About every 114 days, almost like a clockwork, a galaxy 570 million light-years away lights up like a firework. Since at least 2014, our observatories have recorded this strange behavior; now astronomers have put the pieces together to find out why.
In the center of the spiral galaxy, named ESO 253-G003, a supermassive black hole orbits a star that oscillates every 114 days closely enough to suspend some of its material, causing a radiant beam of light over several wavelengths. It then moves away and survives to be slurped again by its next close approach.
Due to the regularity of the flare, astronomers have nicknamed the galaxy “Old Faithful”, like the geyser in Yellowstone National Park.
“These are the most predictable and frequent recurring flare wavelengths we have seen from a galaxy core, and they give us a unique opportunity to study this extragalactic ancient faithful in detail,”
“We believe that a supermassive black hole in the center of the galaxy creates bursts as it partially consumes an orbiting giant star.”
The apertures were first discovered in November 2014, collected by the All-Sky Automated Survey for Supernovae (ASAS-SN). At the time, astronomers believed that light was a supernova occurring in ESO 253-G003.
But in 2020, when Payne was looking at the ASAS-SN data on the ESO 253-G003, she found another glare from the same place. And another. And another.
In total, she identified 17 flares with a distance of about 114 days apart. She and her team then predicted that the galaxy would flare up again on May 17, September 7 and December 26, 2020 – and they were right.
They named the repeatedly flaring ASASSN-14ko, and these accurate predictions meant they were able to take new, more detailed observations of the May flame with NASA’s powerful TESS telescope. Previous observations from other instruments also provided data across a range of wavelengths.
“TESS gave a very thorough picture of the particular glare, but because of the way the mission depicts the sky, it cannot observe them all,” said astronomer Patrick Vallely of Ohio State University. “ASAS-SN collects minor details about individual outbreaks, but provides a longer baseline, which was crucial in this case. The two studies complement each other.”
But a supernova flares only once and then fades as such an event destroys the original star; so whatever caused the light bursts in optical, ultraviolet and X-ray wavelengths, it had to be something else.
A super-massive black hole that emits regular flares as it snacks on an orbiting star is not unheard of – one was identified last year on a nine-hour flare schedule – but the matter was not so simple with the ESO 253-G003.
This is because the ESO 253-G003 are actually two galaxies in the final stages of fusion, meaning that there must be two supermassive black holes in the center.
Recent research has shown that two interacting supermassive black holes can cause repeated burnout, but the objects in the middle of the ESO 253-G003 are thought to be too far apart to interact in this way.
Another possibility raised was a star crashing down through a growth disk of material swirling around and feeding into one of the black holes. This also had to be ruled out. As the star crashed through the disk at different places and angles, the shapes of its flares should have been different – but observations showed that the flares from ESO 253-G003 were too closely matched.
The third option was repeated partial tidal disturbance, where a larger solid object repeatedly removes material from a smaller orbiting end.
If a star was in an eccentric 114-day orbit around the black hole, its dense approach or periastron could see it turn close enough that the material was removed before throwing away again.
When this material collides with the accretion disk, it causes a flare. And that’s what seems to be happening.
With this scenario in mind, the team analyzed observations. They analyzed the light curve for each flare and also compared them with other known black hole tidal disturbance events. And they determined that the star probably orbited a supermassive black hole that lay at 78 million solar masses.
At each closest approach, the star, which lost about 0.3 percent of the sun’s mass – about three Jupiter – to the black hole, would be sufficient to cause the observed flares while the star could live on.
“If a giant star with an inflated envelope travels close, but not too close, on a very elongated orbit, then the black hole can steal some of the outer material without tearing the entire star apart.” said astronomer Benjamin Shappee of the University of Hawai’i Institute of Astronomy. “In that case, the giant star will just return again and again until the star is exhausted.”
It is not clear how long the star and the black hole have sustained this dance, making it difficult to calculate how long the star has left. But the team has predicted when the next two flares will occur – in April and August this year – and plans to take even more observations.
It represents an extremely rare opportunity to understand supermassive growth in black cavity mass.
“In general, we want to really understand the properties of these black holes and how they grow,” said astronomer Kris Stanek of Ohio State University. “The ability to accurately predict the timing of the next episode allows us to take data that we otherwise could not take, and we are already taking such data.”
The research has been presented at the 237th meeting of the American Astronomical Society. It will also be sent to The Astrophysical Journal, and is available on arXiv.