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We investigate the discrepancy between the predicted size of accretion disks(ADs) in quasars and the observed sizes as deduced from gravitationalmicrolensing studies. Specifically, we aim to understand whether thediscrepancy is due to an inadequacy of current AD models or whether it can beaccounted for by the contribution of diffuse broad-line region (BLR) emissionto the observed continuum signal. We employed state-of-the-art emission modelsfor quasars and high-resolution microlensing magnification maps and comparedthe attributes of their magnification-distribution functions to those obtainedfor pure Shakura-Sunyaev disk models. We tested the validity of our detailedmodel predictions by examining their agreement with published microlensingestimates of the half-light radius of the continuum-emitting region in a sampleof lensed quasars. Our findings suggest that the steep disk temperatureprofiles found by microlensing studies are erroneous as the data are largelyaffected by the BLR, which does not obey a temperature-wavelength relation. Weshow with a sample of 12 lenses that the mere contribution of the BLR to thecontinuum signal is able to account for the deduced overestimation factors aswell as the implied size-wavelength relation. Our study points to a likelysolution to the AD size conundrum in lensed quasars, which is related to theinterpretation of the observed signals rather than to disk physics. Ourfindings significantly weaken the tension between AD theory and observations,and suggest that microlensing can provide a new means to probe the hithertopoorly constrained diffuse BLR emission around accreting black holes.

Diffuse emission in microlensed quasars and its implications for accretion-disk physics