The beaming pattern and spectrum of inverse Compton radiation in blazars

By including Klein-Nishina effects, we generalize previous calculations of the beaming pattern of photons produced by inverse Compton scattering. For an isotropic distribution of soft photons upscattered by nonthermal electrons with a power-law density distribution embedded in a plasma moving with relativistic bulk speed we show that the observed radiation intensity is proportional to D^(3+p), where D is the Doppler boosting factor, and p the electron distribution index. This agrees with previous computations performed in the Thomson limit, where the observed spectral index is a=(p-1)/2 and the beaming pattern D^(4+2a). Independent of D, Klein-Nishina effects limit the location of the 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, we demonstrate that the GeV emission of blazars is significantly modified by Klein-Nishina effects, the spectrum being softer than that calculated in the Thomson limit. We further show that the change in spectral index of the inverse Compton emission across the peak energy can exceed the value of 0.5 predicted by computations performed in the Thomson limit. The model spectra agree with OSSE and COMPTEL limits on this break without invoking the effects of differential absorption at the edge of a gamma-ray photosphere.

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