Astronomers capture black hole engulfing star in ‘overfeeding frenzy’

Earlier this year, astronomers detected an unusually bright signal in X-ray, optical and radio modes called AT 2022cmc. They have already determined that the most likely source of this signal is a supermassive black hole engulfing a star in a “superfeeding frenzy,” shooting out jets of matter in what’s known as a tidal disruption event (TDE). According to new paper published in the journal Nature Astronomy, it is one of the record books: the the most distant such event still discovered approximately 8.5 billion light years away.

The authors estimate that the jet from this TDE is traveling at 99.99 percent of the speed of light, meaning the black hole is indeed munching on its stellar snack. “Probably devouring the star at the rate of half the mass of the Sun per year,” said co-author Dheeraj “DJ” Pasham from the University of Birmingham. “Many of these tidal disruptions happen early, and we were able to catch this event at the very beginning, within a week of the black hole starting to feed on the star.”

As do we reported earliera popular misconception is that black holes behave like space vacuum cleaners, voraciously sucking up any matter in their surroundings. In reality, only things that pass beyond the event horizon – including light – are absorbed and cannot escape, although black holes are also messy. This means that some of the matter of an object is ejected in a powerful jet.

If that object is a star, the process of being shredded (or “spaghettified”) by the powerful gravitational forces of a black hole occurs outside the event horizon, and some of the star’s original mass is violently ejected outwards. This in turn can form spinning ring of fabric (also known as an accretion disk) around the black hole, which emits powerful X-rays and visible light, and sometimes radio waves. Physicist John Wheeler once described TDE jets as “a tube of toothpaste squeezed tightly around its middle” so that matter spews out from both ends. TDEs are one way astronomers can indirectly infer the presence of a black hole.

For example, in 2018, astronomers announced the first direct image of the effects of a star being torn apart by a black hole 20 million times more massive than our Sun in a pair of colliding galaxies called Arp 299, about 150 million light-years from Earth. A year later, astronomers recorded final death agony of a star ripped apart by a supermassive black hole called AT 2019qiz, which provided the first direct evidence that gas escaping during collapse and accretion produces the powerful optical and radio emissions previously observed. In January, astronomers spotted a second radio-mode TDE candidate (called J1533+2727) in archive data collected by the Very Large Array (VLA) telescope in New Mexico.

Astronomers first spotted AT 2022cmc in February and immediately reversed multiple telescopes operating over a wide range of wavelengths to the source. These include an X-ray telescope aboard the International Space Station called the Neutron Star Interior Composition Explorer (NICER). It is possible that the bright signal – calculated to be equivalent to the light of 1,000 trillion suns – is a burst of gamma rays from the collapse of a massive star. But the data revealed a source 100 times more powerful than even the strongest known gamma-ray burst.

“Our spectrum told us that the source is hot: about 30,000 degrees, which is typical of TDEs,” said co-author Matt Nichol from the University of Birmingham. “But we also saw some absorption of light from the galaxy where this event happened. These absorption lines were strongly red-shifted, which tells us that this galaxy is much further away than we expected.’

Given the brightness of AT 2022cmc and its longer duration, astronomers concluded that it must be powered by a supermassive black hole. X-ray data also point to an “episode of extreme accretion”. A vortex of debris then forms as the unfortunate star falls into the black hole. But the brightness was still a surprise given how far the source is from Earth. The authors attribute this to so-called “Doppler amplification,” which occurs when the jet points directly toward Earth, similar to how the sound of a passing siren is amplified. AT 2022cmc is only the fourth Doppler-enhanced TDE discovered so far; the last one was opened in 2011.

DOI: Nature Astronomy, 2022 10.1038/s41550-022-01820-x (About DOI).