For the first time, scientists have detected light from behind a black hole, and it fulfills a prediction rooted in Albert Einstein’s general theory of relativity.
Stanford University astrophysicist Dan Wilkins and his colleagues observed X-rays released from a supermassive black hole at the center of a galaxy located 800 million light-years from Earth.
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These bright flashes of light are not uncommon, because while light cannot escape a black hole, the enormous gravity surrounding it can heat material up to millions of degrees. This can release radio waves and X-rays. Sometimes this superheated material is flung into space by fast jets — including X-rays and gamma rays.
But Wilkins saw smaller X-ray flashes that happened later and were different colors — and they came from the other side of the black hole.
“All the light that goes into that black hole isn’t coming out, so we shouldn’t be able to see anything beyond the black hole,” said Wilkins, study author and research scientist at the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University and SLAC National. Accelerator Laboratory, in a statement.
However, the strange nature of the black hole made the observation possible.
“The reason we can see that is because that black hole is distorting space, bending light and spinning magnetic fields around itself,” he said.
“Fifty years ago, when astrophysicists began speculating about how the magnetic field might behave near a black hole, they had no idea that one day we would have the techniques to directly observe this and general relativity of Einstein in action,” Roger Blandford, co-author of the study and Luke Blossom professor in the School of Humanities and Sciences and professor of physics at Stanford University, said in a statement.
Einstein’s theory, or the idea that gravity is matter that distorts space-time, lasted a hundred years as new astronomical discoveries were made.
Some black holes have a corona, or a ring of bright light that forms around a black hole when material falls into them and is heated to extreme temperatures. This X-ray light is a way for scientists to study and map black holes.
When gas falls into a black hole, it can spike to millions of degrees. This extreme heating causes electrons to separate from atoms, creating magnetic plasma. The black hole’s powerful gravity causes this magnetic field high above the black hole to arc and spin until it breaks.
This is no different from the sun’s corona, or the hot atmosphere outside. The sun’s surface is covered in magnetic fields, which create loops and plumes as they interact with charged particles in the sun’s corona. That’s why scientists call the ring around black holes a corona.
“This magnetic field that is tied up and then snaps close to the black hole heats everything around it and produces these high-energy electrons which then produce the X-rays,” Wilkins said.
As he studied the X-ray flames, Wilkins noticed smaller flashes. He and his fellow researchers realized that the larger X-ray flashes were reflected and “curved from the back of the disk around the black hole,” allowing them to see the other side of the black hole.
“I’ve been making theoretical predictions for a few years about how these echoes appear to us,” Wilkins said. “I had already seen them in the theory I was developing, so once I saw them in the telescope observations, I was able to figure out the connection.”
The observations were made using two space-based X-ray telescopes: NASA’s NuSTAR and the European Space Agency’s XMM-Newton.
More observation is needed to understand these black hole coronas, and the European Space Agency’s forthcoming X-ray observatory, called Athena, will be launched in 2031.
“It has a much larger mirror than we’ve ever had on an X-ray telescope and it will allow us to get higher resolution in much shorter observation times,” Wilkins said. “So the picture we’re starting to get from the data at this point is going to be much clearer with these new observatories.”
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