2016ApJ...831...43V

We present new mid-IR observations of the quadruply lensed quasar Q2237+0305 taken with CanariCam on the Gran Telescopio Canarias. Mid-IR emission by hot dust, unlike the optical and near-IR emission from the accretion disk, is unaffected by the interstellar medium (extinction/scattering) or stellar microlensing. We compare these "true" ratios to the (stellar) microlensed flux ratios observed in the optical/near-IR to constrain the structure of the quasar accretion disk. We find a half-light radius of R_1/2=3.4_–2.1^+5.3sqrt(\langleM)>/0.3M_☉ lt-day at λ_rest=1736 \mathringA and an exponent for the temperature profile R∝λ^p of p=0.79±0.55, where p=4/3 for a standard thin-disk model. If we assume that the differences in the mid-IR flux ratios measured over the years are due to microlensing variability, we find a lower limit for the size of the mid-IR-emitting region of R_1/2>rsim200 sqrt(\langleM)>/0.3M_☉ lt-day. We also test for the presence of substructure/satellites by comparing the observed mid-IR flux ratios with those predicted from smooth lens models. We can explain the differences if the surface density fraction in satellites near the lensed images is α=0.033_–0.019^+0.046 for a singular isothermal ellipsoid plus external shear mass model or α=0.013_–0.008^+0.019 for a mass model combining ellipsoidal NFW and de Vaucouleurs profiles in an external shear.