The Spatial Structure of an Accretion Disk

Abstract

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 propto 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 × 10¹⁵ 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 × 10¹⁵ 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.