While analyzing the X-rays from the supermassive black hole at the center of a galaxy 800 million light-years away, scientists at Stanford University noticed a peculiar pattern. It appears that the black hole is ejecting these rays into the universe surrounding it.
Dan Wilkins, who made the observations, saw a number of thrilling, but not unusual, bright X-ray flares. Then, the telescopes caught a surprise: more X-ray flashes that were smaller, later, and had different "colors" than the bright flares.
Such a phenomenon had previously been theorized to exist and is explained by the fact that, just as observed by scientists, dazzling flares of X-ray emissions are produced as gas sinks into a supermassive black hole.
Then, after the flares died down, brief X-ray bursts were seen. These lights were the reflection of flares that came from the disc's farthest edge and were bent around the black hole by its strong gravitational field. Then, as the gas descended toward the black hole, the fireworks echoed off of it.
Although even a rudimentary understanding of black holes reveals that this is a weird place for light to originate from, the theory suggests that these bright echoes are compatible with X-rays reflected from behind the black hole.
“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,” explained Wilkins, who is a research scientist at the Kavli Institute for Particle Astrophysics and Cosmology at Stanford and SLAC National Accelerator Laboratory."
"It is another strange characteristic of the black hole, however, that makes this observation possible. “The reason we can see that is because that black hole is warping space, bending light and twisting magnetic fields around itself,” Wilkins added.
The odd discovery was described in a study that appeared on July 28, 2022, in Nature (linked below). It is the first direct observation of light coming directly from behind a black hole, a scenario that Einstein's theory of general relativity predicted but which had never been verified before.
“Fifty years ago, when astrophysicists starting speculating about how the magnetic field might behave close to a black hole, they 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, a co-author of the paper who is the Luke Blossom Professor in the School of Humanities and Sciences, Stanford Professor of physics, and SLAC Professor of particle physics and astrophysics.
Black hole coronae were first the subject of the study
This study's initial goal was to better understand the corona, a puzzling characteristic of some black holes. The universe's brightest constant sources of light are powered by material falling into supermassive black holes, and as it does so, a corona is created around the black hole. It is possible to map and describe a black hole by analyzing these X-ray photons.
According to the dominant theories, a corona is created when gas enters a black hole and superheats to millions of degrees. A magnetized plasma is produced when electrons split from atoms at that temperature.
The magnetic field becomes completely broken as a result of the black hole's strong spin, which causes it to arch so high above the black hole and spin erratically that it has been given the term "corona." This phenomenon is similar to what occurs around our own Sun.
“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,” explained Wilkins.
Wilkins also noticed a string of smaller flashes as he got closer to the flares to examine their source. The scientists discovered that these are the identical X-ray flares that were reflected from the disk's back, giving them their first look at a black hole's far side.
“I’ve been building theoretical predictions of how these echoes appear to us for a few years,” said Wilkins. “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.”
So what comes next?
More observations will be needed as the effort to describe and comprehend coronas progresses. Athena, an X-ray observatory built by the European Space Agency, will be a part of that future (Advanced Telescope for High-ENergy Astrophysics). Wilkins is working on the Wide Field Imager detector for Athena in the lab of Steve Allen, a professor of physics at Stanford, as well as particle physics and astrophysics at SLAC.
“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,” said Wilkins. “So, the picture we are starting to get from the data at the moment is going to become much clearer with these new observatories.”
You can view the entire study for yourself in the journal Nature.
Reference(s): Stanford University