An extraordinary flash from a black hole pointing straight toward Earth

The observations could illuminate how supermassive black holes feed and grow.

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A black hole can launch a powerful relativistic jet after it tidally disrupts a star. If this jet fortuitously aligns with our line of sight, the brightness is Doppler boosted by several orders of magnitude. Consequently, such on-axis relativistic tidal disruption events have the potential to unveil cosmological quiescent black holes. They are ideal test beds for understanding the radiative mechanisms operating in super-Eddington jets.

Earlier this year, the team- led by researchers at NASA, Caltech, and elsewhere- reported the detection of an extraordinary flash in a part of the sky where no such light had been observed the night before. For more details, multiple telescopes focused on the signal to gather more data across multiple wavelengths in the X-ray, ultraviolet, optical, and radio bands to see what could produce such an enormous amount of light.

Now, MIT astronomers have determined a likely source for the signal. In a new study, they reported that the signal, named AT 2022cmc, originated from a relativistic jet of matter streaking out from a supermassive black hole at close to the speed of light.

The brightness of AT 2022cmc exceeds that of any TDE found to date. The source is also the farthest TDE yet discovered, located more than halfway across the universe at a distance of 8.5 billion light-years. It is the fourth Doppler-boosted TDE ever detected. What’s more, it is also the first TDE discovered using an optical sky survey.

Astronomers noted, “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.”

Co-author Matteo Lucchini, a postdoc at MIT’s Kavli Institute for Astrophysics and Space Research, said, “We know there is one supermassive black hole per galaxy, and they formed very quickly in the universe’s first million years. That tells us they feed very fast, though we don’t know how that feeding process works. So, sources like a TDE can be a good probe for how that process happens.”

After the initial discovery of AT 2022cmc, astronomers focused on the signal using neutron star Interior Composition ExploreR (NICER). What they found was the source was too bright. In particular, the event was 00 times more powerful than the most powerful gamma-ray burst afterglow.

Research Scientist Dheeraj “DJ” Pasham said, “It was extraordinary.”

Later, astronomers gathered observations from other X-ray, radio, optical, and UV telescopes and tracked the signal’s activity over the next few weeks. What they observed was: the signal’s extreme luminosity in the X-ray band. They found that X-ray emissions from AT 2022cmc swung widely by a factor of 500 over a few weeks.

Astronomers noted“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.”

“Indeed, AT 2022cmc’s X-ray luminosity was comparable to, though brighter than, three previously detected TDEs. These bright events happened to generate jets of matter pointing straight toward Earth. The researchers wondered: If AT 2022cmc’s luminosity results from 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.”

The jet speed is 99.99 percent the speed of light, reported astronomers. And to generate such jets, a black hole needs to be in an extremely active phase- what Pasham describes as a “hyper-feeding frenzy.”

Journal Reference:

  1. Pasham, D.R., Lucchini, M., Laskar, T. et al. The Birth of a Relativistic Jet Following the Disruption of a Star by a Cosmological Black Hole. Nat Astron (2022). DOI: 10.1038/s41550-022-01820-x
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