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The four-image gravitationally lensed quasar QSO 2237+0305 is microlensed bystars in the lens galaxy. The amplitude of microlensing variability can be usedto infer the relative size of the quasar as a function of wavelength; thisprovides a test of quasar models. Toward this end, we present Spitzer SpaceTelescope Infrared Spectrograph and Infrared Array Camera (IRAC) observationsof QSO 2237+0305, finding the following. (1) The infrared (IR) spectral energydistribution (SED) is similar to that of other bright radio-quiet quasars,contrary to an earlier claim. (2) A dusty torus model with a small openingangle fits the overall shape of the IR SED well, but the quantitative agreementis poor due to an offset in wavelength of the silicate feature. (3) The fluxratios of the four lensed images can be derived from the IRAC data despitebeing unresolved. We find that the near-IR fluxes are increasingly affected bymicrolensing toward shorter wavelengths. (4) The wavelength dependence of theIRAC flux ratios is consistent with the standard quasar model in which anaccretion disk and a dusty torus both contribute near 1 micron in the restframe. This is also consistent with recent IR spectropolarimetry of nearbyquasars.

Spitzer observations of a gravitationally lensed quasar, QSO 2237+0305