“Astronomers have determined the source of an incredibly bright X-ray, optical and radio signal appearing from halfway across the Universe.
The signal, named AT 2022cmc, was discovered earlier this year by the Zwicky Transient Facility in California. Findings published today in Nature Astronomy, suggest that it is likely from a jet of matter, streaking out from a supermassive black hole at close to the speed of light.
The team, including researchers from MIT and the University of Birmingham, believe the jet is the product of a black hole that suddenly began devouring a nearby star, releasing a huge amount of energy in the process. Their findings could shed new light on how supermassive black holes feed and grow.
Astronomers have observed other such “tidal disruption events,” or TDEs, in which a passing star is torn apart by a black hole’s tidal forces. However AT 2022cmc is brighter than any TDE discovered to date, and is also the farthest TDE ever detected, at some 8.5 billion light years away.
The team measured the distance to the AT 2022cmc using the European Southern Observatory’s Very Large Telescope, in Chile.
Dr Matt Nicholl, associate professor at the University of Birmingham, said: “Our spectrum told us that the source was hot: around 30,000 degrees, which is typical for a TDE. But we also saw some absorption of light by the galaxy where this event occurred. These absorption lines were highly shifted towards redder wavelengths, telling us that this galaxy was much further away than we expected!”
How could such a distant event appear so bright in our sky? The team says the black hole’s jet may be pointing directly toward Earth, making the signal appear brighter than if the jet were pointing in any other direction. The effect is “Doppler boosting,” and is similar to the amped-up sound of a passing siren.
AT 2022cmc is the fourth Doppler-boosted TDE ever detected and the first such event that has been observed since 2011. It is also the first boosted TDE discovered using an optical sky survey. As more powerful telescopes start up in the coming years, they will reveal more TDEs, which can shed light on how supermassive black holes grow and shape the galaxies around them.
Following AT 2022cmc’s initial discovery, the team focused in on the signal using the Neutron star Interior Composition ExploreR (NICER), an X-ray telescope that operates aboard the International Space Station.
“Things looked pretty normal the first three days,” recalls Dheeraj “DJ” Pasham, who was first author on the study. “Then we looked at it with an X-ray telescope, and what we found was, the source was 100 times more powerful than the most powerful gamma-ray burst afterglow.”
Typically, such bright flashes in the sky are gamma-ray bursts — extreme jets of X-ray emissions that spew from the collapse of massive stars.
Dr Benjamin Gompertz, assistant professor at the University of Birmingham, led the gamma-ray burst comparison analysis. “Gamma-ray bursts are the usual suspects for events like this.” he said. “However, as bright as they are, there is only so much light a collapsing star can produce. Because AT 2022cmc was so bright and lasted so long, we knew that something truly gargantuan must be powering it – a supermassive black hole.”
The extreme X-ray activity is believed to be powered by an “extreme accretion episode” when the shredded star creates a whirlpool of debris as it falls into the black hole. Indeed, the team found that AT 2022cmc’s X-ray luminosity was comparable to, though brighter than, three previously detected TDEs.
“It’s probably swallowing the star at the rate of half the mass of the sun per year,” Pasham estimates. “A lot of this tidal disruption happens early on, and we were able to catch this event right at the beginning, within one week of the black hole starting to feed on the star.”
“We expect many more of these TDEs in the future,” co-author Matteo Lucchini adds. “Then we might be able to say, finally, how exactly black holes launch these extremely powerful jets.”
Dr Graham Smith, Reader in Cosmology at the University of Birmingham, said: “The brightness of AT2022cmc caused excitement that it might be gravitationally lensed, making it the first ever discovery of a gravitationally lensed TDE, whilst we have ruled out this exciting interpretation for this particular object we are looking forward to many such discoveries with the upcoming Vera C. Rubin Observatory”.
Other Birmingham scientists who contributed to this paper were Dr Samantha Oates, and PhD researchers Aysha Aamer, Evan Ridley and Xinyue Sheng.”