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

TL;DR

This study measures long time delays in a gravitational lens system, confirming previous results and providing new insights into the lensing galaxy, quasar structure, and microlensing effects over a 5.7-year period.

Contribution

It presents the longest measured delay of 822 days in a gravitational lens and combines extensive data to analyze quasar microlensing and accretion disk size.

Findings

Confirmed the 40.6-day delay between images A and B.

Measured an 821.6-day delay from image C to A.

Estimated the quasar's accretion disk size and black hole mass.

Abstract

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 dt_BA=40.6+-1.8 days, and find that image C leads image A by dt_CA=821.6+-2.1 days. The lower limit on the remaining delay is that image D lags image A by dt_AD>1250 days. Based on the microlensing of images A and B we estimate that the accretion disk size at a rest wavelength of 2300 angstrom is 10^{14.8+-0.3} cm for a disk inclination of cos{i}=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 logM_BH/M_sun=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 gamma = 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.