Department of Astrophysics

A bit of history

From 1975 to 1995, the scientific interests of the department staff were focused on the problems of high-resolution imaging in ground-based observations: speckle-interferometry of red giants and double stars with the 6-m BTA telescope in 1977-1982, restoration of long-exposure images of faint objects with the telescopes of the high-altitude Maidanak Observatory in 1981- 1995. Since 1995, the main efforts of the staff are dedicated to investigation of the gravitational lensing phenomenon. In this year, the first successful CCD image of the gravitationally lensed quasar Q2237+0305 (the Einstein Cross) was obtained with the 1.5-m AZT-22 telescope of the Maidanak Observatory.


In the subsequent years, the observations of gravitationally lensed quasars were initiated and actively supported by Prof. P.Bliokh and Prof. A.Minakov (Institute of Radio Astronomy of the National Ac.Sci. of Ukraine), who are the pioneers of gravitational lensing studies in the former USSR. Their monograph “Gravitational lensing” is the first book on this subject published in Russian. In fact, they established collaboration between the two institutions, which turned out to be stable and fruitful. Our joint team has been awarded with the CRDF (1997-1998) and STCU (2005-2006) grants, which has enabled us to collect observational data of extremely good quality using the Maidanak 1.5-m telescope. The work on these two projects has been fulfilled in collaboration with the Princeton University, (U.S.A.), Harvard-Smithsonian Center for Astrophysics, (U.S.A.), Ulugh-Beg Astronomical Institute (Republic of Uzbekistan), and Sternberg Astronomical Institute (Russia).

Scope of activity

The phenomenon of gravitational lensing (GL) is known to result in forming multiple images of a distant source, e.g. high-redshift quasars. It occurs when a sufficiently massive (e.g. foreground galaxy) happens to be close enough to the line of sight between the quasar and the observer.

Gravitationally lensed quasars are variable objects, with two different sources of variability. The intrinsic quasar variability is responsible for the constituent of variations of individual lensed images, which are the same for all the images, but are shifted in time due to the differences in light paths corresponding to specific lensed images. The other source of variability is due to microlensing events, which occurs when a compact object (say, a star or planet populating a lensing galaxy) passes close to the line of sight of a specific image. The microlensing events disturb the quasar light curve differently for different lensed images. Monitoring of variability in gravitationally lensed quasars is an important tool for solving a set of topical problems of the current astrophysics and cosmology. It allows, in particular:

Principal results and publications

Time delays and the Hubble constant

A very important application of GL is a possibility to determine the value of the Hubble constant which is a measure of the expansion rate of the Universe. If the geometry of GLQ components is known and if one has a good knowledge on the mass distribution of the deflector, an estimate of the absolute scale of the system and thereby the Hubble constant can be obtained from measurements of the time delay differences between the quasar intrinsic brightness variations seen in pairs of quasar images.

Thus, when the time delay difference is measured, the value of H0 can be obtained:

1) within the adopted cosmology,
2) from the observed system geometry,
3) with the known lens and source redshifts, and
4) with the use of physically substantiated model for mass distribution in the lensing galaxy.

Items 2) and 3) are observables that are usually measured accurately enough, while a choice of a cosmological model is in fact a question of agreement. Therefore, the errors of the time delays determination and uncertainties in selection of lens models mostly contribute to estimation of the Hubble constant values.

The problem of determining the Hubble constant from the time delay lenses is known to suffer from the so-called central concentration degeneracy, which means that, given the measured time delay values, the estimates of the Hubble constant turn out to be strongly model-dependent. In particular, models with more centrally concentrated mass distribution (low dark matter content or no dark matter) provide higher values of H0, more consistent with the results of the local H0 measurements than those with lower mass concentration towards the center (dark matter is present, consistent with the modern dark matter concept).

In this respect, our measurements of time delays in the quadruple PG1115+080 system turned out to be important.

They provided, for the lens galaxy model that assumes presence of the dark matter halo, the values of the Hubble constant consistent with the results of other methods:

Schechter et al. (1997): ΔtBC≈25d        H0= 44 km s -1 Мpс-1
Kharkov data (2009): ΔtBC≈16.4d      H0= 63 km s -1 Мpс-1

During the last decade, we determined time delays for several gravitationally lensed quasars, namely for Q2237+0305, SBS 0909+532, PG 1115+080, H1413+115, Q0957+561, UM673. For two of them, Q2237+0305 and SBS 0909+532, the time delays have been measured for the first time.

  1. Koptelova, E.; Chen, W. P.; Chiueh, T.; Artamonov, B. P.; Oknyanskij, V. L.; Nuritdinov, S. N.; Burkhonov, O.; Akhunov, T.; Bruevich, V. V.; Ezhkova, O. V.; Gusev, A. S.; Sergeyev, A. V.; Ehgamberdiev, Sh. A.; Ibragimov, M. A. Time delay between images of the lensed quasar UM673 // Astronomy & Astrophysics, 2012, v. 544, p.51-59
  2. Vakulik, V. G.; Shulga, V. M.; Schild, R. E.; Tsvetkova, V. S.; Dudinov, V. N.; Minakov, A. A.; Nuritdinov, S. N.; Artamonov, B. P.; Kochetov, A. Ye.; Smirnov, G. V.; Sergeyev, A. A.; Konichek, V. V.; Sinelnikov, I. Ye.; Bruevich, V. V.; Akhunov, T.; Burkhonov, O. Time delays in PG 1115+080: new estimates // MNRAS, 2009, v. 400, Iss.1, pp. L90-L93
  3. Ullán, A.; Goicoechea, L. J.; Zheleznyak, A. P.; Koptelova, E.; Bruevich, V. V.; Akhunov, T.; Burkhonov, O. Time delay of SBS 0909+532 // Astronomy & Astrophysics, v. 452, Iss. 1, 2006, pp.25-35
  4. Vakulik, V.; Schild, R.; Dudinov, V.; Nuritdinov, S.; Tsvetkova, V.; Burkhonov, O.; Akhunov, T. Observational determination of the time delays in gravitational lens system Q2237+0305 // Astron. & Astrophys., 2006, v. 447, Iss. 3, pp.905-913
  5. Colley, W.N., Schild R.E., Dudinov V.N., et al. (38 co-authors). Around the clock observations of the Q0957+561 A,B gravitationally lensed quasar II: Results for the second observing season // Astroph. J., 2003, v. 587, p. 71-79.

Microlensing studies as a tool to examine physical properties of source quasars and lensing galaxies.

Examining variability of the lensed quasar components provides valuable information about the lens and the source. For the lens, spatial density of stars and typical masses of microlenses can be estimated, while for the source, the study of the quasar structure over angular scales of the order of arcsec is possible.

Because of proximity of the lensing galaxy to the observer, the gravitational lens Q2237+0305 (the Einstein Cross) is unique for microlensing studies. It can be seen from the light curves shown below, where the Maidanak data are marked with the grey background.

The analysis of the Q2237+0305 microlensing variability has made it possible to estimate typical masses of microlenses and characteristic size of the quasar central engine. It enabled us to propose a two-component model of the source quasar, which reproduces the observed lightcurves in simulations much better as compared to a single accretion disc model. Using our observations in filters V, R and I, we estimated parameters of our two-component model. The results can be seen, e.g. in the following works:

  1. Vakulik, V. G.; Dudinov, V. N.; Zheleznyak, A. P.; Tsvetkova, V. S.; Notni, P.; Shalyapin, V. N.; Artamonov, B. P. VRI photometry of the Einstein Cross Q2237+0305 at Maidanak observatory // Astron.Nachr., 1997, v. 318, no. 2, p. 73-79.
  2. Дудинов В.Н., Вакулик В.Г., Железняк А.П., и др. Вариации блеска и цвета в гравитационно-линзовой системе Q2237+0305 по наблюдениям в 1997 и 1998 гг. // КФНТ, 2000, т.16, № 4, с.346-354.
  3. V.Vakulik, R.Schild, V.Dudinov, et al. Color Effects Associated with the 1999 Microlensing Brightness Peaks in Gravitationally Lensed Quasar Q2237+0305.// Astron.& Astrophys., 2004, v. 420, pp.447-457.
  4. Vakulik, V. G.; Schild, R. E.; Smirnov, G. V.; Dudinov, V. N.; Tsvetkova, V. S. Q2237+0305 source structure and dimensions from light-curve simulation // MNRAS, 2007, v. 382, Iss. 2, pp. 819-825.
  5. Vakulik, V., Shulga, R.Schild, G.Smirnov, V.Tsvetkova, A.Minakov, V.Dudinov, B.Artamonov, S.Nuritdinov, A.Sergeev, V.Konichek, A.Zheleznyak, V.Bruevich, O.Burkhonov, T.Akhunov // Q2237+0305 source structure from observations of chromatic Microlensing, prepared for MNRAS

Monitoring of microlensing activity in other systems provided estimates of the characteristic masses of microlenses in their lensing galaxies. Very detailed and long-term lightcurves for the doubly lensed quasar SBS 1520+530 should be mentioned especially: observations spanned over a decade of its variability history, having revealed several microlensing events of various durations and amplitudes.

  1. A.P.Zheleznyak, A.V.Sergeev, V.V.Konichek, et al. Optical monitoring of gravitationally lensed quasar SBS 1520+530 // Vіsn.Kyiv.Nat.Iniv., 2003, Вып. 39-40, с. 81-85.
  2. Khamitov, I. M.; Bikmaev, I. F.; Aslan, Z.; Sakhibullin, N. A.; Vlasyuk, V. V.; Zheleznyak, A. P.; Zakharov, A. F. Analysis of optical light curves for the components of the gravitationally lensed quasar SBS 1520+530 based on observations with the 1.5-m RTT-150 telescope in 2001-2005 // Astronomy Letters, 2006, v. 32, Iss. 8, pp.514-519
  3. Shalyapin, V. N.; Goicoechea, L. J.; Koptelova, E.; Artamonov, B. P.; Sergeyev, A. V.; Zheleznyak, A. P.; Akhunov, T. A.; Burkhonov, O. A.; Nuritdinov, S. N.; Ullan, A. Microlensing variability in FBQ 0951+2635: short-time-scale events or a long-time-scale fluctuation? // MNRAS, 2009, v. 397, Iss. 4, pp. 1982-1989.

Gravitational lensing as a tool to study dark matter.

The most natural way to study dark matter in the Universe is the search for manifestations of its gravitational interaction with the ordinary matter. Gravitational lensing at various scales in the Universe is the best tool to do this.

Observations of gravitationally lensed quasars provide several approaches to detect dark matter in lensing galaxies. One of them is based on determining time delays and, in short, assumes the following. We may ask a question, what mass distribution the lensing galaxy must have in order to agree, for the measured time delays, with some adopted value of H0, e.g., with local estimates made from Cepheids. This is the way to study a total mass distribution, both luminous and dark. It is worth noting that our measurements of the time delays in PG1115+080 provided adequate estimates for H0 and, at the same time, favored a family of lens models, which contain the cold dark matter halos.

Other approaches assume statistical analysis of variability of quasar’s images, with subsequent simulation of microlensing events and comparison with observarions. In particular, for Q2237+0305+0305 we analyzed probability density distributions of microlensing amplification to conclude that the dark matter fraction in the total surface mass density in Q2237 lens galaxy is <50% at the distance ~25pc from the nucleus.

One more approach we used in the attempts to detect the presence of dark matter in the PG 1115+080 lensing galaxy is based on the analysis of flux ratio anomalies. Anomalous flux ratios are meant when the observed flux ratios of lensed quasar images cannot be reproduced by any reasonable lens model. One of the possible explanations is that the anomalies may be caused by the dark matter substructure somewhere on the sightline of a specific image component. Our analysis if the PG 1115+0305 lightcurves did not reveal such a substructure for the A1 and A2 components, which would exceed 104 – 105 solar mass.

  1. Minakov A.A., Schild R.E., Vakulik V.G., Smirnov G.V., Tsvetkova V.S. Microlensing events in gravitationally lensed quasar Q2237+0305: stars or dark matter // Problems of Practical Cosmology, Proc. of the International Conference, 23-27 June, 2008, St. Petersburg, Russia,. Vol.1, 2008, p.180-186
  2. Tsvetkova, V. S.; Vakulik, V. G.; Shulga, V. M.; Schild, R. E.; Dudinov, V. N.; Minakov, A. A.; Nuritdinov, S. N.; Artamonov, B. P.; Kochetov, A. Ye.; Smirnov, G. V.; Sergeyev, A. A.; Konichek, V. V.; Sinelnikov, I. Ye.; Zheleznyak, A. P.; Bruevich, V. V.; Gaisin, R.; Akhunov, T.; Burkhonov, O. PG1115+080: variations of the A2/A1 flux ratio and new values of the time delays // MNRAS, 2010, v. 406, Iss. 4, pp. 2764-2776.
  3. Цветкова В.С., Шульга В.М., Вакулик B.Г., Смирнов Г.В., Дудинов В.Н., Минаков А.А. Поиск темной материи с использованием явления сильного гравитационного линзирования // Кинемат. и физ. неб. тел., 2009, т. 25, N 1, C. 40-56.

Other research topics

Of other research topics investigation of toroidal structures as possible elements of the quasar spatial structure should be mentioned:

  1. Bannikova, E. Yu.; Vakulik, V. G.; Sergeev, A. V. N-body simulation of a clumpy torus: application to active galactic nuclei // MNRAS 2012, v. 424, Iss. 2, pp. 820-829
  2. Bannikova, E. Yu.; Vakulik, V. G.; Shulga, V. M. Gravitational potential of a homogeneous circular torus: a new approach // MNRAS 2011, v. 411, Issue 1, pp. 557-564.

Also, various aspects of gravitational lensing and micolensing phenomena was being elaborating theoretically. The results have been summarized in the monograph:

А.А.Минаков, В.Г.Вакулик. Статистический анализ гравитационного микрозинзирования. Наукова думка, Киев, 2010 г, 262 с.