* Astronomy

Title: A 5.5-year robotic optical monitoring of Q0957+561: substructure in a non-local cD galaxy
Authors: V. N. Shalyapin, L. J. Goicoechea, R. Gil-Merino (GLENDAMA team)

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.

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Title: New two-colour light curves of Q0957+561: time delays and the origin of intrinsic variations
Authors:  V. N. Shalyapin, L. J. Goicoechea, E. Koptelova, A. Ullán, and R. Gil-Merino

Aims. We extend the gr-band time coverage of the gravitationally lensed double quasar Q0957+561. New gr light curves permit us to detect significant intrinsic fluctuations, to determine new time delays, and thus to gain perspective on the mechanism of intrinsic variability in Q0957+561.
Methods. We use new optical frames of Q0957+561 in the g and r passbands from January 2005 to July 2007. These frames are part of an ongoing long-term monitoring with the Liverpool robotic telescope. We also introduce two photometric pipelines that are applied to the new gr frames of Q0957+561. The transformation pipeline incorporates zero-point, colour, and inhomogeneity corrections to the instrumental magnitudes, so final photometry to the 1- 2% level is achieved for both quasar components. The two-colour final records are then used to measure time delays.
Results. The gr light curves of Q0957+561 show several prominent events and gradients, and some of them (in the g band) lead to a time delay between components \Delta t_ = 417 ± 2 d (1\sigma). We do not find evidence of extrinsic variability in the light curves of Q0957+561. We also explore the possibility of a delay between a large event in the g band and the corresponding event in the r band. The gr cross-correlation reveals a time lag \Delta t_{rg} = 4.0 ±2.0 d (1\sigma; the g-band event is leading) that confirms a previous claim of the existence of a delay between the g and r band in this lensed quasar.
Conclusions. The time delays (between quasar components and between optical bands) from the new records and previous ones in similar bands indicate that most observed variations in Q0957+561 (amplitudes of ~100 mmag and timescales of ~100 d) are very probably due to reverberation within the gas disc around the supermassive black hole.

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