QSO size ratios from multiband monitoring of a microlensing   high-magnification event
  We introduce a new scheme to study the nature of the central engine in a lensed QSO. The compact emission regions could have different sizes in different optical wavelengths, and our framework permits to obtain the source size ratios when a microlensing special high-magnification event (e.g., a caustic crossing event, a two-dimensional maximum crossing event and so on) is produced in one of the QSO components. To infer the source size ratios, only cross-correlations between the brightness records in different optical bands are required. While the deconvolution method leads to a richer information (1D intrinsic luminosity profiles), the new approach is free of the technical problems with complex inversion procedures. Using simulations related to recent VR data of Q2237+0305A, we discuss the ability of the scheme in the determination of the visible-to-red ratio q = $R_V/R_R$. We conclude that extremely accurate fluxes (with a few microJy uncertainties, or equivalently, a few milli-magnitudes errors) can lead to ~10% measurements of q. Taking into account the errors in the fluxes of Q2237+0305A from a normal ground-based telescope, ~10 microJy (~10 mmag), it must be possible the achievement of smaller errors from the current superb-telescopes, and thus, an accurate determination of q. Obviously, to measure the visible-to-red ratio, the light curves cannot be contaminated by an intrinsic event or an important high-frequency intrinsic signal, i.e., exceeding the microJy (mmag) level. For an arbitrary lensed QSO, we finally remark that the framework seems to work better with very fast microlensing events. 
L. J. Goicoechea, V. Shalyapin, J. Gonzalez-Cadelo, A. Oscoz