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 .