Astronomers just observed a black hole devouring a passing star up close: ScienceAlert

In March 2021, a star in a galaxy 250 million light years away was seen having a terrible, horrible, not good, very bad day.

It was there, minding its own business, when it was sucked into the gravity well of a supermassive black hole, and ripped into a thousand pieces. We know this because several telescopes sighted it from Earth, as light from the event spread across the Universe.

It’s the fifth closest such event – known as a tidal disturbance event – on record, and the wealth of data obtained could help scientists better understand how black holes ‘feed’ .

“Tidal disturbance events are a kind of cosmic laboratory,” says astronomer Suvi Gezari of the Space Telescope Science Institute. “They are our window into the real-time feed of a massive black hole lurking at the center of a galaxy.”

Tidal disturbance events are fairly rare, but we’ve seen enough of them to get a fairly detailed understanding of what happens when a star veers a bit too close to a black hole. Once the star is caught in the gravitational field of the black hole, tidal forces stretch it and pull it to the point that it separates (this is the “disturbance” part).

The dismembered star’s innards then circle the black hole chaotically, colliding with itself and generating shocks that glow in multiple wavelengths. This process is not instantaneous, but can take weeks or months and the black hole devours the stellar debris.

The debris forms a disk orbiting the black hole, falling (or “accreting”) onto it from the inner edge. When matter falls on 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.

It is a region of burning electrons thought to be powered by the magnetic field of the black hole, which acts as a synchrotron to accelerate electrons to energies so high that they glow brightly in d x-ray wave.

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Then, powerful plasma jets are launched from the black hole’s polar regions, blasting corona material in opposite directions, sometimes near the speed of light. These astrophysical jets are thought to form when matter is accelerated along magnetic field lines outside the black hole’s event horizon; when it reaches the poles, it explodes.

Coronas and jets are not observed for all tidal disturbance events, but when they do occur they are usually observed together. So when the Zwicky Transient Facility picked up the bright flash of a tidal disturbance event on March 1, 2021, subsequently named AT2021ehb, NASA transformed its NICER X-ray Observatory and Swift Observatory (X-ray , gamma rays and ultraviolet radiation) to observe the evolution of the event in the hope of catching something interesting. Later, 300 days after Zwicky’s detection, the NuSTAR X-ray Observatory joined in the fun.

X-rays, ultraviolet, optical and radio rays emitted from the event over a 430-day period revealed the culprit to be a black hole about 10 million times the mass of the Sun. So far quite normal.

But hey, something was weird. No evidence of jets had been detected by any of the observatories. Yet NuSTAR observations revealed the presence of a corona. And this strange discrepancy, say the scientists, is extremely exciting.

“We’ve never seen a tidal disturbance event with an X-ray emission like this without a jet present, and that’s really spectacular because it means we can potentially sort out what’s causing the jets and what’s causes the coronas,” says astronomer Yuhan Yao of Caltech.

“Our observations of AT2021ehb are consistent with the idea that magnetic fields have something to do with corona formation, and we want to know what makes this magnetic field so strong.”

Targets like AT2021ehb provide 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. Researchers hope they will be able to find more such tidal disturbance events 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 ended up being a very, very good day for human astronomers.

The research has been published in The Astrophysical Journal.

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