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 on July 28 in Nature, is the first direct observation of light from behind a black hole – a scenario predicted by Einstein’s general theory of relativity but never confirmed until now.
“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, a co-author of the paper and the Luke Blossom Professor in the School of Humanities and Sciences, Stanford professor of physics, and SLAC professor of particle physics and astrophysiology.
How to see a black hole
The original goal of this study was to learn more about a mysterious feature of certain black holes known as a corona. Material falling into a supermassive black hole powers the universe’s brightest continuous sources of light, forming a corona around the black hole in the process. This X-ray light can be analysed in order to map and characterise a black hole.
The leading theory for what a corona is begins with gas falling into a black hole and superheating to millions of degrees. Electrons separate from atoms at that temperature, resulting in a magnetised plasma. Caught up in the black hole’s powerful spin, the magnetic field arcs so high above it and twirls so much that it eventually breaks – a situation so similar to 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.
Wilkins noticed a series of smaller flashes as he investigated the source of the flares. These, the researchers discovered, are the same X-ray flares but reflected from the back of the disc – the first glimpse of 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.”
The mission to characterise and understand coronas continues, and more observations are required. Athena, the European Space Agency’s X-ray observatory, will be a part of that future (Advanced Telescope for High-ENergy Astrophysics). Wilkins is working in the lab of Steve Allen, professor of physics at Stanford and particle physics and astrophysics at SLAC, to develop a component 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.”