EDP Sciences
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Volume 420, Number 2, June III 2004
Page(s) 447 - 457
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20034104

A&A 420, 447-457 (2004)
DOI: 10.1051/0004-6361:20034104

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

V. G. Vakulik1, R. E. Schild2, V. N. Dudinov1, A. A. Minakov3, S. N. Nuritdinov4, V. S. Tsvetkova3, A. P. Zheleznyak1, V. V. Konichek1, I. Ye. Sinelnikov1, O. A. Burkhonov4, B. P. Artamonov5 and V. V. Bruevich5

1  Institute of Astronomy of Kharkov National University, Sumskaya 35, 61022 Kharkov, Ukraine
2  Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
    e-mail: rschild@cfa.harvard.edu
3  Institute of Radio Astronomy of Nat. Ac. Sci. of Ukraine, Chervonoznamennaya 4, 61002 Kharkov, Ukraine
    e-mail: minakov@ira.kharkov.ua
4  Ulugh Beg Astronomical Institute of Ac. Sci. of Uzbekistan, Astronomicheskaya 33, 700052, Tashkent, Republic of Uzbekistan
    e-mail: nurit@astrin.uzsci.net
5  Sternberg Astronomical Institute, Universitetski Av. 13, 119899 Moscow, Russia
    e-mail: artamon@sai.msu.ru

(Received 23 July 2003 / Accepted 9 February 2004)

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  $0.3^{\rm m}$ to  $0.12^{\rm m}$ since August 1998, and from  $0.56^{\rm m}$ to  $0.12^{\rm m}$ 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  $0.47^{\rm m}$ brighter in  R and  $0.15^{\rm m}$ 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  $0.08^{\rm m}$ fading of B and C components, and  $0.05^{\rm m}$ 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  $\Delta
(V-I)$ vs.  $\Delta R$, with a regression line slope of  $0.33\pm0.08$ 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  $1.31\pm0.14.$ 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.

Key words: cosmology: gravitational lensing -- galaxies: quasars: individual: QSO 2237+0305 -- methods: observational -- techniques: image processing

Offprint request: V.Vakulik, vakulik@astron.kharkov.ua

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