When a star gets too close to the event horizon of a supermassive black hole, powerful tidal surges
March 1, 2021 during the automatedFrom the Zwicky Transient Facility all-sky survey, researchers detected the tidal disruption event AT2021ehb for the first time. Its source was a supermassive black hole at the center of a galaxy located just 250 million light years from Earth. Astrophysicists began observing almost at the very beginning of the destruction, when it had not yet reached its peak, and studied this event for more than a year.
What happened?
Since the discovery of AT2021ehb, severaltelescopes have observed what happens when a star gets too close to its black hole. This tidal disruption event took place in a galaxy with a central black hole about 10 million times the mass of the Sun.
A system located just 250 million lightyears from Earth, became the fifth farthest known example of the destruction of a star by a supermassive black hole. But, unlike closer events, AT2021ehb was discovered quite early and was studied for the first time simultaneously in the X-ray, ultraviolet, optical and radio ranges.
How does tidal disruption happen?
When a star comes too close to blackhole, strong gravity stretches the star until it becomes a long river of hot gas. The gas flow swirls around the black hole, and its particles collide with each other.
It is believed that the collision of gas creates shockwaves and outward gas flows that generate radiation both in visible light and in other wavelength ranges (ultraviolet radiation and x-rays).
As the tidal disruption event progressesthe material begins to settle in a disk orbiting the black hole like water spinning through a sewer, with the friction generating low-energy X-rays. In the case of AT2021ehb, this entire series of events from the start of spaghettification of the star to the formation of the disk occurred in just 100 days.
What have astrophysicists learned?
Approximately 300 days after the eventtidal disruption was identified for the first time, astrophysicists began using NASA's NuSTAR (Nuclear Spectroscopic Telescopic Array) satellite to observe the black hole. This is the most X-ray sensitive space telescope.
Satellite discovers corona cloud around black holehot plasma left over from the star. This was extremely unusual, since the formation of a corona is usually accompanied by ejections of jets or relativistic jets - streams of plasma escaping from the center of a black hole. However, there were no jets in AT2021ehb.
The authors of the study published on thisweek in the Astrophysical Journal note that coronas emit higher-energy X-rays than any other part of a black hole, but astrophysicists still don't know where the plasma comes from or exactly how it gets so hot.
We have never seen such a tidal eventdestruction with X-rays like this, without the presence of a jet, and that's really impressive because it means that we can potentially separate what causes jets and what causes coronas.
Yuhan Yao, graduate student at Caltech Pasadena and lead author of the study
The researchers note that current observationssuggest that the formation of a black hole's corona "has something to do with magnetic fields." They hope that future observations will help explain the physics of this process.
What is interesting about tidal disruption?
Most supermassive black holes are surrounded byhot gas that has accumulated over thousands and millions of years. It is extremely difficult to study these processes. Tidal disruption events occur near black holes but last only a few weeks or months. At the same time, they are much brighter than individual particles of dust and gas that form a disk near a black hole.
Brightness and short duration of eventstidal disruption is helping astrophysicists learn in a limited amount of time how the gravity of a black hole controls matter particles, what black holes are, and what physics is at work at the event horizon.
Tidal disruption is a kind of space laboratory. They are our real-time window into the massive black hole lurking at the center of the galaxy.
Suvi Gezari, astronomer at the Space Telescope Science Institute in Baltimore and co-author of the study
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On the cover: Artistic illustration of the AT2021ehb tidal disruption event. Image: NASA/JPL-Caltech