  Observations of gravitational microlensing in multiply imaged quasars
currently provide the only direct probe of quasar emission region structure on
sub-microarcsecond scales. Analyses of microlensing variability are
observationally expensive, requiring long-term monitoring of lensed systems.
Here we demonstrate a technique for constraining the size of the quasar
continuum emission region as a function of wavelength using single-epoch
multi-band imaging. We have obtained images of the lensed quasar MG 0414+0534
in five wavelength bands using the Magellan 6.5-metre Baade telescope at Las
Campanas Observatory, Chile. These data, in combination with two existing
epochs of Hubble Space Telescope data, are used to model the size of the
continuum emission region $\sigma$ as a power-law in wavelength,
$\sigma\propto\lambda^\nu$. We place an upper limit on the Gaussian width of
the $r^\prime$-band emission region of $1.80 \times 10^{16} h_{70}^{-1/2}
(<M>/\rmn{M}_{\odot})^{1/2}$cm, and constrain the power-law index to
$0.77\leq\nu\leq2.67$ (95 per cent confidence range). These results can be used
to constrain models of quasar accretion discs. As a example, we find that the
accretion disc in MG 0414+0534 is statistically consistent with a
Shakura-Sunyaev thin disc model.

Astrophysics