HST imaging of four gravitationally lensed quasars
  We present new HST WFPC3 imaging of four gravitationally lensed quasars: MG
0414+0534; RXJ 0911+0551; B 1422+231; WFI J2026-4536. In three of these systems
we detect wavelength-dependent microlensing, which we use to place constraints
on the sizes and temperature profiles of the accretion discs in each quasar.
Accretion disc radius is assumed to vary with wavelength according to the
power-law relationship $r\propto \lambda^p$, equivalent to a radial temperature
profile of $T\propto r^{-1/p}$. The goal of this work is to search for
deviations from standard thin disc theory, which predicts that radius goes as
wavelength to the power $p=4/3$. We find a wide range of power-law indices,
from $p=1.4^{+0.5}_{-0.4}$ in B 1422+231 to $p=2.3^{+0.5}_{-0.4}$ in WFI
J2026-4536. The measured value of $p$ appears to correlate with the strength of
the wavelength-dependent microlensing. We explore this issue with mock
simulations using a fixed accretion disc with $p=1.5$, and find that cases
where wavelength-dependent microlensing is small tend to under-estimate the
value of $p$. This casts doubt on previous ensemble single-epoch measurements
which have favoured low values using samples of lensed quasars that display
only moderate chromatic effects. Using only our systems with strong chromatic
microlensing we prefer $p>4/3$, corresponding to shallower temperature profiles
than expected from standard thin disc theory.
