Constraining Black Hole Accretion Disks by Detecting Time Lags
Between
 Quasar Continuum Emission Bands

Yan-Fei Jiang, University of California, Santa Barbara

Black hole accretion disk that is responsible for the optical/ultraviolet

continuum emission from quasars is usually too small to be resolved
directly.
 As photons with different wavelengths emerge from different
radial ranges in the accretion disk, time lags between different continuum
bands can constrain the sizes of the accretion disks. I will describe the
successful detection of such time lags for a sample of 240 quasars 
based
on more than four years of multi-band (g,r,i,z) light-curve in the
Pan-STARRS Medium Deep fields. We detect typical lags of several days in
the rest frame between the g band and the r,i,z bands. The detected lags
are ~2 to 3 times larger than the light crossing time estimated from the
standard thin disk model, consistent with the recently measured lag in
NGC5548 and micro-lensing measurements of quasars. The lags in our sample
are found to increase with increasing luminosity. Furthermore, the
increase in lags going from g-r to g-i and then to g-z is slower than
predicted in the thin disk model, particularly for high luminosity
quasars. The radial temperature profile in the disk must be different from
what is assumed. We also find evidence that the lags decrease with
increasing line ratios between ultraviolet Fe II lines and Mg II, which
may point to changes in the accretion disk structure at higher
metallicity. I will also briefly show some recent results of global three
dimensional radiation MHD simulations of black hole accretion disks in
quasars including the iron
 opacity peak self-consistently, which are
promising to understand the time lags we have found. By carrying out
similar experiment with LSST data, together with the simulations, we will
be able to significantly improve our understanding of black hole accretion
disks in quasars and go beyond the standard thin disk model.