Sizes and Temperature Profiles of Quasar Accretion Disks from Chromatic Microlensing

TL;DR

This study uses multiwavelength microlensing observations of quasars to measure accretion disk sizes and temperature profiles, revealing they are larger and have different wavelength dependencies than standard models predict.

Contribution

It provides the first large sample analysis constraining quasar accretion disk sizes and temperature profiles using chromatic microlensing, challenging standard thin disk theory.

Findings

Quasar disks are larger than predicted by standard models.

Disk sizes increase with wavelength with a slope of ~0.2.

Standard thin disk models are inconsistent with observed disk sizes.

Abstract

Microlensing perturbations to the flux ratios of gravitationally lensed quasar images can vary with wavelength because of the chromatic dependence of the accretion disk's apparent size. Multiwavelength observations of microlensed quasars can thus constrain the temperature profiles of their accretion disks, a fundamental test of an important astrophysical process which is not currently possible using any other method. We present single-epoch broadband flux ratios for 12 quadruply lensed quasars in eight bands ranging from 0.36 to 2.2 microns, as well as Chandra 0.5--8 keV flux ratios for five of them. We combine the optical/IR and X-ray ratios, together with X-ray ratios from the literature, using a Bayesian approach to constrain the half-light radii of the quasars in each filter. Comparing the overall disk sizes and wavelength slopes to those predicted by the standard thin accretion disk model, we find that on average the disks are larger than predicted by nearly an order of magnitude, with sizes that grow with wavelength with an average slope of ~0.2 rather than the slope of 4/3 predicted by the standard thin disk theory. Though the error bars on the slope are large for individual quasars, the large sample size lends weight to the overall result. Our results present severe difficulties for a standard thin accretion disk as the main source of UV/optical radiation from quasars.