Accretion Disk Sizes and Temperature Profiles in Lensed Quasars: NIR Microlensing Challenges Thin Disk Theory

Abstract

Microlensing and reverberation mapping measurements of quasar accretion disk sizes and temperature gradients disagree with thin disk theory predictions. Previous microlensing results rely on heterogeneous wavelength coverage -primarily UV broad emission lines (BELs) from small samples -probing the disk only out to a typical radius of $\lesssim$5 light days on average. We use microlensing estimates from an homogeneous sample of near-infrared (NIR) observations of lensed quasars (21 image-pairs from 7 lens systems) to extend disk size measurements out to 14 light days. This analysis leverages narrow emission lines (NELs), which provide a more reliable microlensing-free baseline than BEL cores. We derive Bayesian accretion disk size estimates that reproduce the observed microlensing magnifications, as simulated from magnification maps. NEL-based sizes yield a logarithmic slope of $p=0.68\pm0.23$, consistent with prior estimates corresponding to inner disk regions ($r \lesssim$5 light days). Using a new homogeneous NIR dataset that allows us to reach radial distances of up to 14 light days, we find that accretion disks in these previously unexplored regions are also larger and exhibit steeper temperature gradients than thin disk theory predicts. The increased precision allows us to reject the theoretical logarithmic slope $p=4/3$ at the 98\% confidence level. Any hypothesis invoking BLR contamination to explain this discrepancy must account for how such contamination modulates the underlying accretion disk such that the combination of both results in a power law with logarithmic slope $p=0.68\pm0.23$ across a broad wavelength baseline spanning from $\sim$X-Ray to $\sim 5000$Å.