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 } ( \langle M \rangle / \mathrm { M } _ { \odot } ) ^ { 1 / 2 } cm , and constrain the power-law index to 0.77 \leq \nu \leq 2.67 ( 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 .