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Kazan University's telescope assists in discovering a binary star system

February 13, 2020

A co-author from Kazan University, Professor, Corresponding Member of the Tatarstan Academy of Sciences, Chair of the Department of Astronomy and Space Geodesy Ilfan Bikmaev, explains how the new system was found.

"The gravitational lensing method is one of the most powerful space exploration tools. In space, photons deviate from the rectilinear direction when passing near a massive body (star) under the influence of its gravitational field. If we take as a lens a celestial body, which is a sphere, then it will bend the space spherically symmetrically. However, the gravitational fields of many space objects do not have spherical symmetry, so more complex curvatures may appear. After their path has been curved, the photons will be summed up with those that hit the receiver earlier, and, as a result, an increase in the brightness of the star will occur. As a result, an increase in the brightness of the object is displayed on the light curve of the source, and this increase is not associated with a change in the physical parameters of the source itself.

"If between a star of our Galaxy and an observer on Earth a massive object (a star-lens) moves across the line of sight, then when the lens passes exactly upon the line of sight, the effect of gravitational lensing will manifest itself in the form of a short-term (hours to days) brightening of the background star. Such events are called gravitational microlensing events. They are quite rare, isolated, short-lived and unpredictable."

As the interviewee, in order to register a microlensing event in the Milky Way, you need to track the brilliance of hundreds of millions of stars daily. In particular, the space mission of the European Space Agency (GAIA) is engaged in this. Any brightness changes amounting to tens of percents from celestial sources that fall into the field of view of the GAIA space observatory are reported to Earth. And then the international network of telescopes around the globe begins to track these objects and identify the nature of variability.

"Since 2016, astronomers of Kazan Federal University, together with Turkish colleagues, have been participating in the GAIA satellite object classification program. The vast majority of variable objects are cataclysmic variables, some are supernovae, and some are active galactic nuclei, which change their brightness from time to time. But there are objects that, while not being a variable, change their brightness for a short period of time, and then it attenuates. Such cases are unique," says Bikmaev. "So, in August 2016, the GAIA satellite discovered an object that received the designation Gaia16aye, the brightness change of which exceeded the accuracy of registration of the telescope and continued to increase. Turkish colleagues, analyzing the nature of the brightness change, suggested that this is not a variable object, but the microlensing effect. Polish colleagues, experts in the field of research on the effects of microlensing, organized an international campaign on photometry of this source, which was soon joined by Kazan Federal University. Observations of this unique object were carried out both in Turkey with the RTT 150 telescope and at the North Caucasian Astronomical Station.

"The data obtained make it possible for the first time to simulate a situation where an observer on Earth makes a yearly motion around the Sun, a gravitating body moves in the form of a binary system around the center of mass, and the binary system has its own motion in the Galaxy. This is a rather complex kinematic movement. Therefore, the system of these maxima is complex. And what we can do is accurately measure the brightness change.

"With a single passage, a single maximum is observed, and then the brightness curve of the object drops to the initial level. In the case of the Gaia16aye event, after the first maximum, the light curve did not drop to the initial level. Therefore, astronomers have made the assumption that the gravitational lens is not a single object, but a binary system. And then the third peak appeared and everyone understood that it was, without a doubt, a binary system. Perhaps the geometry of the system is even more complex. In this article, a group of Polish scientists, based on international cooperative observations and their own theoretical calculations, built a geometric picture of the occurrence of the Gaia16aye microlensing phenomenon," concludes Professor Bikmaev.

Kazan Federal University

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