Emission from Hot Dust in the Infrared Spectra of Gamma-ray Bright
  Blazars
  A possible source of $\gamma$-ray photons observed from the jets of blazars
is inverse Compton scattering by relativistic electrons of infrared seed
photons from a hot, dusty torus in the nucleus. We use observations from the
Spitzer Space Telescope to search for signatures of such dust in the infrared
spectra of four $\gamma$-ray bright blazars, the quasars 4C 21.35, CTA102, and
PKS 1510$-$089, and the BL Lacertae object ON231. The spectral energy
distribution (SED) of 4C 21.35 contains a prominent infrared excess indicative
of dust emission. After subtracting a non-thermal component with a power-law
spectrum, we fit a dust model to the residual SED. The model consists of a
blackbody with temperature $\sim1200$ K, plus a much weaker optically thin
component at $\sim660$ K. The total luminosity of the thermal dust emission is
$7.9\pm0.2 \times 10^{45}$ erg s$^{-1}$. If the dust lies in an equatorial
torus, the density of IR photons from the torus is sufficient to explain the
$\gamma$-ray flux from 4C 21.35 as long as the scattering occurs within a few
parsecs of the central engine. We also report a tentative detection of dust in
the quasar CTA102, in which the luminosity of the infrared excess is $7 \pm 2
\times 10^{45}$ erg s$^{-1}$. However, in CTA102 the far-IR spectra are too
noisy to detect the $10 \mu$m silicate feature. Upper limits to the luminosity
from thermal emission from dust in PKS 1510-089, and ON231, are,
$2.3\times10^{45}$, and $6.6\times10^{43}$ erg s$^{-1}$, respectively. These
upper limits do not rule out the possibility of inverse Compton up-scattering
of IR photons to $\gamma$-ray energies in these two sources. The estimated
covering factor of the hot dust in 4C 21.35, 22%, is similar to that of
non-blazar quasars; however, 4C 21.35 is deficient in cooler dust.
