Measuring Microlensing using Spectra of Multiply Lensed Quasars
  We report on a program of spectroscopic observations of
gravitationally-lensed QSOs with multiple images. We seek to establish whether
microlensing is occurring in each QSO image using only single-epoch
observations. We calculate flux ratios for the cores of emission lines in image
pairs to set a baseline for no microlensing. The offset of the continuum flux
ratios relative to this baseline yields the microlensing magnification free
from extinction, as extinction affects the continuum and the lines equally.
When we find chromatic microlensing, we attempt to constrain the size of the
QSO accretion disk. SDSSJ1004+4112 and HE1104-1805 show chromatic microlensing
with amplitudes $0.2< |\Delta m| < 0.6$ and $0.2< |\Delta m| < 0.4$ mag,
respectively. Modeling the accretion disk with a Gaussian source ($I\propto
\exp(-R^2/2r_s^2)$) of size $r_s\propto \lambda^p$ and using magnification maps
to simulate microlensing we find $r_s(\lambda 3363)=7\pm3 light-days
(18.1\pm7.8 \times 10^{15} cm$) and $p=1.1\pm 0.4$ for SDSS1004+4112, and
$r_s(\lambda 3363)=6\pm2 light-days (15.5\pm5.2 \times 10^{15} cm$) and
$p=0.7\pm0.1$ for HE1104-1805. For SDSSJ1029+2623 we find strong chromaticity
of $\sim 0.4$ mag in the continuum flux ratio, which probably arises from
microlensing although not all the available data fit within this explanation.
For Q0957+561 we measure B-A magnitude differences of 0.4 mag, much greater
than the $\sim$0.05 mag amplitude usually inferred from lightcurve variability.
It may substantially modify the current interpretations of microlensing in this
system, likely favoring the hypothesis of smaller sources and/or larger
microdeflectors. For HS0818+1227, our data yield posible evidence of
microlensing.
