THE STRUCTURE OF THE ACCRETION DISK IN THE LENSED QUASAR SBS 0909+532

Abstract

We derive the size and temperature profile of the accretion disk of the lensed quasar SBS 0909+532 by measuring the wavelength dependence (chromaticity) of the microlensing magnification produced by the stars in the lens galaxy. After correcting for extinction using the flux ratios of 14 emission lines, we observe a marked change in the B-A flux ratio with wavelength, varying from –0.67 ± 0.05 mag at (rest frame) ~1460 A to –0.24 ± 0.07 mag at ~6560 A. For λ 7000 A both effects, extinction and microlensing, look minimal. Simulations indicate that image B rather than A is strongly microlensed. If we model the change in disk size from 1460 A to 6560 A using a Gaussian source (I ∝ exp(–R 2/2r 2 s )) with a disk size scaling with wavelength as rs ∝ λ p , we find rs = 7+5 –3 light-days at 1460 A and p = 0.9+0.6 –0.3 for uniform priors on rs and p, and rs = 4+3 –3 light-days and p = 1.0+0.6 –0.4 for a logarithmic prior on rs . The disk temperature profile T ∝ R –1/p is consistent with thin disk theory (T ∝ R –3/4), given the uncertainties. The estimates of rs are also in agreement with the size inferred from thin disk theory using the estimated black hole mass (M BH 2 × 109 M ☉) but not with the smaller size estimated from thin disk theory and the optical flux. We also use the flux ratios of the unmicrolensed emission lines to determine the extinction curve of the dust in the lens galaxy, finding that it is similar to that of the LMC2 Supershell.