Tanmoy Laskar
Mysterious bright flash is a black hole jet pointing straight at Earth.
Earlier this year, astronomers at the Palomar Observatory detected an extraordinary flash in a part of the sky where no such light had been observed the night before. From a rough calculation, the flash appeared to give off more light than 1,000 trillion suns.
The team, led by researchers at NASA, Caltech, and elsewhere, posted their discovery to an astronomy newsletter, where the signal drew the attention of astronomers around the world, including scientists at MIT and the University of Utah. Over the next few days, multiple telescopes focused in on the signal to gather more data across multiple wavelengths in the X-ray, ultraviolet, optical and radio bands, to see what could possibly produce such an enormous amount of light.
Now, the U and MIT astronomers and collaborators have determined a likely source for the signal. Tanmoy Laskar, Assistant Professor in the Department of Physics and Astronomy at the U, was co-author of a study that appeared on Nov. 30 in Nature Astronomy. The scientists report that the signal, named AT 2022cmc, likely comes from a relativistic jet of matter launched by a supermassive black hole at close to the speed of light. They 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.
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. AT 2022cmc is brighter than any TDE discovered to date. The source is also the farthest TDE ever detected, at some 8.5 billion lights years away—more than halfway across the universe.
Palomar Observatory
How could such a distant event appear so bright in our sky? The team said 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 called “Doppler boosting.”
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 TDE discovered using an optical sky survey.
“One of the tell-tale signatures of the presence of such a jet is powerful radio emission from a small volume of space,” said Laskar. A preliminary report alerted the team that this event might have detectable radio emissions. “So, we followed it up with the Karl G. Jansky Very Large Array in New Mexico, and boom, there it was! Bright radio emission signaling a compact, Doppler-boosted jet.”
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.
“We know there is one supermassive black hole per galaxy, and they formed very quickly in the universe’s first million years,” said co-author Matteo Lucchini, a postdoc in MIT’s Kavli Institute for Astrophysics and Space Research. “That tells us they feed very fast, though we don’t know how that feeding process works. So, sources like a TDE can actually be a really good probe for how that process happens.”
Feeding frenzy
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,” recalled the study’s lead author Dheeraj “DJ” Pasham, who is an Einstein Fellow at MIT. “Then we looked at it with an X-ray telescope, and what we found was, the source was too bright.”
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.
“Both GRBs and TDEs are events that have superfast jets pointed at Earth,” said Laskar. “One of the key ways to distinguish between them is in the X-rays. Jetted TDEs seem to also have strongly variable X-ray emission.” Indeed, the team found that X-ray emissions from AT 2022cmc swung widely by a factor of 500 over a few weeks.
The team then gathered observations from other X-ray, radio, optical and UV telescopes and tracked the signal’s activity over the next few weeks. Another remarkable property they observed was the signal’s extreme luminosity in the X-ray band.
“This particular event was 100 times more powerful than the most powerful gamma-ray burst afterglow,” Pasham said. “It was something extraordinary.”
They suspected that such extreme X-ray activity must be powered by an extreme accretion episode—an event that generates a huge churning disk, such as from a tidal disruption event, in which a shredded star creates a whirlpool of debris as it falls into a black hole.
The team found that AT 2022cmc’s X-ray luminosity was comparable to, though brighter than, three previously detected jetted TDEs. These bright events happened to generate jets of matter pointing straight toward Earth. The researchers wondered: If AT 2022cmc’s luminosity is the result of a similar Earth-targeting jet, how fast must the jet be moving to generate such a bright signal? To answer this, Lucchini modeled the signal’s data, assuming the event involved a jet headed straight toward Earth.
“We found that the jet speed is 99.99% the speed of light,” Lucchini said.
To produce such an intense jet, the black hole must be in an extremely active phase—what Pasham described as a “hyper-feeding frenzy.”
“It’s probably swallowing the star at the rate of half the mass of the sun per year,” Pasham estimated. “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,” Lucchini added. “Then we might be able to say, finally, how exactly black holes launch these extremely powerful jets.”
“When the next TDE is discovered, we will again be ready to catch its light from X-rays to radio waves,” Laskar said. “By combining such data with physical models, we hope to build a full picture of how supermassive black holes at the centers of galaxies grow, evolve, and shape their environments over cosmic time.”
