About every 114 days, almost like clockwork, a 570 million light-year galaxy burns away like a firework. Since at least 2014, our observatory has recorded this strange behavior; now astronomers have put the pieces together to find out why.
At the center of the spiral system, called ESO 253-G003, a supermassive black hole orbits a star that swings close enough every 114 days to swallow some of its material, causing a dazzling ray of light of several wavelengths. Then it moves away and survives to be sneaked up again with its next close approach.
Due to the regularity of the torches, astronomers have given the nickname “Old Faithful”, like the geyser in Yellowstone National Park.
“It is the most predictable and frequently repeating torch with more wavelengths we have seen from the core of a galaxy, and it gives us a unique opportunity to study this extragalactic Old Faithful in detail,” said the first author of the study, astronomer Anna Payne of the University, said of Hawai’i at Mānoa.
“We think a supermassive black hole in the center of the galaxy is causing the bursts because it partially digests a giant star.”
The torches were first detected in November 2014, recorded by the All-Sky Automated Survey for Supernovae (ASAS-SN). At the time, astronomers thought that the luminosity was a supernova found in ESO 253-G003.
But in 2020, when Payne was examining the ASAS-SN data on ESO 253-G003, she found another torch from the same spot. And another. And another.
In total, she identified 17 torches, with 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 mention the repeated flaring ASASSN-14ko, and the accurate predictions meant they were able to take new, more detailed observations of the May torch with NASA’s powerful TESS telescope. Previous observations of other instruments have also provided data on a range of wavelengths.
“TESS has given a very thorough picture of the specific torch, but because of the way the mission images the sky, not everyone can observe it,” said astronomer Patrick Vallely of Ohio State University. “ASAS-SN collects less detail about individual outbreaks, but provides a longer baseline, which was crucial in this case. The two surveys complement each other.”
But a supernova flares only once and then fades, as such an event destroys the original star; whatever caused the bursts of light in optical, ultraviolet and X-ray wavelengths had to be something else.
A supermassive black hole emitting ordinary torches as it orbits a star is not unheard of – one was identified last year under a nine-hour scheme – but the case was not so simple with ESO 253-G003.
This is because ESO 253-G003 are actually two galaxies in the final phase of fusion, which means there should be two supermassive black holes in the middle.
Recent research has shown that two interaction supermassive black holes can flare up repeatedly, but the objects in the middle of ESO 253-G003 are presumably too far apart to communicate in this way.
Another possibility raised was a star circling through a pile of material and feeding into one of the black holes. It also had to be excluded. While the star collided with the disk at different places and angles, the shapes of its torches must have been different – but observations showed that the torches of ESO 253-G003 corresponded too closely.
The third possibility was repeated partial tidal disruption, where a larger massive object repeatedly removed material from a smaller orbit.
If a star was on an eccentric 114-day orbit around the black hole, its close-up approach, or periastron, could see it rotate close enough to have material stripped before blowing away again.
When this material collides with the growth disk, it causes a flare. And that’s what’s apparently happening.
In view of this scenario, the team analyzed the observations. They analyzed the light curve of each torch and also compared it to other known events in the disruption of the tides in the black hole. And they determined that the star was probably orbiting a supermassive black hole that turned in 78 million solar masses.
At each closest approach, the star would lose about 0.3 percent of the mass of the sun – about three Jupiters – to the black hole would be sufficient to cause the observed flares, while allowing the star to survive.
“If a giant star with an inflated envelope orbits close, but not too close, on a very long orbit, then the black hole can steal from the outer material without tearing the whole star apart.” said astronomer Benjamin Shappee of the Institute of Astronomy of the University of Hawaii. “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 retain this dance, making it difficult to calculate how long the star is left. But the team predicted when the next two torches would take place – in April and August this year – and plans to take even more observations.
This is an extremely rare opportunity to understand supermassive growth of black holes.
“Overall, we really want to 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 would not be able to take, and we take in all such data.”
The research was presented at the 237th meeting of the American Astronomical Society. It will also be submitted to The Astrophysical Journal, and is available on arXiv.