NASA gets an unusually close glimpse of a black hole devouring a wandering star

Observations may help scientists understand more complex black hole-feeding behaviors.

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A star getting too close to a massive black hole (MBH) can get disrupted by the tidal forces in a tidal disruption event (TDE). The first observational evidence for TDEs came from detecting X-ray flares from the centers of quiescent galaxies in 1990-1991.

Recently, multiple NASA telescopes observed a massive black hole tearing apart an unlucky star that wandered too close. It was the fifth-closest instance of a black hole obliterating a star, and it was situated around 250 million light-years away from Earth in the center of another galaxy.

Astronomers noticed a sharp increase in high-energy X-ray photons around the black hole after the star had been shattered by the black hole’s gravity. This demonstrated that the star material created a corona, a very hot structure above the black hole, as it was drawn into its destruction.

This event, called AT2021ehb, took place in a galaxy with a central black hole about 10 million times the mass of our Sun. During this TDE, the star was torn apart and reduced to nothing but a long noodle of hot gas because the side of the star closest to the black hole was tugged more strongly than the opposite side.

Because of the event’s close vicinity, NASA’s NuSTAR (Nuclear Spectroscopic Telescopic Array) satellite—the most sensitive space telescope capable of monitoring these wavelengths of light—got an unprecedented look into the corona’s development and evolution.

NASA said, “The work demonstrates how the destruction of a star by a black hole – a process formally known as a tidal disruption event – could be used to understand better what happens to material captured by one of these behemoths before it fully devoured.”

The event was first spotted on March 1, 2021, by the Zwicky Transient Facility (ZTF), located at the Palomar Observatory in Southern California. It was subsequently studied by NASA’s Neil Gehrels Swift Observatory and Neutron star Interior Composition Explorer (NICER) telescope (which observes longer X-ray wavelengths than Swift).

Around 300 days after the first discovery, NASA’s NuSTAR started monitoring the system. Since coronae typically form with jets of gas flowing opposite directions from a black hole, scientists were surprised when NuSTAR identified a corona – a cloud of hot plasma, or gas atoms with their electrons stripped away.

Scientists noted, “However, with the AT2021ehb tidal event, there were no jets, which made the corona observation unexpected. Coronae emit higher-energy X-rays than any other part of a black hole, but scientists don’t know where the plasma comes from or exactly how it gets so hot.”

Yuhan Yao, a graduate student at Caltech in Pasadena, California, said“We’ve never seen a tidal disruption event with X-ray emission like this without a jet present, and that’s spectacular because it means we can potentially disentangle what causes jets and what causes coronae. Our observations of AT2021ehb are in agreement with the idea that magnetic fields have something to do with how the corona forms, and we want to know what’s causing that magnetic field to get so strong.”

According to scientists, during such events, the stream of gas is driven around a black hole and collides with it. This is believed to produce shock waves and outward flows of gas that produce visible light, as well as light at wavelengths that are invisible to the human eye, like X-rays and ultraviolet light.

The material then starts to settle into a disk rotating around the black hole like water circling a drain, with friction generating low-energy X-rays. In the case of AT2021ehb, this series of events took place over just 100 days.

Journal Reference:

  1. Yuhan Yao, Wenbin Lu et al. The Tidal Disruption Event AT2021ehb: Evidence of Relativistic Disk Reflection, and Rapid Evolution of the Disk–Corona System. The Astrophysical Journal. DOI 10.3847/1538-4357/ac898a