First light detected from behind a black hole

For the first time, astrophysicists have caught sight of light reflected from behind a black hole, proving Einstein right yet again.

You may have heard that nothing – not even light – can escape a black hole, but this isn’t strictly true. Anything that crosses the event horizon is forever lost, but the hot disc of matter swirling around the black hole can emit dazzlingly powerful X-rays visible from Earth.

However, not all of this light escapes easily.

While watching X-rays streaming out from a supermassive black hole at the heart of a galaxy 800 million light-years away, Stanford University astrophysicist Dan Wilkins noticed something odd – extra flashes of X-rays. They were smaller, came later and had different wavelengths to the normal, more luminous emissions, as though they were echoes.

As described in a study led by Wilkins in Nature, these flashes seemed to be reflected from behind the black hole – a weird place for light to be coming 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,” Wilkins explains.

“The reason we can see that is because that black hole is warping space, bending light and twisting magnetic fields around itself.”

As a black hole spins, its incredibly strong magnetic field arcs high above it and become so tangled that the field lines eventually break – similar to what happens on the surface of our 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,” says Wilkins.

These X-rays try to escape the black hole’s massive gravitational pull, but some end up being pulled back – then reflected off the back of the disc and out into space. Some of these ‘echoes’ from behind the black hole are bent around it by extreme gravity, creating the light seen by Wilkins and his team.

This is the first time astronomers have directly spotted light from behind a black hole, building on research published last year that found “imprints” of such reflected light.

These observations also confirm predictions made based on Einstein’s theory of general relativity.

Michael Cowley, an astrophysicist from the Queensland University of Technology (who was not involved in the study), explains that the theory shows that massive objects can warp spacetime itself, causing light to travel along these bent paths.

“This theory has been proven experimentally, first by the English astronomer Arthur Eddington in 1919 after he performed observations of starlight bending around our own Sun during a solar eclipse,” Cowley notes.

By accounting for this effect, the authors could theoretically predict when the reflected X-ray signals should appear, as well as what they should look like.

But as James Miller-Jones – an astrophysicist at Curtin University who was also not part of the research team – points out, this effect is challenging to observe.

“It requires separating out the response of the disc to being lit up by the X-ray flares from a region above the black hole, from all the other emission in this active and high-energy region,” he explains. “So it’s an impressive piece of work, to get such a well-defined signal.”

Cowley agrees: “This new work continues to bridge the divide between observational and theoretical research of active supermassive black holes and provide us with more insight into how they can generate such awesome amounts of power.”

Observing these kinds of signals will enable astronomers to build a better understanding of black holes themselves.

“One of the things that’s really cool about this latest paper is that the researchers are able to probe the environment around a black hole devouring hot gas,” says Eric Thrane, an astrophysicist at Monash University and OzGrav.

“This environment is dynamic, so they see it changing with time. From this they worked out the mass of the black hole and the behaviour of the surrounding gas.

“One of the interesting questions in cosmology is how the most massive black holes got to be so big so quickly. I hope that studies like this will eventually shed light on how black holes grow over cosmic time.”

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