On the Location of the Acceleration and Emission Sites in Gamma-Ray Blazars

Abstract

Compton scattering of external radiation by nonthermal particles in outflowing blazar jets is dominated by accretion-disk photons rather than scattered radiation to distances $\sim 0.01-0.1$ pc from the central engine for standard parameters, thus clarifying the limits of validity of the model by the present authors and the model of Sikora, Begelman, \& Rees. On the basis of contemporaneous Ginga X-ray and EGRET gamma-ray observations, we estimate the radius of 3C 279's gamma-ray photosphere to be smaller than estimated by Blandford. There is thus no need to require that the acceleration and emission sites of gamma-ray blazars to be located farther than $\sim 10^{2-3}$ gravitational radii from the central engine. We argue that lineless BL Lac objects, rather than quasars, are more likely to be detected in the TeV energy range.