Quasar Microlensing Variability Studies Favor Shallow Accretion Disk Temperature Profiles

TL;DR

This study compares microlensing and luminosity-based size measurements of quasar accretion disks, finding evidence for shallower temperature profiles than the standard model, suggesting alternative disk structures.

Contribution

It provides the first observational evidence that quasar accretion disks have shallower temperature profiles than predicted by the standard thin disk model.

Findings

Microlensing sizes are smaller than luminosity-based estimates by a factor of about 3.

The temperature profile slope $eta$ is constrained between 0.37 and 0.56, shallower than the standard 0.75.

Alternative models like disk winds or non-blackbody atmospheres can explain the observed size scaling.

Abstract

We compare the microlensing-based continuum emission region size measurements in a sample of 15 gravitationally lensed quasars with estimates of luminosity-based thin disk sizes to constrain the temperature profile of the quasar continuum accretion region. If we adopt the standard thin disk model, we find a significant discrepancy between sizes estimated using the luminosity and those measured by microlensing of $\log(r_{L}/r_{\mu})=-0.57\pm0.08\,\text{dex}$. If quasar continuum sources are simple, optically thick accretion disks with a generalized temperature profile $T(r) \propto r^{-\beta}$, the discrepancy between the microlensing measurements and the luminosity-based size estimates can be resolved by a temperature profile slope $0.37 < \beta < 0.56$ at $1\,\sigma$ confidence. This is shallower than the standard thin disk model ($\beta=0.75$) at $3\,\sigma$ significance. We consider alternate accretion disk models that could produce such a temperature profile and reproduce the empirical continuum size scaling with black hole mass, including disk winds or disks with non-blackbody atmospheres.