The brightening of the star,located in the Cygnus constellation, was first spotted in August 2016 by the Gaia Photometric Science Alerts programme. This system, maintained by the Institute of Astronomy at the University of Cambridge, UK, scans daily the huge amount of data coming from Gaia and alerts astronomers to the appearance of new sources or unusual brightness variations in known ones, so that they can quickly point other ground and space-based telescopes to study them in detail. The phenomena may include supernova explosions and other stellar outbursts.
In this particular instance, follow-up observations performed with more than 50 telescopes worldwide revealed that the source – since then named Gaia16aye – was behaving in a rather strange way. The pair consists of two rather small stars, with 0.57 and 0.36 times the mass of our Sun, respectively. Separated by roughly twice th Earth-Sun distance, the stars orbit around their mutual centre of mass in less than three years.
We saw the star getting brighter and brighter and then, within one day, its brightness suddenly dropped,” says Lukasz Wyrzykowski from the Astronomical Observatory at the University of Warsaw , who is one of the scientists behind the Gaia Photometric Science Alert programme. “This was a very unusual behaviour. Hardly any type of supernova or other star does this.”
Lukasz and collaborators soon realized that this brightening was caused by gravitational microlensing – an effect predicted by Einstein’s theory of general relativity, caused by the bending of space-timenear very massive objects, like stars or black holes.
When such a massive object, which may be too faint to be observed from Earth, passes in front of another, more distant source of light, its gravity bends the fabric of spacetime in its vicinity. This distorts the path of light rays coming from the background source – essentially behaving like a giant magnifying glass. Gaia16aye is the second micro-lensing event detected by ESA’s star surveyor. However, the astronomers noticed it behaved strangely even for this type of event.
“If you have a single lens, caused by a single object, there would be just a small, steady rise in brightness and then there would be a smooth decline as the lens passes in front of the distant source and then moves away,” says Lukasz. “In this case, not only did the star brightness drop sharply rather than smoothly, but after a couple of weeks it brightened up again, which is very unusual. Over the 500 days of observation, we have seen it brighten up and decline five times.”
This sudden and sharp drop in brightness suggested that the gravitational lens causing the brightening must consist of a binary system – a pair of stars, or other celestial objects, bound to one another by mutual gravity. The combined gravitational fields of the two objects produce a lens with a rather intricate network of high magnification regions. When a background source passes through such regions on the plane of the sky, it lights up, and then dims immediately upon exiting it. From the pattern of subsequent brightenings and dimmings, the astronomers were able to deduce that the binary system was rotating at a rather fast pace.
The long period of observations,which lasted until the end of 2017, and the extensive participation of ground-based telescopes from around the globe enabled the astronomers to gather a large amount of data – almost 25 000 individual data points. In addition, the team also made use of dozens of observations of this star collected by Gaia as it kept scanning the sky over the months. These data have undergone preliminary calibration and were made public as part of the Gaia Science Alerts programme.
ICCUB researcher Josep Manel Carrasco has coordinated the participation of the Observatori del Montsec in the Gaia Science Alerts programme since 2005. The group has contributed with up to 2000 observations, obtained with the Joan Oró Telescope (TJO). That represents the 9% of the total photometric observations of Gaia16aye, which were obtained through more than 50 observatories. “The detection of binary systems through microlensing – such as Gaia16aye – allows measuring the mass and orbital parameters of those binary systems”, Carrasco explains. “The same method that we used in Gaia16aye could be applied in the future to search for new multiple systems, regardless of their mass, so it might lead to new discoveries of very low-mass bodies such as planets, or very high-mass objects as black holes, that are currently invisible for the optical telescopes.”
“If it wasn’t for Gaia scanning the whole sky and then sending the alerts straight away, we would never have known about this microlensing event,” says co-author Simon Hodgkin from the University of Cambridge, who leads the Gaia Science Alerts programme. “Maybe we would have found it later, but then it might have been too late.”