An example of an entirely new type of cosmic explosion that vastly outpowers most supernovas could be the result of a small or medium-sized black hole destroying a star.
The explosion, which has been named AT2022aedm, was seen emerging from a red galaxy located around 2 billion light-years from Earth by astronomers using the ATLAS network of robotic telescopes located in Hawaii, Chile, and South Africa. It was quickly recognized as something never seen before.
"We're always on the lookout for things that are a bit weird and different from standard kinds of supernovas, of which we find hundreds or even thousands per year," Matt Nicholl, leader of the team behind the discovery and an astrophysicist at Queen’s University Belfast, told Space.com. "AT2022aedm stood out because it was one of the brightest explosions that we've ever seen, and it was also one of the fastest to fade away after its peak."
The explosion spotted by Nicholl and the team emitted as much as 100 times more energy than an average supernova. Plus, while supernovas fade over the course of months, Nicholl noted that AT2022aedm faded to 1% of its original brightness in just 14 days, after which it completely disappeared. That means, in just two weeks, AT2022aedm emitted as much energy as the sun will in its entire 10 billion-year lifetime.
It is of little wonder why AT2022aedm sent shockwaves through the team and earned itself a category of its own, with the scientists behind the discovery defining it as the first "Luminous Fast Cooler" or "LFC." That name is a nod to the explosion’s qualities as well as to Nicholl and colleagues' love for the English Premier League soccer team Liverpool Football Club, which also goes by the acronym "LFC."
"I think probably the most promising explanation for LFCs like AT2022aedm are models involving the destruction of stars by a black hole," Nicholl explained.
This was a conclusion he and colleagues arrived at by first eliminating some other prime suspects.
The unusual suspects: How the finger got pointed at destructive black holes
One of the first steps for Nicholl and the Queen’s University Belfast scientists to take was to eliminate some of the usual culprits for cosmic cataclysms.
The explosion already didn’t present like a supernova, as it was too powerful and too fast, but the location at which it originated also helped distinguish this LFC as something entirely new.
One of the most common types of supernova is a core-collapse supernova formed when huge stars with masses over 8 times that of the sun run out of fuel for nuclear fusion. The stars' cores become unable to battle gravity any longer and ultimately collapse. This leaves behind a black hole or a neutron star at the heart of stellar wreckage from the outer layers of the star.
"AT2022aedm can’t be a normal core-collapse supernova because the galaxy it is seen in only has old low-mass stars; it doesn’t have anything more than eight times the mass of the sun, and that’s what you need to have to get to get a supernova," Nicholl explained.
Alternatively, another common space blast, a Type-Ia supernova, happens when stellar remnants called white dwarfs strip matter from a companion star. This stripping of matter tips the white dwarf over the mass limit needed to trigger a supernova and create a neutron star or black hole, but these events create a uniform output of radiation. For this reason, astronomers call them "standard candles" and use them to accurately measure cosmic distances.
AT2022aedm, however, doesn’t look like those at all.
That led to the team pointing the finger at black holes. But even then, they were able to clear the usual suspects.
Supermassive black holes get cleared
Events that see black holes rip up stars and then feast on the stellar remains are rare, but not unknown. Astronomers have spotted many examples of these so-called "Tidal Disruption Events" or "TDEs" as well as the light emitted during the violent proceedings.
TDEs usually occur when a star ventures too close to a huge supermassive black hole sitting at the heart of a galaxy. This black hole can have masses millions, or even billions, of times that of our sun. The gravitational influences of these monster black holes generate huge tidal forces in their star subjects that stretch and squeeze the stellar bodies, ripping them apart in a process called "spaghettification."
Yet, Nicholl and his colleagues immediately saw that this LFC couldn’t be the result of just any TDA driven by a supermassive black hole. Again, this is due partially to where the LFC appeared to originate from. Supermassive black holes sit at the heart of galaxies, and Nicholl said AT2022aedm was seen away from the center of its home galaxy. This means a smaller black hole (not at the heart of a galaxy) could be the culprit for this LFC.
"If you had a lower mass black hole that was in a dense environment with lots of stars, and one of those stars got very, very close to the black hole, even a stellar black hole with a mass 10 to 100 times that of the sun would still be able to potentially tear up and consume one of the stars," he continued.
Nicholl added that he and the team haven’t yet ruled out a more intriguing suspect, however.
There remains a chance the LFC could be the work of a "medium-sized," or intermediate-mass black hole that sits between stellar mass black holes and supermassive black holes in terms of dimension, possessing between 100 and a few thousand times the mass of the sun.
This is a tantalizing prospect not only because intermediate-mass black holes have remained elusive, but also because studying them could help explain how supermassive black holes grew to such intimidating sizes early in cosmic history.
"Intermediate-mass black holes are expected to consume stars, and they don’t have to be the center of the galaxies because they could have been kicked out of the center by a bigger black hole," Nicholl said. "LFCs could potentially be associated with intermediate-mass black holes, and if so, they would give us a new way to try to find and account for medium-sized black holes.
"This is probably the most important thing you can do in terms of trying to understand how supermassive black holes got to be so big."
The team has already made considerable progress on its LFC investigation, searching through archival data to find two "cold cases" that match AT2022aedm, indicating this class of powerful cosmic explosions has been seen before but was buried in data and likely missed.
The next step for Nicholl is to investigate globular clusters, which are incredibly dense groupings of stars that could provide conditions needed for small or medium black holes to destroy a star and fire off an LFC.
Even if this search is a success, the thrill of discovering something entirely new is unlikely to have diminished for the astrophysicist.
"We’ve been looking at the sky for a very long time, and sometimes people maybe think that we’ve seen all there is to see out there," Nicholl concluded. "I think things like this are really exciting because they remind us that the universe still has a lot of surprises in store, and when we build a new telescope, we will find new things, and that is going to help us to understand our universe better."
The team’s research was published on Sept. 1 in The Astrophysical Journal Letters.