  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.

Astrophysics