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 μm, we have measured the size and wavelength-dependent structure of the quasar accretion disk. Modeled as a power law in temperature, T ∝ R −β , we measure a B -band (0.13 μm in the rest frame) half-light radius of R 1/2,B = 6.7 + 6.2 −3.2 × 10 15 cm (68% confidence level) and a logarithmic slope of β = 0.61 + 0.21 −0.17 (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 β = 3/4 and R 1/2,B = 5.9 × 10 15 cm for a Shakura-Sunyaev disk radiating at the Eddington limit with 10% efficiency. The observed fluxes favor a slightly shallower slope, β = 0.55 + 0.03 −0.02 , and a significantly smaller size for β = 3/4.