Quantitative Interpretation of Quasar Microlensing Light Curves

We develop a general method for analyzing the light curves of microlensedquasars and apply it to the OGLE light curves of the four-image lensQ2237+0305. We simultaneously estimate the effective source velocity, theaverage stellar mass, the stellar mass function, and the size and structure ofthe quasar accretion disk. The light curves imply an effective source planevelocity of 10200 km/s < v_e h / sqrt() < 39600 km/s (68% confidence). Givenan independent estimate for the source velocity, found by combining estimatesfor the peculiar velocity of the lens galaxy with its measured stellar velocitydispersion, we obtain a mean stellar mass of =0.037h^2 solar masses(0.0059h^2 < /Msun < 0.20h^2). We were unable to distinguish a Salpeter massfunction from one in which all stars had the same mass, but we do find a stronglower bound of 50% on the fraction of the surface mass density represented bythe microlenses. Our models favor a standard thin accretion disk model as thesource structure over a simple Gaussian source. For a face-on, thin diskradiating as a black body with temperature profile T_s ~ R^(-3/4), the radiusr_s where the temperature matches the filter pass band (2000 Angstroms orT_s(r_s)=70000K) is (1.4 x 10^15)/h cm < r_s < (4.5 x 10^15)/h cm. The fluxpredicted by the disk model agrees with the observed flux of the quasar, sonon-thermal or optically thin emission processes are not required. From thedisk structure we estimate a black hole mass of M_BH = (1.1_(-0.7)^(+1.4) x10^9) h^(-3/2) (L/L_E)^(-1/2) solar masses, consistent with the mass estimatedunder the assumption that the quasar is radiating at the Eddington luminosity(L/L_E=1).Comment: 39 pages, 11 figures (some only in jpeg format), submitted to Ap