Quasar Accretion Disk Sizes From Continuum Reverberation Mapping From the Dark Energy Survey

TL;DR

This study measures accretion disk sizes of 15 quasars using continuum reverberation mapping from the Dark Energy Survey, finding results consistent with thin disk models and microlensing studies, despite large uncertainties.

Contribution

First application of continuum reverberation mapping to measure quasar accretion disk sizes at redshifts 0.7-1.9 using Dark Energy Survey data.

Findings

Disk sizes agree with thin disk model predictions for certain accretion rates.

Results are consistent with microlensing measurements showing larger disk sizes.

Many measured lags are only upper limits due to uncertainties.

Abstract

We present accretion disk size measurements for 15 luminous quasars at $0.7 \leq z \leq 1.9$ derived from $griz$ light curves from the Dark Energy Survey. We measure the disk sizes with continuum reverberation mapping using two methods, both of which are derived from the expectation that accretion disks have a radial temperature gradient and the continuum emission at a given radius is well-described by a single blackbody. In the first method we measure the relative lags between the multiband light curves, which provides the relative time lag between shorter and longer wavelength variations. From this, we are only able to constrain upper limits on disk sizes, as many are consistent with no lag the 2$\sigma$ level. The second method fits the model parameters for the canonical thin disk directly rather than solving for the individual time lags between the light curves. Our measurements demonstrate good agreement with the sizes predicted by this model for accretion rates between 0.3-1 times the Eddington rate. Given our large uncertainties, our measurements are also consistent with disk size measurements from gravitational microlensing studies of strongly lensed quasars, as well as other photometric reverberation mapping results, that find disk sizes that are a factor of a few ($\sim$3) larger than predictions.