Black hole observed ripping apart a star

An international team of astronomers has determined the source of an incredibly bright X-ray, optical and radio signal appearing from halfway across the universe, claiming it is a luminous jet of matter streaking out from a supermassive black hole at close to the speed of light.

The event, named AT 2022cmc, was discovered earlier this year by the Zwicky Transient Facility in California. The researchers 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, published in the journal Nature Astronomy, could shed new light on how supermassive black holes feed and grow.

When a star gets too close to a supermassive black hole, the star is violently ripped apart by tidal forces, with pieces drawn into orbit around the black hole and eventually completely consumed by it. In rare instances (around 1% of the time), these so-called tidal disruption events (TDEs) also launch luminous jets of material moving almost at the speed of light. AT 2022cmc is brighter than any TDE discovered to date, and is also the furthest TDE ever detected — around 8.5 billion light years away, or more than halfway across the observable universe — according to measurements from the European Southern Observatory’s Very Large Telescope in Chile.

“Our spectrum told us that the source was hot: around 30,000 degrees, which is typical for a TDE,” said Dr Matt Nicholl from the University of Birmingham. “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!”

So how could such a distant event appear so bright in our sky? The team says the black hole’s jet may be pointing directly towards 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, the first such event that has been observed since 2011 and the first boosted TDE discovered using an optical sky survey.

Video credit: Dheeraj Pasham (MIT), Matteo Lucchini (MIT) and Margaret Trippe.

Following its 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. As explained by first author Dheeraj Pasham, from Massachusetts Institute of Technology (MIT), bright flashes in the sky are typically gamma-ray bursts — extreme jets of X-ray emissions that spew from the collapse of massive stars — but the source of this signal was 100 times more powerful than the most powerful gamma-ray burst afterglow.

“As bright as they are, there is only so much light a collapsing star can produce,” said Birmingham’s Dr Benjamin Gompertz, who led the gamma-ray burst comparison analysis. “Because AT 2022cmc was so bright and lasted so long, we knew that something truly gargantuan must be powering it — a supermassive black hole.”

AT2022cmc is believed to be at the centre of a galaxy that is not yet visible because the intense light from the flash still outshines it; future observations may unveil the galaxy when the signal eventually fades away. The extreme X-ray activity is meanwhile 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.

“[The black hole is] probably swallowing the star at the rate of half the mass of the sun per year,” Pasham said. “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.”

It is still a mystery why some TDEs launch jets while others do not appear to. From their observations, the researchers concluded that the black holes associated with AT2022cmc and other similarly jetted TDEs are likely spinning rapidly. MIT co-author Matteo Lucchini said the team expects to encounter more of these TDEs in the future as more powerful telescopes come online: “Then we might be able to say, finally, how exactly black holes launch these extremely powerful jets.”

Image caption: A supermassive black hole rips apart a star, causing a bright optical flare to emerge. Image credit: Carl Knox (OzGrav, ARC Centre of Excellence for Gravitational Wave Discovery, Swinburne University of Technology).

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