A 5.5-year robotic optical monitoring of Q0957+561: substructure in a
  non-local cD galaxy
  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 2sigma
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.
