Updated: Sep 27, 2021
A 100 years ago, Einstein predicted that due to the immense gravitational pull of black holes light should bend around them. What this essentially means is that black holes trap light and should not emit it. This was true till a study by Stanford astrophysicist, Dan Wilkins et al, was published in the journal, Nature, proved otherwise. While studying X-rays flung out of a supermassive black hole 800 million light years away, Dan Wilkins observed an unusual sight. He saw series of bright flashes of X-rays and then later observed more flashes of X-rays, this time of different "colours" than the initial flashes, a completely unexpected observation.
Anyone with a basic knowledge of black holes would know that light should not be observed from behind a black hole. But this new observation says otherwise. "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 Wilkins, who is a research scientist at the Kavli Institute for Particle Astrophysics and Cosmology at Stanford and SLAC National Accelerator Laboratory.
This observation is kind of an accident. The original substance of this research was to study one of the most mysterious part of a black hole, the corona. Scientists believe that the corona is formed as result of gas continuously falling into the black hole, ultimately forming a disc that gets heated up and emits X-rays. This is something that can help analyze and map the characteristics of 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 magnetized 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 -- a situation so reminiscent of what happens around our own Sun that it borrowed the name "corona."
"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.
As Wilkins and his team observed these flashes of X-rays closely, the team observed several smaller flashes of X-rays that were the same but were reflected from behind the black hole, the first ever observation of the far side of a black hole. "I've been building theoretical predictions of how these echoes appear to us for a few years," said Wilkins. "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."
Further research on the nature and characteristics of the corona requires better means of observation. Part of that future will be the European Space Agency's X-ray observatory, Athena (Advanced Telescope for High-ENergy Astrophysics). As a member of the lab of Steve Allen, professor of physics at Stanford and of particle physics and astrophysics at SLAC, Wilkins is helping to develop part of the Wide Field Imager detector for Athena.
"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. "So, the picture we are starting to get from the data at the moment is going to become much clearer with these new observatories."