ACCEPTED IN APJ Preprint typeset using LATEX style emulateapj v. 11/26/03 DIFFERENTIAL MICROLENSING OF THE CONTINUUM AND BROAD EMISSION LINES IN SDSS J0924+0219, THE MOST ANOMALOUS LENSED QUASAR ∗

SDSSJ092455.87+021924.9isthemostglaringexampleofagravitationallenswithanomalousfluxratios;optical broadband photometry shows image D to be a factor of 12 fainter than expected for smooth lens potentials. We report spectroscopy showing that the anomaly is present in the broad emission line flux ratios as well. There are differences between the emission-line and continuumflux ratios; the image A/image D ratio is 10 in the broad Lyline and 19 in theassociatedcontinuum. Knownvariabilityarguesforthepresenceof microlensing.Weshowthat microlensingcan accountforboththecontinuumandemission-linefluxratios,ifthebroademissionlineregioniscomparableinsizeto the Einstein radii of the microlenses. Specifically, we need the half-light radius of the broad-line region to be RBLR P 0:4RE � 9 lt-days, which is small but reasonable. If the broad-line region is that large, then stars can contribute only 15%-20% of the surface mass density at the positions of the images. While we cannot exclude the possibility that millilensing by dark matter substructure is also present, we conclude that microlensing is present and sufficient to explainexistingdata.Underthishypothesis,theA/Dfluxratioshouldreturntoavalueclosetounityonatimescaleof years rather than millennia. Subject headingg cosmology: theory — gravitational lensing — quasars: individual (SDSS J0924+0219) Online material: color figures