While observing the powerful X-ray emissions launched into space by the supermassive black hole at the center of a galaxy 800 million light-years away, Stanford University astrophysicist Dan Wilkins realized something truly intriguing. After observing several ‘normal’ X-ray flashes, the telescopes detected something that took the scientist by surprise: a series of additional flashes, less intense than the first ones, but which appeared to come from just behind the black hole and that Wilkins apparently , I was seeing ‘through’ it.

No one until now had been able to observe anything like it. Even the most basic knowledge of black holes tells us that no light can come from inside them.

“Any light that enters a black hole cannot come out again,” says Wilkins, “so we should not be able to see anything behind it.”

But he saw it, and according to the researcher, what made this unusual observation possible is another outstanding characteristic of black holes: “The reason we were able to see that is that this black hole is warping space, bending light and twisting the holes. magnetic fields around itself ”.

The strange discovery, published today in Nature, is the first direct observation of light behind a black hole, a scenario that was predicted by Einstein’s theory of general relativity but has never been confirmed until now.

A relativistic ‘trick’
“Fifty years ago,” explains Roger Blandford, co-author of the paper, “when astrophysicists began to speculate about how the magnetic field might behave near 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. “

The original purpose of this research was to learn more about a mysterious feature of certain black holes, called a corona. Material that falls into a supermassive black hole becomes one of the brightest sources of light in the Universe, in the form of a rapidly rotating ‘corona’ around it. This light, which is X-ray light, can be analyzed to map and characterize black holes.

The best theory for what one of these crowns is starts with the gas sliding into the black hole, where it is superheated to millions of degrees. At that temperature, the electrons separate from the atoms, creating a magnetized plasma. Caught in the black hole’s powerful gravitational vortex, the magnetic field arcs above it, spinning so fast and so fast that it sometimes breaks completely. A ‘magnetic lash’ very similar to those that take place around our own Sun. Hence the name ‘corona’.

“This magnetic field that first gets stuck and then breaks next to the black hole – Wilkins continues – heats everything around it and produces these high-energy electrons, which in turn generate X-rays.”

Reflected sparkles

When the scientist looked closer to investigate the origin of those flashes, he saw a series of smaller flashes. As the researchers explain in their study, these are the same X-ray flashes, but reflected from the back of the disk, a first look at the far side of a black hole.

“I’ve been building theoretical predictions of how we might see these echoes for a few years,” says Wilkins. “I’d already seen them in the theory I’ve been developing, so once I saw them through the telescope, I immediately found out the connection.” .

The mission to characterize and understand the crowns of black holes continues, and will require more observation. Part of that future will be the European Space Agency’s Athena X-ray Observatory (Advanced Telescope for High Energy Astrophysics). Wilkins is helping develop some of the instrumentation for Athena.

“It will have a much larger mirror than we have ever had in an X-ray telescope,” Wilkins says, “and it will allow us to obtain higher resolution images in much shorter observation times. So, the image that we are beginning to obtain from the data at this point will become much clearer. “