Observations of the Lensed Quasar Q2237+0305 with CanariCam at GTC

Abstract

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 $R_{1/2}=3.4_{-2.1}^{+5.3}\sqrt{⟨M⟩/0.3{\mathrm{M}}_{\odot }}$ light-days at ${\lambda }_{rest}=1736$ Å, and an exponent for the temperature profile $R\propto {\lambda }^p$ of $p=0.79\pm 0.55$, 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 $R_{1/2}\gtrsim 200\sqrt{⟨M⟩/0.3{\mathrm{M}}_{\odot }}$ light-days. 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 $\alpha =0.033_{-0.019}^{+0.046}$ for a singular isothermal ellipsoid plus external shear mass model or $\alpha =0.013_{-0.008}^{+0.019}$ for a mass model combining ellipsoidal NFW and de Vaucouleurs profiles in an external shear.

19 pages, 3 figures, 2 tables; ApJ (in press)

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