For the first time, scientists have detected light behind a black hole, and it responds to a prediction rooted in Albert Einstein’s theory of general relativity.
Stanford University astrophysicist Dan Wilkins and his colleagues observed X-rays emitted from a supermassive black hole at the center of a galaxy 800 million light years from Earth.
These flares of bright light are not unusual because although light cannot escape from a black hole, the enormous gravity surrounding it can heat matter to millions of degrees. This can release radio waves and x-rays. Sometimes this superheated material is thrown into space by rapid jets, including x-rays and gamma rays.
But Wilkins noticed smaller x-ray flashes that happened later and were different colors – and they were coming from across the black hole.
“Not all of the light that enters this black hole comes out of it, so we shouldn’t be able to see anything behind the black hole,” said Wilkins, study author and Kavli Institute researcher. particle astrophysics and cosmology at Stanford University and SLAC National Accelerator Laboratory, in a statement.
However, the eerie nature of the black hole actually made observation possible.
“The reason we can see it is that this black hole distorts space, bends light and twists the magnetic fields around it,” he said.
“Fifty years ago, when astrophysicists began to speculate on the behavior of the magnetic field near a black hole, they had no idea that one day we could have the techniques to observe it directly and see the Einstein’s general theory of relativity in action, “Roger Blandford, study co-author and Luke Blossom professor at the School of Humanities and Sciences and physics professor at Stanford University, said in a statement.
Einstein’s theory, or the idea that gravity is space-time-distorting matter, persisted for a hundred years as new astronomical discoveries were made.
Some black holes have a ring or ring of bright light that forms around a black hole when matter falls into it and becomes 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 reach millions of degrees. This extreme heating causes the separation of electrons from atoms, which creates a magnetic plasma. The black hole’s powerful gravitational forces cause this magnetic field to arc above the black hole and swirl around until it shatters.
It is reminiscent of the solar corona, or the warm outer atmosphere. The sun’s surface is covered with magnetic fields, which cause loops and plumes to form when they interact with charged particles in the solar corona. This is why scientists call the ring around black holes a crown.
“This magnetic field that sets and then moves closer to the black hole heats everything around it and produces these high-energy electrons which then produce the x-rays,” Wilkins said.
While studying the x-ray flares, Wilkins spotted smaller lightning bolts. He and his fellow researchers realized that the larger X-ray flares were reflected and “curved around the black hole from the back of the disc”, allowing them to see the other side of the black hole.
“I’ve been building theoretical predictions on how these echoes appear to us for the past few years,” Wilkins said. “I had already seen them in the theory I had developed, so once I saw them in the telescope observations, I was able to understand the connection.”
The observations were made using two X-ray space telescopes: NASA’s NuSTAR and the European Space Agency’s XMM-Newton.
More observations will be needed to understand these crowns of black holes and the European Space Agency’s next 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’s going to allow us to get higher resolution images in much shorter observation times,” Wilkins said. “So the picture we’re starting to get from the data right now is going to become a lot clearer with these new observatories.”