A 5.5-year robotic optical monitoring of Q0957+561: Substructure in a non-local cD galaxy

Abstract

New light curves of the gravitationally lensed double quasar Q0957+561 in the gr bands during 2008-2010 include densely sampled, sharp intrinsic fluctuations with unprecedentedly high signal-to-noise ratio. These relatively violent flux variations allow us to very accurately measure the g-band and r-band time delays between the two quasar images A and B. Using correlation functions, we obtain that the two time delays are inconsistent with each other at the 2σ level, with the r-band delay exceeding the 417-day delay in the g band by about 3 days. We also studied the long-term evolution of the delay-corrected flux ratio B/A from our homogeneous two-band monitoring with the Liverpool Robotic Telescope between 2005 and 2010. This ratio B/A slightly increases in periods of violent activity, which seems to be correlated with the flux level in these periods. The presence of the previously reported dense cloud within the cD lensing galaxy, along the line of sight to the A image, could account for the observed time delay and flux ratio anomalies