Despite their name, black holes can give off bright flares of light, usually as they shred stars that wander too close. Most of those are one-off events, but now astronomers have discovered an extragalactic Old Faithful that fires up like clockwork every 114 days.
The flares are coming from a galaxy about 570 million light-years away, and have been named ASASSN-14ko, after the All-Sky Automated Survey for Supernovae (ASAS-SN) that helped uncover them. The first detection occurred on November 14, 2014, and at the time it was presumed to be a run-of-the-mill supernova.
But a few years later, astronomers on the new study examined ASAS-SN data and discovered 17 other flares that originated in the same location. Weirder still, they were each spaced 114 days apart, creating a strangely regular pattern for this kind of cosmic occurrence.
That ruled out supernovae – those are one-and-done events (with perhaps very rare exceptions). Instead, the researchers hypothesized that the flares were coming from a star trapped in a deadly orbit around a supermassive black hole.
Rather than the roughly circular path that Earth takes around the Sun, this star’s orbit would be extremely elongated, skimming past very close to the black hole before being flicked way out, then dragged back in to repeat the cycle every 114 days. It’s those close encounters that are likely behind the flares, as material is slurped off the surface of the star.
Astronomers have spotted plenty of these kinds of flares, known as tidal disruption events, in the past but usually they only happen once, before the star is ripped apart. A repeating event, let alone one on such a predictable cycle, is a new discovery.
“It’s really exciting, because we’ve seen black holes do a lot of things, but we’ve never seen them do something like this – cause this regular eruption of light – before,” says Patrick Vallely, co-author of the study. “It’s like an extra-galactic Old Faithful.”
To validate the cycle, the researchers predicted the dates throughout 2020 that the flares should be seen: May 17, September 7 and December 26. And sure enough, the flashes appeared right on cue each time.
The discovery could help astronomers spot more of these events out there in the cosmos, as well as give new hints about stars and black holes.
The study is due to be published in the Astrophysical Journal. An animation of the event can be seen in the video below.
Source: Ohio State University
