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Just 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 recorded this strange behavior; now astronomers have put the pieces together to figure out why.
At the center of the spiral galaxy, named ESO 253-G003, a supermassive black hole is put into orbit by a star that, every 114 days, oscillates close enough that some of its material is sucked in, causing a bright light to through several wavelengths. Then it walks away, surviving to be sucked again on its next close approach.
Due to the regularity of the flares, astronomers have dubbed the galaxy “Old Faithful”, after the geyser of Yellowstone National Park.
“These are the most predictable and frequent recurring multi-wavelength eruptions that we have seen from the heart of a galaxy, and they give us a unique opportunity to study in detail this ancient extragalactic devotee,” he said. said the study’s first author, astronomer Anna Payne of the University of Hawai’i at Mānoa.
“We believe that a supermassive black hole in the center of the galaxy creates the bursts because it partially consumes a giant star in orbit.”
The eruptions were first detected in November 2014, detected by the All-Sky Automated Survey for Supernovae (ASAS-SN). At the time, astronomers believed the enlightenment was a supernova occurring in ESO 253-G003.
But in 2020, when Payne examined the ASAS-SN data on the ESO 253-G003, she found another flare in the same spot. And another. And another.
In total, she identified 17 outbreaks, spaced approximately 114 days apart. She and her team then predicted that the galaxy would erupt again on May 17, September 7, and December 26, 2020 – and they were right.
They called the repeated ASASSN-14kb flaring, and those precise predictions meant they were able to take new, more detailed observations of May’s flare with NASA’s powerful TESS telescope. Previous observations from other instruments have also provided data over a range of wavelengths.
“TESS provided a very complete picture of this particular flare, but because of the way the mission imagines the skies, it can’t see them all,” said astronomer Patrick Vallely of Ohio State University. “ASAS-SN collects less detail on individual explosions, but provides a longer baseline, which was crucial in this case. The two investigations complement each other.”
But a supernova erupts only once, then disappears, since such an event destroys the original star; so whatever was causing the flares of light in optical, ultraviolet, and X wavelengths had to be something else.
A supermassive black hole emitting regular flares as it nibbles at an orbiting star is not unheard of – one was identified last year, on a nine-hour flaring schedule – but the case was not so simple with ESO 253-G003.
That’s because ESO 253-G003 is actually two galaxies in the final stages of merging, meaning there should be two supermassive black holes at its center.
Recent research has shown that two interacting supermassive black holes can cause repeated flares, but objects at the center of ESO 253-G003 are considered too far apart to interact in this way.
Another possibility raised was a star crashing through an accreting disc of swirling material and feeding one of the black holes. This should also be ruled out. As the star struck the disc at different places and angles, the shapes of its rockets should have been different – but observations showed that the rockets from ESO 253-G003 were too close.
The third possibility was a repeated partial disturbance of the tide, where a larger massive object repeatedly removes material from a smaller orbiting object.
If a star were in an eccentric 114-day orbit around the black hole, its close approach, or periastron, could see it veer close enough that matter was stripped before it flies away again.
When this material collides with the accretion disc, it causes flaring. And that’s what seems to be happening.
With this scenario in mind, the team analyzed the observations. They analyzed the light curve of each eruption and also compared them to other known black hole tidal disturbance events. And they determined that the star was probably orbiting a supermassive black hole with 78 million solar masses.
With each closest approach, the star losing about 0.3% of the Sun’s mass – about three Jupiters – to the black hole would be enough to cause the observed flares while still allowing the star to live.
“If a giant star with a swollen envelope wanders near, but not too near, in a very elongated orbit, then the black hole can steal some of the outer material without tearing the entire star.” said astronomer Benjamin Shappee of the Institute of Astronomy at the University of Hawaii. “In this case, the giant star will keep coming back again and again until the star is exhausted.”
It is not known how long the star and the black hole have kept this dance going, making it difficult to calculate how long the star has left. But the team has predicted when the next two eruptions are expected to occur – in April and August of this year – and plan to take even more sightings.
It represents an extremely rare opportunity to understand the supermassive mass accretion of black holes.
“In general, we really want to understand the properties of these black holes and how they develop,” said astronomer Kris Stanek of Ohio State University. “The ability to predict exactly when the next episode is going allows us to take data that we might not otherwise be able to take, and we are already taking that data.”
The research was presented at the 237th meeting of the American Astronomical Society. It will also be subject to The astrophysical journal, and is available on arXiv.
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