The Rewards of Patience: An 822 Day Time Delay in the Gravitational Lens SDSS J1004+4112

We present 107 new epochs of optical monitoring data for the four brightest images of the gravitational lens SDSS J1004+4112 observed between October 2006 and June 2007. Combining this data with the previously obtained light curves, we determine the time delays between images A, B and C. We confirm our previous measurement finding that A leads B by ∆tBA = 40.6 ± 1.8 days, and find that image C leads image A by ∆τCA = 821.6±2.1 days. The lower limit on the remaining delay is that image D lags image A by ∆τAD> 1250 days. Based on the microlensing of images A and B we estimate that the accretion disk size at a rest wavelength of 2300˚A is 10 14.8±0.3 cm for a disk inclination of cosi = 1/2, which is consistent with the microlensing disk size-black hole mass correlation function given our estimate of the black hole mass from the MgII line width of log MBH/M ⊙ = 8.44±0.14. The long delays allow us to fill in the seasonal gaps and assemble a continuous, densely sampled light curve spanning 5.7 years whose variability implies a structure function with a logarithmic slope of γ = 0.35 ± 0.02. As C is the leading image, sharp features in the C light curve can be intensively studied 2.3 years later in the A/B pair, potentially allowing detailed reverberation mapping studies of a quasar at minimal cost.