This graph shows the variation of brightness of the a distant star caused by a microlensing event, referred to as Gaia16aye, as a foreground massive object – a binary system of stars – passed across the distant star's line of sight. The brightness is indicated on the vertical axis in terms of the astronomical magnitude, with smaller values (towards the top) indicating higher brightness; time is indicated on the horizontal axis.
The graph shows data collected over a period of almost two years with more than 50 telescopes worldwide, as part of a global observation campaign spearheaded by ESA’s Gaia satellite. Brightness measurements obtained with Gaia are shown as large black diamond symbols, whereas ground-based observations performed with a variety of telescopes across the world are shown as smaller circles, squares, diamonds and triangles of different colours.
The first detection of the source’s sudden brightening, made by Gaia in August 2016, is marked with a red arrow. The detection was made as part of the Gaia Photometric Science Alerts programme, a system that 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.
Follow-up observations of the source, since then named Gaia16aye, revealed that the source was behaving in a rather strange way: after getting brighter and brighter, its luminosity suddenly dropped within one day.
Astronomers soon realised that this brightening was caused by gravitational microlensing – an effect predicted by Einstein’s theory of general relativity, caused by the bending of spacetime in the vicinity of very massive objects, like stars or black holes.
The Gaia alert led to a 500-day long global observation campaign, which lasted until the end of 2017, monitoring the background star as it brightened up and subsequently declined five times. Extensive participation of ground-based telescopes from around the world enabled astronomers to gather a large amount of data – almost 25 000 individual data points – which were used to study the foreground object that was acting as a lens on the more distant star.
The data revealed the gravitational lens is actually a binary stellar system, consisting of two rather small stars, with 0.57 and 0.36 times the mass of our Sun, respectively. Separated by roughly twice the Earth-Sun distance, the stars orbit around their mutual centre of mass in less than three years.
Gaia16aye is the second microlensing event detected by the Gaia mission.
The ground-based observations were performed with different filters at a variety of visible and near-infrared wavelengths. In the graph, the different filters are indicated with the following letters: B standing for blue; g for green; V for visual (corresponding to intermediate wavelengths between green and red); R and r for red; I and i for infrared.