In March 2021, a star in a galaxy 250 million light years away was seen having a horrible, horrible, not good, really, really bad day.
There it was, minding its own business, as it was sucked into the gravity well of a supermassive massif black hole, and torn to pieces. We know this because multiple telescopes saw it from Earth as the event’s light shone across the universe.
It’s the fifth closest such event — known as a tidal disturbance event — ever recorded, and the wealth of data recovered could help scientists better understand how black holes ‘feed’.
“Tid disturbances are a kind of cosmic laboratory”, says the astronomer Suvi Gezari of the Space Telescope Science Institute. “They are our window into real-time feeding of a massive black hole lurking at the center of a galaxy.”
Tidal disturbances are fairly rare, but we’ve seen enough of them to have a fairly detailed understanding of what happens when a star gets a little too close to a black hole. Once the star is trapped in the black hole’s gravitational field, tidal forces stretch and pull it to the point where it’s falling apart (that’s the “disorder” part).
The guts of the dismembered star then spill chaotically around the black hole, colliding with itself and creating shocks that glow in multiple wavelengths. This process is not instantaneous, but can take weeks or months as the black hole engulfs the stellar debris.
The debris forms a disk orbiting the black hole and falls (or “accretes”) onto it from the inner rim. When material falls onto the black hole, a structure called a corona can form between the inner edge of the accretion disk and the black hole’s event horizon.
This is a region of scorching hot electrons thought to be powered by the black hole’s magnetic field, which acts like a synchrotron to accelerate the electrons to energies so high that they glow brightly at X-ray wavelengths.
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Then, powerful jets of plasma launch from the black hole’s polar regions and shoot corona material in opposite directions, sometimes at almost the speed of light. These astrophysical jets are thought to form when material is accelerated along magnetic field lines outside the black hole’s event horizon; When it reaches the poles, it explodes.
Coronae and jets are not observed in all tidal disturbance events, but when they do occur they are usually seen together. When the Zwicky Transient Facility caught the bright flash of a tidal disturbance event later dubbed AT2021ehb on March 1, 2021, NASA turned around MORE BEAUTIFUL X-ray observatory and its Swift Observatory (X-rays, gamma rays, and ultraviolet rays) to watch the event unfold in hopes of catching something of interest. Later, 300 days after the Zwicky proof, the X-ray observatory NuSTAR joined the fun.
X-ray, ultraviolet, optical and radio light emitted by the event over a 430-day period showed that the culprit was a black hole about 10 million times the mass of the Sun. So far so normal.
But, well, something was strange. No evidence of jets had been detected by any of the observatories. However, NuSTAR’s observations indicated the presence of a corona. And the strange Discrepancy, scientists say, is tremendously exciting.
“We’ve never seen an X-ray emission tidal disturbance event like this without a jet, and it’s really spectacular because it means we can potentially unravel what’s causing jets and what’s causing coronae,” says astronomer Yuhan Yao by Caltech.
“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 is causing this magnetic field to become so strong.”
Targets like AT2021ehb are excellent laboratories for studying the formation and evolution of accretion disks and coronas in real time; and where there is one, there can be more. The researchers hope to find more such tidal disturbances in the future, leading to answers about the role magnetic fields play in the formation of coronas and jets.
A bad day for a star 250 million years ago turned into a very, very good day for human astronomers.
The research was published in The Astrophysical Journal.
