A disc of hot gas swirls around a black hole in this picture. The black hole tore apart some of the gas from a star. This created a long stream of hot gas on the right, which flowed into the disc. These things are called TDEs, which stand for “tidal disruption events.” The star’s death to the disc’s birth can happen in just a few weeks or months.
The gas gets hotter as it gets closer to the black hole, but the material above it is the hottest. This hottest is a cloud of plasma made up of gas atoms that have lost their electrons. This cloud is called a corona. Most TDEs that cause a corona to form also cause jets of material to shoot away from the black hole’s poles and into space. A tidal disruption event (TDE) called AT2021ehb is the first confirmed case of a corona forming without jets.
Scientists may be able to learn more about how black holes eat by looking at how they ate a wandering star recently.
Recently, several NASA telescopes saw a huge black hole ripping apart a star that got too close. It was in the center of another galaxy, about 250 million light-years from Earth. It was the fifth-closest time a black hole was seen destroying a star.
Astronomers saw a sharp rise in high-energy X-ray light around the black hole once the star had been completely torn apart by the black hole’s gravity. As the star’s matter was pulled toward its death, it formed a corona structure above the black hole.
NASA’s NuSTAR (Nuclear Spectroscopic Telescopic Array) satellite is the most sensitive space telescope that can see these wavelengths of light. The event’s closeness gave scientists their best look yet at how the corona forms and changes, according to a new study in the Astrophysical Journal.
The work shows how a black hole’s destruction of a star, called a “tidal disruption event,” could be used to learn more about what happens to matter that gets sucked into one of these monsters before it’s completely eaten.
Most black holes that scientists can study are surrounded by huge discs of hot gas that have been building up for years or even millennia. Some of these discs shine brighter than galaxies as a whole. Even near these bright sources, but especially near black holes that are much less active, a single star being torn apart and eaten stands out.
And the whole thing usually doesn’t take more than a few weeks or months. Astronomers are especially interested in tidal disruption events because they can be seen and last only a short time. This lets them figure out how the black hole’s gravity affects the matter around it, creating amazing light shows and new physical features.
Suvi Gezari, an astronomer at the Space Telescope Science Institute in Baltimore and co-author of the study, said, “Tidal disruption events are like a laboratory in space.” “They show us how a huge black hole at the centre of a galaxy is getting food in real-time.”
When a star gets too close to a black hole, the strong gravity will stretch the star out until it becomes a long river of hot gas, as this animation shows. The gas is then whipped around the black hole and slowly pulled into orbit, making a bright disc.
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A Surprising Signal
The focus of the new study is an event called AT2021ehb, which happened 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 harder than the side farthest from the black hole. This pulled the star apart and left a long, hot gas noodle.
Scientists think that when these things happen, a gas stream gets whipped around a black hole and crashes into itself. This is thought to make shock waves and outward flows of gas that create visible light and wavelengths that can’t be seen by the human eye, like ultraviolet light and X-rays. Then, the matter starts to fall into a disc that spins around the black hole like water around a drain. Friction between the disc and the black hole makes low-energy X-rays. In the case of AT2021ehb, all of these things happened in only 100 days.
The Zwicky Transient Facility (ZTF), at the Palomar Observatory in Southern California was the first to notice the event on March 1, 2021. After that, the Neil Gehrels Swift Observatory and Neutron star Interior Composition Explorer (NICER) telescopes at NASA looked at it (which observes longer X-ray wavelengths than Swift).
Then, about 300 days after the first sign of the event, NASA’s NuSTAR started watching the system. Scientists were surprised when NuSTAR found a corona, a cloud of hot plasma or gas atoms that have lost their electrons. Usually, coronae appear with jets of gas that move in opposite directions from a black hole.
But there were no jets during the AT2021ehb tidal event, which made the corona observation a surprise. Coronae give off more powerful X-rays than any other part of a black hole, but scientists don’t know where the plasma comes from or how it gets so hot.
“We’ve never seen a tidal disruption event with X-ray emission like this without a jet,” said Yuhan Yao, a graduate student at Caltech in Pasadena, California, and the lead author of the new study. “That’s exciting because it means we might be able to figure out what makes jets and coronae,” he said. “Our observations of AT2021ehb support 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.”
Yao is also in charge of a project to look for more ZTF-identified tidal disruption events and then watch them with telescopes like Swift, NICER, and NuSTAR. Each new observation gives us a chance to learn something new or confirm what we already know about AT2021ehb and other tidal disruption events. Yao said, “We want to find as many as possible.”
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