Zooming in on quasar accretion disks using chromatic microlensing

Abstract

Observing the temperature profiles of accretion disks around black holes is a fundamental test of an important astrophysical process. However, angular resolution limitations have prevented such a measurement for distant quasars. We present a new method for determining the size of quasar accretion disks at a range of wavelengths, thus constraining their temperature profiles. The technique uses single-epoch, multi-wavelength optical and nearinfrared imaging of gravitationally lensed quasars in conjunction with X-ray imaging, and takes advantage of the presence of microlensing perturbations to the magnifications of the lensed images. The dependence of these perturbations on the angular size of the source, combined with the temperature structure of quasar accretion disks, causes the flux ratio anomalies due to microlensing to appear chromatic. This allows us to probe regions of the quasar that are too small to be measured by any other technique. We apply this method to observations of 12 lensed quasars, and measure the size of the accretion disk of each in 8 broadband filters between 0.36 and 2.2 microns (in the observed frame). We find that the overall sizes are larger by factors of 3 to 30 than predicted by the standard thin accretion disk model, and that the logarithmic slope of the wavelength-dependent size is ~ 0.2 on average, much shallower than the predicted slope of 4/3. This implies that the temperature is a steeper function of radius than the thin disk model predicts. With this new approach to determining quasar accretion disk sizes, we are thus able to rule out the standard thin disk model as the source of the (rest-frame) ultraviolet and optical continuum in these bright quasars.