Two stars, too dim for telescopes to notice, gave themselves out as gravitational lens, distorted the light of a farther and brighter star.

The Gaia European Space Telescope conducts detailed astrometric observations of the stars of our Galaxy, accurately capturing their luminosities, positions and movements. In 2016, the device noticed that one of them behaves very unusual, quickly and dramatically changing the brightness. This behavior is not associated with the features of the star itself, but with a massive object located between us and her. For telescopes, it remains invisible, but strong gravity distorts the light of a farther star, betraying its presence.

The gravitational lens was called Gaia16aye, and the authors of the new articles published in the journal Astronomy & astrophysics, were able to find out its nature. The approach used by Lukasz Wyrzykowski from Warsaw University and his colleagues may be useful for studying other objects inaccessible to conventional methods of observation, primarily black holes.

“A single lens created by a single object gives just a small, gradual increase in brightness and then a gradual decline, as it passes against the background of a distant source, – explains Vyrzykowski. “In this case, the brightness not only dropped sharply, but after a couple of weeks it also increased sharply, which is very unusual.”

© ESA, Mellinger, Digital Sky Survey, Pan-STARRS1, Wyrzykowski et al., 2020

For additional study of an unusual object, ground-based telescopes were used. During 500 days of observation, scientists saw five such cycles. Researchers associated them with a gravitational lens created by a double star. While rotating, such a system creates a complex system of small and fast-moving gravitational lenses that cause sharp changes in the brightness of a farther star.

500 days of observation for Gaia16aye / © ESA, Wyrzykowski et al., 2020

These changes allowed astronomers to calculate the characteristics of the double, which received the index 2MASS19400112 + 3007533, without observing it directly. According to scientists, it includes two red dwarfs with masses of 57 and 36 percent of the mass of the Sun, which revolve around a common center of gravity for 2.88 Earth years.

“We were able to determine the period of rotation of the system, the mass of its components, the size and shape of their orbits, literally everything,” says Lukasz Wyrzykowski, “without even seeing it.” The authors believe that applying this approach to other Gaia data will allow us to find and describe many other objects that are visible only as gravitational lenses and inaccessible to other methods.