9 Scientists take a peek behind a black hole

EINSTEIN’S THEORY OF general relativity has been confirmed several times in recent years. And last year, it was reinforced yet again, with the first observation of X-rays coming from behind a black hole, bent in our direction as the black hole’s gravity warps the shape of space-time around it.

Although light cannot escape from inside a black hole’s event horizon, we can see signals emitted from outside this boundary, where a superheated, fastmoving accretion disk of material swirls inward. This is the region astronomers were looking at within the heart of galaxy I Zwicky 1, located 800 million light-years away, using two X-ray space telescopes: NASA’s NuStar and the European Space Agency’s XMM-Newton. There, they were watching the supermassive black hole for bright flares of X-ray light from its corona — a spherical region of energetic particles around the black hole.

But a short time after each flare, the telescopes recorded a fainter X-ray flash at a different wavelength. These flashes, the team determined, were the echoes of each flare reflecting off the portion of the accretion disk blocked from us by the black hole’s shadow. The change in wavelength of the light indicated the reflections had been twisted and stretched by gravity, until they’d arced around the black hole and back toward Earth.

It is the first time such behavior, long predicted by general relativity, has been directly observed. “We think that anytime we’ve seen these reflections coming from the accretion disk, there should be some part of that signal that is coming from behind the black hole, but it’s not been possible to separate it out,” says

Dan Wilkins of Stanford University, lead author of a paper published July 28 in Nature describing the observations.

“Now we’ve been able to actually tease out that signal.”

Seeing that signal gives astronomers one more peek into how black holes truly work. Most of these extreme objects cannot be imaged directly, so the trick is working out ways to see them without actually seeing them. And that’s where this new find fits in: X-ray flares provide key information about the region immediately around the black hole, Wilkins says. “What we’re hoping is that we’ll be able to take what we’ve learned and use this to actually be able to reconstruct an image of other black holes right across the universe.”

DAN WILKINS A black hole sits in the middle of a swirling maelstrom of hot gas (red) in this artist’s concept. In 2021, researchers captured X-ray light, shown in white, reflected off gas on the farside of a black hole. The black hole’s extreme gravity had warped space-time around it, bending the X-rays around itself and into our line of sight.