From the Astrophysics Group: Accretion disks around lensed supermassive black holes may be significantly smaller than previously estimated

In a new paper by C. Fian et al. we investigate the discrepancy between the predicted size of accretion disks (AD) in quasars and the observed sizes as deduced from gravitational microlensing studies. We employ state-of-the-art emission models for quasars and high-resolution microlensing magnification maps (top figure) and compare attributes of their magnification-distribution functions to those obtained for pure Shakura-Sunyaev disk models. We test the validity of our detailed model predictions by examining their agreement with published microlensing estimates of the half-light radius of the continuum-emitting region in a sample of lensed quasars.

Our findings suggest that the steep disk temperature profiles found by microlensing studies are erroneous as the data are largely affected by the BLR, which does not obey a temperature-wavelength relation (bottom figure). We show that the mere contribution of the BLR to the continuum signal is able to account for the deduced over-estimation factors as well as the implied size-wavelength relation in a sample of 12 lenses. Our findings significantly weaken the tension between AD theory and observations, and suggest that microlensing provides a new means to probe the hitherto poorly constrained diffuse BLR emission around accreting black holes.