Aims. We study brightness variations in the double lensed quasar UM673 (Q0142-100) with the aim of measuring the time delay between its two images. Methods. We combined our previously published observational data of UM673 obtained during the 2003-2005 seasons at the Maidanak Observatory with archival and recently observed Maidanak and CTIO UM673 data. We analyzed the V, R and I-band light curves of the A and B images of UM673, which cover ten observational seasons from August 2001 to November 2010. We also analyzed the time evolution of the difference in magnitudes (flux ratio) between images A and B of UM673 over more than ten years. Results. We find that the quasar exhibits both short-term (with an amplitude of ∼0.1 mag in the R band) and long-term (with an amplitude of ∼0.3 mag) variability on timescales of about several months and several years, respectively. These brightness variations are used to constrain the time delay between the images of UM673. From a cross-correlation analysis of the A and B quasar light curves and an error analysis we measure a mean time delay of 89 days with an rms error of 11 days. Given the input time delay of 88 days, the most probable value of the delay that can be recovered from light curves with the same statistical properties as the observed R-band light curves of UM673, is 95 +5 −16 +14−29 days (68% and 95% confidence intervals). Analysis of the V − I color variations and the V, R and I-band magnitude differences of the quasar images does not show clear evidence for microlensing variations between 1998 and 2010.

Abstract. We present photometry of the Q2237+0305 gravitational lens system in VRI spectral bands with the 1.5-m telescope of the high-altitude Maidanak observatory in 1995−2000. The time interval includes the epoch of the dramatic brightness peaks discovered previously in the A and C image components (Wozniak et al. 2000a,b). By good luck three nights of observation in 1999 were almost at the time of the strong brightness peak of image C, and approximately in the middle of the ascending slope of the brightness peak of image A. Having reached its brightness maximum at the very end of June 1999, the C component had changed its (V − I) color from 0.3 m to 0.12 m since August 1998, and from 0.56 m to 0.12 m since August 1997. It was the bluest component in the system in 1998 and 1999, but by October 2000 that was no longer the case. We do not know the color of the A component exactly at its brightness peak, but we do know that it became 0.47 m brighter in R and 0.15 m bluer in (V − R) between August 1998 and August 2000, about three months before the peak. More intensive monitoring of Q2237+0305 in July-October 2000, made on a nearly daily basis, did not reveal rapid (night-to-night and intranight) brightness variations of the components during this time period, exceeding the photometry error bars. Rather slow changes of magnitudes of the components were observed, in particular, nearly synchronous 0.08 m fading of B and C components, and 0.05 m brightening of D in the R band during July 23−October 7, 2000, while the B component had become the faintest in all filters by the end of this time period. The behavior of the colors of the components was analyzed on the basis of all our VRI observations, made in 1995−2000 on Maidanak. A qualitative tendency of the components to become bluer as their brightness increases, noted in our previous works, was confirmed quantitatively. A correlation between the color variations and variations of magnitudes of the components is demonstrated to be significant and reaches 0.75 for ∆(V − I) vs. ∆R, with a regression line slope of 0.33± 0.08 for these quantities. A plot of (V − I) vs. (V − R) shows the components settled in a cluster, stretchng along a line with a slope of 1.31 ± 0.14. Both slopes are noticeably less steep than those expected if a standard galactic interstellar reddening law were responsible for the differences between the colors of images and their variations over time. We attribute the brightness and color changes to microlensing of the quasar's structure, which we conclude to be more compact at shorter wavelengths, as predicted by most quasar models featuring an energizing central source.

Aims. The time delays between the components of a lensed quasar are basic tools for analysing the expansion of the Universe and the structure of the main lens galaxy halo. In this paper, we focus on the variability and time delay of the double system SBS 0909+532A,B as well as the time behaviour of the field stars. Methods. We use VR optical observations of SBS 0909+532A, B and the field stars in 2003. The frames were taken at Calar Alto, Maidanak, and Wise observatories, and the VR light curves of the field stars and quasar components were derived from aperture and point-spread function fitting methods. We measured the R-band time delay of the system from the χ 2 and dispersion techniques and 1000 synthetic light curves based on the observed records. Results. One nearby field star (SBS 0909+532c) was found to be variable, and the other two nearby field stars are non-variable sources. With respect to the quasar components, the R-band records seem more reliable and are more densely populated than the V-band ones. The observed R-band fluctuations permit a pre-conditioned measurement of the time delay. From the χ 2 minimization, if we assume that the quasar emission is observed first in B and afterwards in A (in agreement with basic observations of the system and the corresponding predictions), we obtain ∆τ BA = −45 +1 −11 days (95% confidence interval). The dispersion technique leads to a similar delay range. A by-product of the analysis is the determination of a totally corrected flux ratio in the R band (corrected by the time delay and the contamination due to the galaxy light). Our 95% measurement ∆m BA = m B (t + ∆τ BA ) − m A (t) = 0.575 ± 0.014 mag is in excellent agreement with previous results from contaminated fluxes at the same time of observation.

We have observed the gravitational lens system Q2237+0305 from the Maidanak Observatory over the period from 2002 August to 2003 November. Here we report the results of our observations. We implemented a two‐stage technique that has been developed specifically for the purpose of gravitational lens image reconstruction. The technique is based on the Tikhonov regularization approach and allows one to obtain astrometric and photometric characteristics of the gravitational lens system. Light curves with 78 data points for the four quasar components are obtained. Slow brightness variations over the observational period are found in all components. Images A, C and D have a tendency to decrease in brightness. Image B does not vary more than 0.05 mag. The observations did not reveal evidence for large variations in brightness of the components due to microlensing effects. To provide an overall picture of the photometry behaviour, our data are combined with the Maidanak observations published for 1995–2000.

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