limit, where the observed spectral index is a=(p-1)/2 and the beamingpattern D^(4+2a). Independent of D, Klein-Nishina effects limit the location ofthe peak energy of the observed spectral energy distribution such that e_{peak}less than 1/e_0, where e_0 is the energy of the seed photons in units of mc^2.Assuming that the seed photons originate in the broad line region, wedemonstrate that the GeV emission of blazars is significantly modified byKlein-Nishina effects, the spectrum being softer than that calculated in theThomson limit. We further show that the change in spectral index of the inverseCompton emission across the peak energy can exceed the value of 0.5 predictedby computations performed in the Thomson limit. The model spectra agree withOSSE and COMPTEL limits on this break without invoking the effects ofdifferential absorption at the edge of a gamma-ray photosphere.Comment: Accepted in ApJ, Nov 200