First light detected from behind a black hole

First light detected from behind a black hole

Astrophysicists have observed light reflected from behind a black hole for the first time, confirming Einstein correct once more.

Nothing, not even light, can escape a black hole, but this isn’t strictly true. Anything that passes the event horizon is lost for all time, but the hot disc of matter swirling around the black hole can emit dazzlingly powerful X-rays that can be seen from Earth.

However, not all of this light is easily emitted. Stanford University astrophysicist Dan Wilkins observed something unusual while viewing X-rays shooting out from a supermassive black hole at the core of a galaxy 800 million light-years away: extra X-ray flashes. They were smaller, came later, and had different wavelengths than the normal, stronger emissions, as if 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 very strong magnetic field arcs high above it, becoming so tangled that the field lines finally break, similar to what happens on our Sun’s surface. “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 attempt to escape the black hole’s massive gravitational force, but some are lured back and reflected off the back of the disc 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 that found “imprints” of such reflected light.

Illustration of how light echoes from behind a black hole. Credit: ESA

These findings also back up predictions based on Einstein’s theory of general relativity. According to Michael Cowley, an astrophysicist at Queensland University of Technology (who was not involved in the study), the theory shows that big objects can twist spacetime itself, causing light to flow along 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 potentially predict when and what the reflected X-ray signals would 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 kind of signals can help astronomers gain a better understanding of black holes.

“One of the most cool things about this new paper is that the researchers can probe the environment around a black hole devouring hot gas,” explains Eric Thrane, an astronomer 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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