Program: GN-2020A-Q-105

We propose to conclude in 2020A (+B) our reverberation mapping campaign to measure the size and structure of the broad emission line region and thus the black hole mass in a quasar at redshift 2.82. The quasar is lensed by an intervening galaxy cluster into six images, with three signficantly magnified outer images, for which accurate time delays and magnifications have now been established in part using Gemini photometry. The three images have undergone extreme variability in the past 5 years, increasing and then decreasing in brightness by a magnitude in g-band, and the leading image shows that the two trailing images will continue this behavior over the next year. We have monitored the three images for the last 3 1/2 years and verified that the CIV emission line is responding to the continuum variations as expected. The cross-correlation function is double-peaked, indicating a lag of either 80 or 280 days. Simulations show that one additional year of data will provide enough information to give us a conclusive lag measurement.