NASA gets an unusually close glimpse of a black hole snacking on stars

A disk of hot gas orbits a black hole in this illustration. Some of the gas came from a star that was ripped apart by the black hole, forming a long stream of hot gas on the right, feeding the disk. These events are formally known as tidal disturbance events, or TDEs. It can take just a matter, weeks or months from the destruction of the star to the formation of the disk. Gas gets hotter the closer it gets to the black hole, but hotter material can be found above the black hole. This hotter substance is a cloud of plasma (gas atoms with their electrons stripped) known as the corona. Most of the TDEs that lead to corona formation also produce jets of material that are blasted into space away from the black hole at its poles. A TDE named AT2021ehb is the first confirmed example of halo formation without jets in a tidal disturbance event. Observing AT2021ehb allows scientists to study the formation of the jets and corona separately. Credit: NASA/JPL-Caltech

Recent observations of a black hole devouring a wandering star may help scientists understand the black hole’s more complex feeding behaviors.

Several NASA telescopes recently noticed a supermassive black hole tearing apart an unlucky star that was wandering too close. Located about 250 million light-years from Earth in the center of another galaxy, it was the fifth-closest example of a star-destroying black hole ever observed.

Once the star was completely torn apart by the black hole’s gravity, astronomers saw a spike in high-energy X-ray light around the black hole. This indicates that as stellar matter was pulled toward its destruction, it formed a superheated structure on top of the black hole called the corona.

NASA’s NuSTAR (Nuclear Telescope Spectroscopic Array) satellite is the most sensitive space telescope capable of observing these wavelengths of light, and its proximity to the event provided unprecedented insight into the formation and evolution of the corona, according to a new study published in The Guardian. Astrophysical Journal.

The work shows how the destruction of a star by a black hole – a process formally known as a tidal perturbation event – can be used to better understand what happens to matter captured by one of these behemoths before being completely devoured.

Most black holes that scientists can study are surrounded by hot gas that has accumulated over many years, sometimes thousands of years, forming disks billions of miles wide. In some cases, these disks shine brighter than entire galaxies. Even around these bright sources, but especially around less energetic black holes, one star stands out that gets ripped apart and consumed.

And from start to finish, the process often takes only weeks or months. The observability and short duration of tidal disturbance events make them particularly attractive to astronomers, who can decipher how a black hole’s gravity manipulates the material around it, creating stunning light shows and new physical properties.

“Tidal disturbance events are a kind of cosmic laboratory,” said study co-author Suvi Gezari, an astronomer at the Space Telescope Science Institute in Baltimore. “It is our window into the real-time feeding of a supermassive black hole lurking at the center of a galaxy.”

When a star wanders too close to a black hole, the intense gravity will stretch the star until it becomes a long river of hot gas, as shown in this animation. The gas is then slammed around the black hole and gradually pulled into orbit, forming a bright disk. Credit: Science Communication Lab/DESY

sudden signal

The new study focuses on an event called AT2021ehb, which occurred in a galaxy with a central black hole about 10 million times the mass of our Sun (about the difference between a bowling ball and the Titanic). During this tidal disruption event, the side of the star closest to the black hole was pulled in more forcefully than the other side of the star, pulling everything apart and leaving only a long filament of hot gas.

Scientists believe that the stream of gas fluctuates around the black hole during such events, colliding with itself. This is thought to create shock waves and an outflow of gas that generates visible light, as well as wavelengths invisible to the human eye, such as ultraviolet and X-rays. The matter then begins to settle into a disk that spins around the black hole like water spins around a drain, with friction generating low-energy X-rays. In the case of AT2021ehb, this series of events occurred over the course of only 100 days.

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

Then, about 300 days after the event was first detected, NASA’s NuSTAR began monitoring the system. Scientists were surprised when NuSTAR detected a halo — a cloud of hot plasma, or atoms of gas with their electrons stripped — because a corona usually appears with jets of gas shooting in opposite directions from a black hole.

However, with the AT2021ehb tidal event, there were no aircraft, which made observing the corona unpredictable. The Coronae emits more high-energy X-rays than any other part of the black hole, but scientists don’t know where the plasma comes from or exactly how it gets so hot.

“We’ve never seen a tidal disturbance event with X-ray emission like this without a jet, and that’s really amazing because it means we can separate what causes the jets and what causes the corona,” said Yuhan Yao, a graduate student at Yuhan Yao University. Caltech in Pasadena, Calif., and lead author of the new study. “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 this magnetic field so strong.”

Yao is also leading an effort to search for more tidal disturbance events identified by the ZTF and then monitor them with telescopes such as Swift, NICER, and NuSTAR. Each new observation provides the potential for new insights or opportunities to confirm what has been observed in AT2021ehb and other tidal disturbance events. “We want to find as many as possible,” Yao said.

more information:
Yuhan Yao et al., Tidal Disturbance Event AT2021ehb: Evidence for Relative Disc Reversal, and Rapid Evolution of the Disc-Corona System, Astrophysical Journal (2022). DOI: 10.3847/1538-4357/ac898a

Provided by JPL

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