Scientists have observed light from behind a black hole for the first time, confirming a prediction based on Albert Einstein’s theory of general relativity.
Dan Wilkins of Stanford University and his colleagues observed X-rays emitted from a supermassive black hole in the center of a galaxy 800 million light-years from Earth.
These bright light flares are not unusual because, while light can escape a black hole, the enormous gravity around it can heat material to millions of degrees. This can cause radio waves and X-rays to be produced. This super-heated material is sometimes thrown into space by rapid jets, which include X-rays and gamma rays.
Wilkins, on the other hand, noticed tiny X-ray flashes that happened later and were different colors – and they were coming from the far side of the black hole.
In a statement, Wilkins, study author and research scientist at Stanford University’s Kavli Institute for Particle Astrophysics and Cosmology and SLAC National Accelerator Laboratory, said, “Any light that goes into that black hole doesn’t come out, so we shouldn’t be able to see anything that’s behind the black hole.”
The strange nature of the black hole, on the other hand, made the observation possible.
“The reason we can see that is because that black hole is warping space, bending light and twisting magnetic fields around itself,” he said.
“Fifty years ago, astrophysicists had no idea that one day we might have the techniques to observe this directly and see Einstein’s general theory of relativity in action,” said Roger Blandford, study coauthor and the Luke Blossom Professor in the School of Humanities and Sciences and professor of physics at Stanford University, in a statement.
Einstein’s theory, or the idea that gravity is caused by matter warping space-time, has maintained for a decade despite new astronomical discoveries.
Some black holes have a corona, or a ring of bright light that forms around them as material falls into them and heats up to extreme temperatures. Scientists may investigate and map black holes using this X-ray light.
As gas falls into a black hole, it can spike to millions of degrees. Because of the extreme heating, electrons detach from atoms, resulting in magnetic plasma. The black hole’s enormous gravitational forces cause this magnetic field to arc high above the black hole and twirl until it breaks.
This is comparable to the corona, or hot outer atmosphere, of the sun. The sun’s surface is covered in magnetic fields, which cause loops and plumes to form as they interact with charged particles in the sun’s corona. This is why scientists refer to the ring around black holes as a corona.
“This magnetic field getting tied up and then snapping close to the black hole heats everything around it and produces these high energy electrons that then go on to produce the X-rays,” Wilkins said.
Wilkins observed smaller flashes when studying the X-ray flares. He and his fellow researchers realized the larger X-ray flares were being reflected and “bent around the black hole from the back of the disk,” allowing them to see the far side of the black hole.
“I’ve been building theoretical predictions of how these echoes appear to us for a few years,” Wilkins said. “I’d already seen them in the theory I’ve been developing, so once I saw them in the telescope observations, I could figure out the connection.”
NASA’s NuSTAR and the European Space Agency’s XMM-Newton space-based X-ray telescopes were used to make the observations.
More observations will be needed to fully understand these black hole coronas, and the European Space Agency’s upcoming X-ray observatory, Athena, will launch in 2031.
“It’s got a much bigger mirror than we’ve ever had on an X-ray telescope and it’s going to let us get higher resolution looks in much shorter observation times,” Wilkins said. “So, the picture we are starting to get from the data at the moment is going to become much clearer with these new observatories.”
