Illustration of how light echoes from behind a black hole. Credit: ESA
For the first time, astronomers have singled out light coming from behind a 
Animation showing how light echoes from behind a black hole. Credit: ESA
The discovery began with the search to find out more about the mysterious ‘corona’ of the black hole, which is the source of the bright X-ray light. Astronomers think that the corona is a result of gas that falls continuously into the black hole, where it forms a spinning disk around it – like water flushing down a drain.
This gas disk is heated up to millions of degrees and generates magnetic fields that get twisted into knots by the spinning black hole. When the magnetic field gets tied up, it eventually snaps, releasing the energy stored within it. This heats everything around it and produces the corona of high energy electrons that produce the X-ray light.
The X-ray flare observed from I Zwicky 1 was so bright that some of the X-rays shone down onto the disk of gas falling into the black hole. The X-rays that reflected on the gas behind the black hole were bent around the black hole, and these smaller flashes arrived at the telescopes with a delay. These observations match Einstein’s predictions of how gravity bends light around black holes, as described in his theory of General Relativity.
The echoes of X-rays from the disk have specific ‘colors’ of light and as the X-rays travel around the black hole, their colors change slightly. Because the X-ray echoes have different colors and are seen at different times, depending on where on the disk they reflected from, they contain a lot of information about what is happening around a black hole. The astronomers want to use this technique to create a 3D map of the black hole’s surroundings.
Another mystery to be solved in future studies is how the corona produces such bright X-ray flares. The mission to characterize and understand black hole coronas will continue with XMM-Newton and ESA’s future X-ray observatory, Athena (Advanced Telescope for High-ENergy Astrophysics).
For more on this discovery, read Strange Black Hole Discovery Confirms Einstein’s Theory of General Relativity.
Reference: “Light bending and X-ray echoes from behind a supermassive black hole” by D. R. Wilkins, L. C. Gallo, E. Costantini, W. N. Brandt and R. D. Blandford, 28 July 2021, Nature.
DOI: 10.1038/s41586-021-03667-0