We present and analyse new R-band frames of the gravitationally lensed double quasar FBQ 0951+2635. These images were obtained with the 1.5m AZT-22 Telescope at Maidanak (Uzbekistan) in the 2001-2006 period. Previous results in the R band (1999-2001
period) and the new data allow us to discuss the dominant kind of microlensing variability in FBQ 0951+2635. The time evolution of the flux ratio A/B does not favour the continuous production of short-timescale (months) flares in the faintest quasar component B (crossing the central region of the lensing galaxy). Instead of a rapid variability scenario, the observations are consistent with the existence of a long-timescale fluctuation. The flux ratio shows a bump in the 2003-2004 period and a quasi-flat trend in more recent epochs. Apart from the global behaviour of A/B, we study the intra-year variability over the first semester of 2004, which is reasonably well sampled. Short-timescale microlensing is not detected in that period. Additional data in the i band (from new i-band images taken in 2007 with the 2m Liverpool Robotic Telescope at La Palma, Canary Islands) also indicate the absence of short-timescale events in 2007.
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 intr
insic variability in Q0957+561. 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. 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 of 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 of 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. 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 about 100 mmag and timescales of about 100 d) are very probably due to reverberation within the gas disc around the supermassive black hole.
To go into the details about the variability of the double quasar SBS 0909+532, we designed a monitoring programme with the 2 m Liverpool Robotic Telescope in the r Sloan filter, spanning 1.5 years from 2005 January to 2006 June. The r-band light cur
ves of the A and B components, several cross-correlation techniques and a large number of simulations (synthetic light curves) lead to a robust delay of 49 +/- 6 days (1-sigma interval) that agrees with our previous results (the B component is leading). Once the time delay and the magnitude offset are known, the magnitude- and time-shifted light curve of image A is subtracted from the light curve of image B. This difference light curve of SBS 0909+532 is consistent with zero, so any possible extrinsic signal must be very weak, i.e., the observed variability in A and B is basically due to observational noise and intrinsic signal. We then make the combined light curve and analyse its statistical properties (structure functions). The structure function of the intrinsic luminosity is fitted to predictions of simple models of two physical scenarios: accretion disc instabilities and nuclear starbursts. Although no simple model is able to accurately reproduce the observed trend, symmetric triangular flares in an accretion disc seems to be the best option to account for it.
