Spitzer observations of a gravitationally lensed quasar, QSO 2237+0305
  The four-image gravitationally lensed quasar QSO 2237+0305 is microlensed by
stars in the lens galaxy. The amplitude of microlensing variability can be used
to infer the relative size of the quasar as a function of wavelength; this
provides a test of quasar models. Toward this end, we present Spitzer Space
Telescope Infrared Spectrograph and Infrared Array Camera (IRAC) observations
of QSO 2237+0305, finding the following. (1) The infrared (IR) spectral energy
distribution (SED) is similar to that of other bright radio-quiet quasars,
contrary to an earlier claim. (2) A dusty torus model with a small opening
angle fits the overall shape of the IR SED well, but the quantitative agreement
is poor due to an offset in wavelength of the silicate feature. (3) The flux
ratios of the four lensed images can be derived from the IRAC data despite
being unresolved. We find that the near-IR fluxes are increasingly affected by
microlensing toward shorter wavelengths. (4) The wavelength dependence of the
IRAC flux ratios is consistent with the standard quasar model in which an
accretion disk and a dusty torus both contribute near 1 micron in the rest
frame. This is also consistent with recent IR spectropolarimetry of nearby
quasars.
