About every 114 days, almost like clockwork, a galaxy 570 million light-years away lights up like fireworks. Since at least 2014, our observatories have been recording this strange behavior; now astronomers have put the pieces together to find out why.
At the center of the spiral galaxy, called ESO 253-G003, a supermassive black hole is orbited by a star that swings so close every 114 days that some of its material can be swallowed up, causing a brilliant flash of light . multiple wavelengths. Then it moves away, survives to be slurped again on its next approach.
Due to the regularity of the flares, astronomers have nicknamed the galaxy “Old Faithful”, as has the geyser in Yellowstone National Park.
“These are the most predictable and frequently recurring multi-wavelength bursts we’ve seen from the core of a galaxy, and they give us a unique opportunity to study this extragalactic Old Faithful in detail,” said study lead author, astronomer Anna Payne from college. from Hawai’i in Mānoa.
“We think a supermassive black hole in the center of the galaxy is causing the eruptions because it partially consumes an orbiting giant star.”
The flares were first detected in November 2014, picked up by the All-Sky Automated Survey for Supernovae (ASAS-SN). At the time, astronomers thought the clearing up was a supernova that took place in ESO 253-G003.
But in 2020, when Payne looked at the ASAS-SN data on ESO 253-G003, she found another torch from the same location. And another. And another.
In total, she identified 17 flares, spaced approximately 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 called the repeated flare ASASSN-14ko, and those accurate predictions meant they could make new, more detailed observations of the May eruption with NASA’s powerful TESS telescope. Previous observations from other instruments also provided data over a range of wavelengths.
“TESS gave a very thorough picture of that particular eruption, but because of the way the mission is mapping the sky, it cannot detect them all,” said Ohio State University astronomer Patrick Vallely. “ASAS-SN collects less detail about individual bursts, but provides a longer baseline, which was crucial in this case. The two studies are complementary.”
But a supernova flares up and then fades only once, as such an event destroys the original star; so what caused the bursts of light in optical, ultraviolet, and X-ray wavelengths had to be something else.
A supermassive black hole regularly emitting flares as it faltering on a rotating star is not unheard of – one was identified last year according to a nine-hour flaring schedule – but the matter was not that simple with ESO 253-G003.
That’s because ESO 253-G003 are actually two galaxies that are in the final stage of amalgamation, which means there should be two supermassive black holes in the center.
Recent research has shown that two interacting supermassive black holes can cause repeated flares, but the objects in the center of ESO 253-G003 are thought to be too far apart to interact in this way.
Another possibility that emerged was a star crashing through an accretion disk of material spinning and ending up in one of the black holes. This also had to be ruled out. Because the star collided through the disk at different locations and from different angles, the flares’ shapes should have been different – but the observations showed that the flares from ESO 253-G003 were too close together.
The third possibility was repeated partial tidal disturbance, in which a larger massive object repeatedly strips material from a smaller one in orbit.
If a star was in an eccentric orbit around the black hole for 114 days, its close-up approach, or periastron, could see it spin close enough to strip material before ejecting again.
When this material collides with the accretion disk, it causes a flare. And this is what seems to be happening.
With this scenario in mind, the team analyzed the observations. They analyzed the light curve of each flare and also compared them with other known tidal disturbances in the black hole. And they determined that the star was likely orbiting a supermassive black hole at a speed of 78 million solar masses.
At any closest approach, the loss of about 0.3 percent of the Sun’s mass – about three Jupiters – to the black hole would be enough to cause the observed eruptions while the star could survive.
“If a giant star with a swollen envelope wanders close, but not too close, in a very elongated orbit, 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 for Astronomy. “In that case, the giant star will just return over and over until the star is exhausted.”
It is not clear how long the star and black hole lasted this dance, making it difficult to calculate how long the star will be left. But the team has predicted when the next two flares will occur – in April and August of this year – and plans to make even more sightings.
It is an extremely rare opportunity to understand the mass growth of supermassive black holes.
“In general, we really want to understand the properties of these black holes and how they grow,” said Ohio State University astronomer Kris Stanek. “The ability to accurately predict the timing of the next episode allows us to collect data that we otherwise wouldn’t be able to use, and we are already incorporating such data.”
The research was presented at the 237th meeting of the American Astronomical Society. It will also be filed with The Astrophysical Journal, and is available on arXiv.