Nothing escapes a black hole, not even light. The extreme light we see around these cosmic objects is formed at their edges from material falling in, powering the brightest continuous light sources in the universe, and forming a corona. Due to their huge gravity, black holes warp space-time in such a way that it is possible to see light from directly behind them. However, this had never been directly observed – until now.
As reported in Nature, researchers have seen X-ray emissions coming from directly behind the supermassive black hole at the center of galaxy I Zwicky 1, an active cosmic object about 800 million light-years away.
“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,” lead author Dr Dan Wilkins, from the Kavli Institute for Particle Astrophysics and Cosmology at Stanford and SLAC National Accelerator Laboratory, said in a statement. “The reason we can see that is because that black hole is warping space, bending light and twisting magnetic fields around itself.”
Such an incredible phenomenon has been predicted for decades by Einstein’s theory of general relativity, but this is the first time the light from behind a black hole has been directly seen.
The team did not set out to look for this extreme warping. They were studying the supermassive black hole's corona, a region of high-energy particles trapped in a magnetic field near the black hole. Recently, astronomers have started to realize that these coronae are more dynamic than previously thought, even disappearing in some extreme cases.
The team was originally looking at how the corona produces X-rays in flares. While investigating the origins of these flares, they noticed a bunch of smaller flashes. These, the team says, are the same X-rays but reflected from behind the disk, the closest we've got to glimpsing the far side of a black hole.
“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. “I’ve been building theoretical predictions of how these echoes appear to us for a few years. 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.”
“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,” added co-author Roger Blandford.
Even more observations of Einstein's theory in action should occur in the future thanks to the European Space Agency's new X-ray telescope Athena. Thanks to its giant mirror, bigger than any X-ray telescope before it, it will be able to provide high-resolution observations of such events in much shorter times, revealing more of the mysteries surrounding black holes.