Fulfilling a prediction of Einstein’s Theory of General Relativity, researchers report the first-ever recordings of X-ray emissions from the far side of a black hole.
Saint Mary’s University researcher Dr. Luigi Gallo contributed to the analysis and interpretation of this collaborative research project, which was recently published in Nature.
“While we have seen X-ray flares before and we have seen them “reflect” off the accretion disc around the black hole, this is really the first time we have been able to isolate individual events (flares) as originating from behind the black hole,” said Dr. Gallo, who has been working on this type of research for 20 years.
“This is really important because these regions are so small in size and dynamic on such rapid time scales that it is impossible to take an image. Events like this allow us to determine what the region closest to the black hole looks like,” he said.
Watching X-rays flung out into the universe by the supermassive black hole at the center of a galaxy 800 million light-years away, Stanford University astrophysicist Dan Wilkins noticed an intriguing pattern. He observed a series of bright flares of X-rays – exciting, but not unprecedented – and then, the telescopes recorded something unexpected: additional flashes of X-rays that were smaller, later and of different “colors” than the bright flares.
According to theory, these luminous echoes were consistent with X-rays reflected from behind the black hole – but even a basic understanding of black holes tells us that is a strange place for light to come from.
“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,” said 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 explained.
The strange discovery, detailed in a paper published in Nature, is the first direct observation of light from behind a black hole – a scenario that was predicted by Einstein’s Theory of General Relativity but never confirmed, until now.
“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 and Stanford and SLAC professor of physics and particle physics.
How to see a black hole
The original motivation behind this research was to learn more about a mysterious feature of certain black holes, called a corona. When material is falling into a supermassive black hole, it powers the brightest continuous sources of light in the Universe, and as it does so, forms a corona around the black hole. This light – which is X-ray light – can be analyzed to map and characterize a black hole.
The leading theory for what a corona is starts with gas sliding into the center of the black hole where it superheats to millions of degrees. At that temperature, electrons separate from atoms, creating a magnetized plasma. Caught up in the powerful spin of the black hole, the magnetic field arcs so high above the black hole, and twirls about itself so much, that it eventually breaks altogether – a situation so reminiscent of what happens around our own Sun that it borrowed the name “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,” said Wilkins.
For this project, the researchers trained two space-based X-ray telescopes, NASA’s NuSTAR and the European Space Agency’s XMM-Newton, on the galaxy known as I Zwicky 1. The two bright flares that they observed are only the second example of flares that can be associated with a corona being launched away from a black hole.
They processed the observations with a new technique, which takes advantage of the fact that the immense gravity around the black hole shifts the wavelength of light. By accounting for that shift and the time delay between the initial flash and when it bounces off the spinning disc of superhot gas encircling the black hole – known as an accretion disk – the researchers were able to translate the X-rays into a map of the environment just outside the event horizon of the black hole.
As Wilkins took a closer look to investigate the origin of the flares, he saw the series of smaller flashes. These, the researchers determined, are the same X-ray flares but reflected from the back of the disk – a first glimpse at the far side of a black hole.
“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, I could figure out the connection.”
Future observations
The mission to characterize and understand coronas continues and will require more observation. Part of that future will be the European Space Agency’s X-ray observatory, Athena (Advanced Telescope for High-ENergy Astrophysics). As a member of the lab of Steve Allen, professor of physics at Stanford and of particle physics and astrophysics at SLAC, Wilkins is helping to developing part of the Wide Field Imager detector for Athena.
“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.”
