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 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.
Highlights
The optical continuum variability of the gravitationally lensed double quasar Q0957+561 at redshift z = 1.41 has been widely studied since its discovery by Walsh et al (1979)
Several monitoring campaigns focused on the determination of the time delay between the two quasar images A and B (e.g., Vanderriest et al 1989; Kundicet al. 1997; Serra-Ricart et al 1999), where a major breakthrough occurred in Kundicet al. (1997), who used Apache Point Observatory (APO) data
While we obtain a delay near to 417 d in the g band, there is an extra delay of about three days in the r band
Summary
The 1.5-year monitoring programme with the APO 3.5 m telescope led to an accurate time delay ΔtBA = 417 ± 3 d (2σ confidence interval; A leading) in the g band. Using the APO light curves for the two quasar images, Collier (2001) found that the r-band main fluctuations lag with respect to those in the g-band by. Not suffer significant dust extinction, the optical continuum light of the A image is affected by a dense dusty cloud inside the cD galaxy (Goicoechea et al 2005a,b) This differential extinction produces a chromatic flux ratio, whose R-band value was basically constant from 1987 through 2000 (e.g., see Fig. 3 of Oscoz et al 2002).
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