VRI photometry of the Einstein Cross Q2237+0305 at Maidanak observatory

Abstract

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)

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

“…No regular multi-filter monitoring of Q2237+0305 is reported between 1996 and 1999, excepting our results of VRI photometry for three nights on 17−19 September 1995, (Vakulik et al 1997), and the similar results by Burud et al (1998), obtained with the Nordic Optical Telescope for a nearby epoch, 10−11 October 1995. No night-to-night or intranight brightness variations of the four components was found for these time periods, while a noticeable change in the component B color as compared to the observations by Yee in 1987Yee in (1988 was reported in both studies.…”

“…There is also photometry in the V and I bands for 17 dates in 1995−2000 (Table 6). Some results were presented in our previous publications (Vakulik et al 1997;Bliokh et al 1999;Dudinov et al 2000a,b). We present here the results of all our observations, including those which have never been published.…”

“…Meanwhile, Vakulik et al (1997) and Burud et al (1998) reported that the B component became the bluest in the system in 1995, as compared to observations by Yee (1988).…”

“…The difficulties inherent in accurate photometry of groundbased images of Q2237+0305, have been noted by many authors (Burud et al 1998;Corrigan et al 1991;Vakulik et al 1997;Yee 1988). They are due mainly to its extremely compact spatial structure, with the wings of the quasar images overlapping even under good seeing conditions.…”

Optical monitoring of the gravitationally lensed quasar Q2237+0305 from APO between June 1995 and January 1998

“…For image B, we used m r = m V − 0.01 in order to roughly account for the colour difference between the two quasar images. This value was determined using the R-band results from Vakulik et al (1997) Overall there is rather good agreement between our data and these independent observations. In 1995, our results seem to favour a slightly fainter image B.…”