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.

We present new brightness monitoring observations of the 4 components of gravitationally lensed system Q2237+0305, which show detection of an intrinsic quasar brightness fluctuation at a time of subdued microlensing activity, between June 27 and October 12, 2003. These data were used to determine the time delays between the arrivals of the four images. The measured delays are τ BA ≈ −6, τ CA ≈ 35, and τ DA ≈ 2 h, so they confirm that the long history of brightness monitoring has produced significant detection of microlensing. However the error bars associated with the delays, of order 2 days, are too large to discriminate between competing macro-imaging models. Moreover, our simulations show that for the amplitude of this intrinsic fluctuation and for photometric errors intrinsic to optical monitoring from our 1.5-m telescope or from the OGLE monitoring, a daily sampled brightness record cannot produce reliable lags for model discrimination. We use our simulations to devise a strategy for future delay determination with optical data. Nevertheless, we regard these first estimates to be significant, since they are the first direct measurements of time delays made for this system from ground-based observations in the visual wavelengths. The detected highly correlated fluctuations of the four quasar images provide an extra confirmation of the gravitational-lens nature of Q2237+0305, and give observational justification to the extensive literature which attributes the quasar's previously observed brightness fluctuations to microlensing.

We report on an observing campaign in March 2001 to monitor the brightness of the later arriving Q0957+561 B image in order to compare with the previously published brightness observations of the (first arriving) A image. The 12 participating observatories provided 3543 image frames which we have analyzed for brightness fluctuations. From our classical methods for time delay determination, we find a 417.09 ± 0.07 day time delay which should be free of effects due to incomplete sampling. During the campaign period, the quasar brightness was relatively constant and only small fluctuations were found; we compare the struc--3ture function for the new data with structure function estimates for the 1995-6 epoch, and show that the structure function is statistically non-stationary. We also examine the data for any evidence of correlated fluctuations at zero lag. We discuss the limits to our ability to measure the cosmological time delay if the quasar's emitting surface is time resolved, as seems likely.

Assuming a two-component quasar structure model consisting of a central compact source and an extended outer feature, we produce microlensing simulations for a population of compact masses in the lensing galaxy of Q2237+0305. Such a model is a simplified version of that adopted to explain the brightness variations observed in Q0957. The microlensing light curves generated for a range of source parameters were compared to the light curves obtained in the framework of the Optical Gravitational Lensing Experiment program. With a large number of trials, we built, in the domain of the source structure parameters, probability distributions to find 'good' realizations of light curves. The values of the source parameters which provide the maximum of the joint probability distribution calculated for all the image components have been accepted as estimates for the source structure parameters. The results favour the twocomponent model of the quasar brightness structure over a single compact central source model, and in general the simulations confirm the Schild-Vakulik model that previously described successfully the microlensing and other properties of Q0957. Adopting 3300 km s −1 for the transverse velocity of the source, the effective size of the central source was determined to be about 2 × 10 15 cm, and ε ≈ 2 was obtained for the ratio of the integral luminosity of the outer feature to that of the central source.

Observations of tlie gravitationally lensed quasar Q?737+0305 were made with the 1.5-meter AZT-22 telescope a t Maidanak (Uzbekistan) on 17-19 September 1995. All four components of the quasar are clearly resolved. T h e results of photometric measurements of the components are presented. It is confirmed that the component A again became the brightest one in the system. A noticeable change of t,he mutual colours of the components A and B in comparison with previous observations is detected and discussed. We cannot decide, on the basis of existing observational data, whether varying extinction or "chromatic" microlensing is the cause. Stronger, correlated colour changes i n the system are probably caused by quasar variability. T h e dominant time scale of the microlensing-governed brightness variations since the discovery is of the order of 10 years. A correlation between radio brightness and optical coloiir is found but cannot be explained.Key words; gravitational lensing -quasars: Q7237+0305 -techniques: photometric A A A subject classification: 160; 161 IiitrotliictionThe object Q2237+0305 discovered in 1985 by FIuchra et al.( 1 9 8~4 , is a quasar (to = 1.695), gravitationally lensed by a nearby galaxy (zg = 0.039). Because of the small tlist.ance to tlie lensing galaxy, the object. is very promising for investigations of gravitat.ioiial lens phenomena. A t the same time, the geometry of the object (four star-like quasar components in a circle of approximately 1.8" in diameter around the centre of the lensing galaxy), makes it rather difficult for observations. Nevertheless, a large amount of observational data on the object is accumulated by now, as well as a variety of theoretical investigations.Coordinates of the four quasar components, measiiretl from ground-based observations (Yee 1988;Racine 1991), are in excellent agreement with Ifubble Space Telescope dat.a (Crane et al. 1991;Rix et al. 1992). A number of successful models of the system (Kent & Falco, 3988; Schtieider et al. 1988, Wambsganss and Paczynski 1994) allowed to obtain estimates of the rnass distribution in the lensiiig galaxy and to predict quite well the observed locations and, to a lesser degree, the mutual briglit,iiesses of the quasar cotnponents. Yee (1988), in his pioiieering work, discovered colour difTcrences of the components and explained them by different amounts of extinction in the lensing galaxy. Observations in IR (Nadeau et al. 1991)