A star that gets too close to a massive black hole (MBH) can be disturbed by the tidal forces during a tidal disturbance (TDE). The first observational evidence for TDEs came from detecting X-ray bursts in the centers of dormant galaxies in 1990-1991.

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

Astronomers saw a sharp increase in high-energy X-rays around the black hole after the star was shattered by the black hole’s gravity. This showed that the star material created a corona, a very hot structure above the black hole, as it was pulled into its destruction.

This event, called AT2021ehb, occurred in a galaxy with a central black hole about 10 million times the mass of our sun. During this TDE, the star was ripped apart and reduced to nothing but a long noodle of hot gas because the side of the star closest to the black hole was pulled more strongly than the other side.

Due to its proximity to the event, NASA’s NuSTAR (Nuclear Spectroscopic Telescopic Array) satellite – the most sensitive space telescope capable of tracking these wavelengths of light – got an unprecedented glimpse into the development and evolution of the corona.

NASA said, “The work shows how the destruction of a star by a black hole – a process formally known as a tidal disturbance – can be used to better understand what happens to material captured by one of these behemoths before it becomes 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 then studied by NASA’s Neil Gehrels Swift Observatory and Neutron star Interior Composition Explorer (NICER) telescope (which observes longer X-ray wavelengths than Swift).

About 300 days after the initial discovery, NASA’s NuSTAR began monitoring the system. Since coronas usually form with jets of gas flowing in 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 removed.

Scientists noted, “However, at the tidal event AT2021ehb, there were no jets, so the corona sighting was unexpected. Coronae emit X-rays with higher energy 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 have never seen a tidal disturbance with such an emission of X-rays without a jet present, and that is spectacular because it means we can potentially untangle what causes jets and what causes coronae. Our observations of AT2021ehb are consistent with the idea that magnetic fields have something to do with how the corona forms, and we want to know what makes that magnetic field so strong.”

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

The material then begins to settle in a disk that revolves around the black hole like water flowing down a drain, with friction generating low-energy X-rays. In the case of AT2021ehb, this chain of events occurred over just 100 days.

Magazine 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