The accretion disc in the quasar SDSS J0924+0219☆

We present single-epoch multiwavelength optical-near-infrared observations of the 'anomalous' lensed quasar SDSS J0924+0219, made using the Magellan 6.5-m Baade Telescope at Las Campanas Observatory, Chile. The data clearly resolve the anomalous bright image pair in the lensed system, and exhibit a strong decrease in the anomalous flux ratio with decreasing wavelength. This is interpreted as a result of microlensing of a source of decreasing size in the core of the lensed quasar. We model the radius of the continuum emission region, Sigma, as a power law in wavelength, Sigma proportional to lambda(zeta). We place an upper limit on the Gaussian radius of the u'-band emission region of 3.04 x 10(16) h(70)(-1/2) (<< M >>/M-circle dot)(1/2) cm, and constrain the size-wavelength power-law index to zeta < 1.34 at 95 per cent confidence. These observations rule out an alpha-disc prescription for the accretion disc in SDSS J0924+0219 with 94 per cent confidence.