Galaxy ESO 253-G003, located 570 million light-years away, flares regularly every 114 days. Astronomers have now worked out why: a supermassive black hole is slowly tearing a star apart. The star is orbiting the supermassive black hole and every 114 days, when it gets close enough for the black hole to strip some of its material, a flash of light is released.
The flaring event known as ASASSN-14ko was first detected in 2014 and was at first thought to be a weird supernova. Last year, lead researcher Anna Payne looked at the data from the All-Sky Automated Survey for Supernovae (ASAS-SN) and discovered the regular flaring, counting 17 flares evenly spaced every 114 days. The peculiar interaction was announced virtually at the recent 237th meeting of the American Astronomical Society, with a paper submitted to The Astrophysical Journal.
Payne and her team expected new flaring in May, September, and December 2020, which occurred as predicted. Having now witnessed 20 repeating flares, the team likens this regularity to the Old Faithful geyser in Yellowstone National Park in the US. This regularity could open a window into the extreme environments created by supermassive black holes at the center of galaxies.
"These are the most predictable and frequent recurring multiwavelength flares we've seen from a galaxy's core, and they give us a unique opportunity to study this extragalactic Old Faithful in detail," Payne, a NASA Graduate Fellow at the University of Hawai'i at Mānoa, said in a statement. "We think a supermassive black hole at the galaxy's center creates the bursts as it partially consumes an orbiting giant star."
The team also investigated two alternative hypotheses. In one, the flare could have been the product of the interaction between the supermassive black hole in question and a second supermassive black hole. While ESO 253-G003 does have a second supermassive black hole, it is currently too far away to be interacting like this.
The other possibility involved a star and the disk of material that typically surrounds a supermassive black hole. A star with an inclined orbit would cross this disk sending material spiraling into the black hole. However, this also doesn’t fit because the star would have to cross the disk twice in its orbit, producing slightly different alternating flares, and no differences have been seen in the flares.
Hence, they argue, the most likely explanation is that the supermassive black hole, which weighs a whopping 78 million solar masses, is slowly robbing a star of material. With each passage, the star loses the equivalent of three Jupiters to the black hole. The team doesn’t know how long this has been going on or for how long this will continue but at some point, the star will run out of mass.
Payne and her team are now analyzing the data from the December flare and will continue observing the event's predicted outbursts coming in April and August of this year. An incredible tool in this study has been the Transiting Exoplanet Survey Satellite (TESS), NASA's planet-hunter. While TESS looks for planets beyond the Solar System, its keen instruments have provided some crucial insights into the flare.