Color effects associated with the 1999 microlensing brightness peaks in gravitationally lensed quasar Q2237+0305

Abstract

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.

Accurate photometric light curves of the lensed components of Q2237+0305 derived with an optimal image subtraction technique: Evidence for microlensing in image A

“…The trends of the V − R colour observed for the lensed components A, B, C and D as a function of V (see Fig. 8) are reminiscent of those reported over longer time scales by Vakulik et al (2004). These authors also interpret the observed colour trends as being caused by microlensing effects.…”

Observational determination of the time delays in gravitational lens system Q2237+0305

“…-since variability of quasars tends to increase towards the shorter wavelength, that is found for a sample of quasars by de Vries et al (2005), and also detected for the Q2237 system, (Vakulik et al 2004), observations should be shifted as much as possible to shorter optical wavelengths; -monitoring should be in sets with durations of several weeks and sampling intervals of hours; -observations should be made under the image quality of 0.6−0.7 and better, and with the photometry accuracy of 0.005 m and higher.…”

“…And nightly monitoring by Vakulik et al (2004) for 46 nights in 2000 with the R filter precludes fluctuations of more than 1%.…”

PG 1115+080: variations of the A2/A1 flux ratio and new values of the time delays

“…Our algorithm for photometric image processing is similar to that applied to Q2237+0305 and described in detail by Vakulik et al (2004). The light curves of PG 1115+080A1,A2,B,C in filter R for the time period from 2001 April to 2006 June are shown in Fig.…”

“…We have also made use of our multicolour observations to analyse behaviours of the V − I colour indices of image components, which were shown to be indicative of the microlensing nature of brightness changes for at least the Q2237+0305 quasar (Vakulik et al 2004). Since only 16 data points were available to build the V − I versus R dependency for each image component, we did not build them for the components separately, but combined the data into two sets, for A1+A2 and B+C image pairs.…”