Einstein’s theory confirmed after light seen emerging from ‘inside’ black hole

Bright X-ray flares were being observed emerging from a supermassive black hole which lies at the centre of a galaxy 800 million light-years away, an intriguing but not unique phenomenon.

However, additional flashes of X-rays that were smaller, later and of different 'colours' than the bright flares were later detected that were much more unexpected by the scientists. These were dubbed “luminous echoes”, which according to theory are consistent 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,” said Stanford University astrophysicist Dan Wilkins. “The reason we can see that is because that black hole is warping space, bending light and twisting magnetic fields around itself.”

The strange discovery is believed to be 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 before 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.

The original motivation behind the research was to learn more about a mysterious feature of certain black holes, called a corona. Material falling into a supermassive black hole powers the brightest continuous sources of light in the universe and, as it does so, forms a corona around it. This light, which falls into the X-ray area of the spectrum, can be analysed to map and characterise a black hole.

The leading theory for what a corona is starts with gas sliding into the black hole where it superheats to millions of degrees. At that temperature, electrons separate from atoms, creating a magnetised 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.

“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.

Further observation of the black hole will make use of the European Space Agency’s X-ray observatory, Athena (Advanced Telescope for High-ENergy Astrophysics).

“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. “The picture we are starting to get from the data at the moment is going to become much clearer with these new observatories.”