The size of the mid-IR emission region of a quasar inferred from microlensed images of Q2237+0305

Abstract

We use published mid-IR and V-band flux ratios for images A and B of Q2237+0305 obtained in 1999 September to demonstrate that the size of the mid-IR emission region has a scale comparable to or larger than the microlensEinstein radius (ER) (∼10 1 7 cm for microlensing by solar mass stars). Q2237+0305 has been monitored extensively in the R and V bands for ∼15 yr. The variability record shows significant microlensing variability of the optical emission region, and has been used by several studies to demonstrate that the optical emission region is much smaller than the ER for solar-mass objects. For the majority of the monitoring history, the optical flux ratios have differed significantly from those predicted by macro-models. In contrast, recent observations in mid-IR show flux ratios similar to those measured in the radio, and to predictions of some lens models, implying that the mid-IR flux is emitted from a region that is at least two orders of magnitude larger than the optical emission region. We have calculated the likelihood of the observed mid-IR flux ratio as a function of mid-IR source size given the observed V-band flux ratio. The expected flux ratio for a source having dimensions of ∼ 1 ER is a sensitive function of the macro-model adopted. However, we find that the probability of source size given the observed flux ratios is primarily sensitive to the ratio of the macro-model magnifications. Limits on the mid-IR source size can therefore be considered as a function of a one-dimensional, rather than a four-dimensional (two optical depths plus two shears) class of models. By combining probabilities from the ratios A/B and C/D we infer that the diameter of a circular IR emission region is > 1 ER with >95 per cent confidence. For sources of this size, other geometries, specifically an annular geometry, appropriate for a dusty torus, yield the same limit if the projected area rather than radius is considered. For microlensing by low-mass stars, this source size limit rules out non-thermal processes such as synchrotron as mechanisms for mid-IR emission.