X-ray echoes behind black holes provide “extreme” proof Einstein was right

The massive gravity of a black hole actually bent X-ray echoes from around its back.

Updated versión of a Black hole with accretion disk
Daniel Rocal - PHOTOGRAPHY/Moment/Getty Images

When scientists first turned the X-Ray space telescopes NuSTAR and XMM-Newton toward a supermassive black hole in the center of a distant galaxy with the memorable name of IZwicky1, they knew their mission:

How could it be that some of nature’s darkest forces are also some of its brightest? Black holes are called black holes for a reason. Yet, the hot gasses that fall into black holes become superheated and glow intensely.

This was a question the international team aimed to solve. Instead, they wound up confirming Albert Einstein’s theory of general relativity in one of its most extreme tests to date.

Through a serendipitous observation of exceptionally bright X-ray flares around the supermassive black hole and careful analysis of the flare’s “echoes,” researchers were able to prove that what they were actually seeing were X-ray reflections from behind the black hole.

The black hole’s massive gravity was bending the X-ray light around the corner, so to speak.

“It’s really further confirmation that this light bending is still working as Einstein predicted it, even when the gravity gets really strong and really close to a black hole,” Dan Wilkins tells Inverse. Wilkins is the paper’s first author and a Stanford University research scientist.

“It’s confirming what we already knew,” he says, “but at a more extreme scale.”

These findings were published Wednesday in the journal Nature.

What’s new — In his Relativity: The Special and General Theory, Einstein discusses two interrelated theories — one being general relativity, a concept proposed in 1915.

The theory of general relativity suggests that a massive object can warp space-time around it, creating what we experience as gravity. This facet of the theory has been proven many times, perhaps most famously during a solar eclipse in 1919, when astronomer Arthur Eddington confirmed that the sun’s gravity bent the light from stars — just as Einstein predicted.

“It’s gravitational lensing at its most extreme.”

Astronomers have since used the gravity of distant galaxies to see around them toward even more distant objects, a phenomenon known as “gravitational lensing.”

But this new work takes gravitational lensing to a whole new level, according to Wilkins.

“It’s not just light being reflected a little bit. It’s coming from all the way behind the black hole, being bent all the way around into our line of sight,” he says. “It’s gravitational lensing at its most extreme.”

How they did it — As stellar gas and dust fall into a supermassive black hole, they flatten out and spin around it in a disk, like water going down a drain, Wilkins says. This creates a high-intensity glow in the visual and X-ray parts of the electromagnetic spectrum.

His team was measuring the X-rays when “all of a sudden, it started emitting what we refer to as X-ray flares,” he says. “Suddenly the X-rays got about 2.5 times brighter for a very short period of time.”

An ESA graphic describing the phenomenon of X-ray echoes around a supermassive black hole.


The extremely bright flares reflect, or echo, off of the spinning disk of hot gas around the black hole.

Echoes, and the flares that cause them, are known phenomena, but Wilkins says the team began noticing extra echoes they hadn’t anticipated.

“These were the echoes from the backside of the disk,” he says. “The echoes that come off the far side of the disk — the part that is hidden by the shadow of the black hole — actually get bent around the edge of the black hole.”

Why it matters — In the big picture, the new findings add one more brick in the column of evidence supporting general relativity. There are also implications for better understanding galaxies, stars, and black holes themselves.

It’s thought that most galaxies contain a supermassive black hole at their center, including our own Milky Way. Better understanding how these supermassive black holes function could help scientists better understand their possible role in galaxy formation, according to Wilkins.

And in the process of verifying the gravitational bending of the X-rays around this particular supermassive black hole, the researchers also developed a new tool to help them study other black holes. Most such objects are too distant to photograph, but Wilkins says they can now use measurements of X-ray echoes as a sort of sonar “to reconstruct that picture, of that map, of the extreme environment outside a black hole.”

What’s next — The immediate next step for Wilkins and other researchers is to refine the new techniques to get better and better measurements of X-ray echoes, as well as a better picture of what the area around a black hole actually looks like.

An artist's simulation of the ATHENA X-ray telescope’s view of the merger of two supermassive black holes.


New X-ray space telescopes, such as the European Space Agency’s Advanced Telescope for High-Energy Astrophysics (ATHENA) — planned for the early 2030s — could be a big help.

“It will be the biggest X-ray telescope we’ve ever launched and having a bigger telescope, we will have a much more detailed view,” Wilkins says. “We will get a clearer and clearer picture of this extreme environment just outside the black hole and find out what is happening to this gas in its final moments before it falls in.”

Abstract: The innermost regions of accretion disks around black holes are strongly irradiated by X-rays that are emitted from a highly variable, compact corona, in the immediate vicinity of the black hole. The X-rays that are seen reflected from the disk, and the time delays, as variations in the X-ray emission echo or ‘reverberate’ off the disk provide a view of the environment just outside the event horizon. I Zwicky 1 (I Zw 1) is a nearby narrow-line Seyfert 1 galaxy. Previous studies of the reverberation of X-rays from its accretion disk revealed that the corona is composed of two components: an extended, slowly varying component extending over the surface of the inner accretion disk, and a collimated core, with luminosity fluctuations propagating upwards from its base, which dominates the more rapid variability. Here we report observations of X-ray flares emitted from around the supermassive black hole in I Zw 1. X-ray reflection from the accretion disk is detected through a relativistically broadened iron K line and Compton hump in the X-ray emission spectrum. Analysis of the X-ray flares reveals short flashes of photons consistent with the re-emergence of emission from behind the black hole. The energy shifts of these photons identify their origins from different parts of the disk. These are photons that reverberate off the far side of the disk, and are bent around the black hole and magnified by the strong gravitational field. Observing photons bent around the black hole confirms a key prediction of general relativity.
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