The spatial structure of an accretion disk

Based on the microlensing variability of the two-image gravitational lens HE 1104-1805 observed between 0.4 and 8 mu m, we have measured the size and wavelength-dependent structure of the quasar accretion disk. Modeled as a power law in temperature, T alpha R-beta, we measure a B-band (0.13 mu m in the rest frame) half-light radius of R-1/2,R-B = 6.7(-3.2)(+6.2) x 10(15) cm (68% confidence level) and a logarithmic slope of beta = 0.61(-0.17)(+0.21)(68% confidence level) for our standard model with a logarithmic prior on the disk size. Both the scale and the slope are consistent with simple thin disk models where beta = 3/4 and R-1/2,R-B = 5.9 x 10(15) cm for a Shakura-Sunyaev disk radiating at the Eddington limit with 10% efficiency. The observed fluxes favor a slightly shallower slope, beta = 0.55(-0.02)(+0.03), and a significantly smaller size for beta = 3/4.