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Tidal Disruption Events

Earlier this year, astronomers were monitoring data from the Zwicky Transient Facility, an all-sky survey based at the Palomar Observatory in California, when they detected an extraordinary flash in a part of the sky where no such light had been observed the night before. From initial calculations, the flash appeared to give off more light than 1,000 trillion suns. 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. “They are a consequence of a star being ripped apart by a super massive black hole living in the center of a galaxy,” Dr Castro Segura said. 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. 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 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. Co-author Matteo Lucchini, from 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 actually be a really good probe for how that process happens.”

Feeding frenzy

Following AT 2022cmc’s initial discovery, MIT focused in on the signal using the Neutron star Interior Composition ExploreR (NICER), an X-ray telescope that operates aboard the International Space Station. Dheeraj “DJ” Pasham, co-author from MIT, said: “Things looked pretty normal the first three days. Then we looked at it with an X-ray telescope, and what we found was, the source was too bright.” He continued: "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. This particular event was 100 times more powerful than the most powerful gamma-ray burst afterglow. 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. The most remarkable property, 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. 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.

How could such a distant event appear so bright in our sky?

The team found that 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 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, the MIT team modeled the signal’s data, assuming the event involved a jet headed straight toward Earth. “We found that the jet speed is 99.99 percent the speed of light,” Lucchini said. “To produce such an intense jet, the black hole must be in an extremely active phase — a 'hyper-feeding frenzy'. Pasham estimates: “It’s probably swallowing the star at the rate of half the mass of the sun per year. “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.” Lucchini added: “We expect many more of these TDEs in the future. Then we might be able to say, finally, how exactly black holes launch these extremely powerful jets.”

Watch: Extraordinarily bright cosmic explosion

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