Black hole observed spraying plasma into space


Thursday, 02 May, 2019



Black hole observed spraying plasma into space

Astronomers have discovered rapidly swinging jets coming from a black hole almost 8000 light-years from Earth — behaviour that has never before been observed on such short timescales.

The jets appear to be rapidly rotating with high-speed clouds of plasma — potentially just minutes apart — shooting out of V404 Cygni’s black hole in different directions. The research has been published in the journal Nature.

“This is one of the most extraordinary black hole systems I’ve ever come across,” said lead author Associate Professor James Miller-Jones, from the Curtin University node of the International Centre for Radio Astronomy Research (ICRAR).

“Like many black holes, it’s feeding on a nearby star, pulling gas away from the star and forming a disk of material that encircles the black hole and spirals towards it under gravity.

“What’s different in V404 Cygni is that we think the disk of material and the black hole are misaligned.”

V404 Cygni was first identified as a black hole in 1989 when it released a big outburst of jets and radiation. Astronomers looking at archival photographic plates then found previous outbursts in observations from 1938 and 1956.

Assoc Prof Miller-Jones said that when V404 Cygni experienced another very bright outburst in 2015, lasting for two weeks, telescopes around the world tuned in to study what was going on. Yet when he and his team studied the black hole, they saw its jets behaving in a way never seen before.

Where jets are usually thought to shoot straight out from the poles of black holes, these jets were shooting out in different directions at different times. And they were changing direction very quickly — over no more than a couple of hours.

Assoc Prof Miller-Jones said the change in the movement of the jets was because of the accretion disk — the rotating disk of matter around a black hole. He said V404 Cygni’s accretion disk is 10 million kilometres wide, and the inner few thousand kilometres was puffed up and wobbling during the bright outburst.

“The inner part of the accretion disk was precessing and effectively pulling the jets around with it,” Assoc Prof Miller-Jones said.

“You can think of it like the wobble of a spinning top as it slows down — only in this case, the wobble is caused by Einstein’s theory of general relativity.”

Artist’s impression of V404 Cygni seen close up. The binary star system consists of a normal star in orbit with a black hole. Material from the star falls towards the black hole and spirals inwards in an accretion disk, with powerful jets being launched from the inner regions close to the black hole. Image credit: ICRAR.

The research used observations from the Very Long Baseline Array, a continent-sized radio telescope made up of 10 dishes across the United States, from the Virgin Islands in the Caribbean to Hawaii. But the scientists had to use a very different approach to most radio observations, as explained by study co-author Alex Tetarenko.

“Typically, radio telescopes produce a single image from several hours of observation,” said Dr Tetarenko, a recent PhD graduate from the University of Alberta who is currently working in Hawaii.

“But these jets were changing so fast that in a four-hour image we just saw a blur. It was like trying to take a picture of a waterfall with a one-second shutter speed.”

Instead, the researchers produced 103 individual images, each about 70 seconds long, and joined them together into a movie. According to Dr Tetarenko, “It was only by doing this that we were able to see these changes over a very short time period.”

Study co-author Dr Gemma Anderson, who is also based at ICRAR’s Curtin University node, said the wobble of the inner accretion disk could happen in other extreme events in the universe too.

“Anytime you get a misalignment between the spin of a black hole and the material falling in, you would expect to see this when a black hole starts feeding very rapidly,” Dr Anderson said.

“That could include a whole bunch of other bright, explosive events in the universe, such as supermassive black holes feeding very quickly or tidal disruption events, when a black hole shreds a star.”

The news comes just a few weeks after scientists unveiled the first direct visual evidence of a supermassive black hole and its shadow.

Top image caption: Artist’s impression of the accretion disk around the black hole. During a powerful outburst in 2015, intense radiation caused the inner few thousand kilometres of the accretion disk to ‘puff up’ into a doughnut-shaped structure. Image credit: ICRAR.

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