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For the first time ever, scientists have glimpsed “echoes” of light from the far side of a black hole, revealing an unprecedented glimpse of the environment behind these extreme objects relative to our perspective on Earth.
It’s well-known that no light escapes from a black hole, but the mind-boggling discovery of light coming from behind one confirms a key prediction of Einstein’s theory of general relativity, which states that photons can be bent around black holes due to their intense gravitational forces, which warp spacetime and anything in it, including light.
A team led by Dan Wilkins, an astrophysicist at Stanford University’s Kavli Institute for Particle Astrophysics & Cosmology, spotted these never-before-seen “X-ray echoes” around a supermassive black hole at the center of a galaxy called I Zwicky 1, which is located about 800 million light years from Earth, according to a study published on Wednesday in Nature.
“We have thought for a long time that these echoes from the far side of the black hole should be present, but we didn’t previously think that using our current telescopes we would be able to pick out the signal from everything else that is going on; the bright flash of X-rays from the corona and the echo from all of the material on the front side of the disk,” said Wilkins in an email.
Nevertheless, the bizarre effect was captured in January 2020 by two specialized space telescopes: NASA’s NuSTAR and the European Space Agency (ESA) observatory XMM-Newton. Their observations revealed precise details of the black hole’s corona, which is a ring of ultra-hot gas particles that surrounds some of these extreme objects.
“When we got the data downloaded from the two space telescopes we were using in this study, it was really exciting to see, a short time into our observations, that I Zwicky 1 started emitting some really bright X-ray flares,” Wilkins recalled. “We knew that these bright flashes of X-ray emission would be a prime opportunity to look for the echoes from the gas that’s falling into the black hole and to learn more about what happens as gas falls into a supermassive black hole.”
Light (or anything else) that crosses the border of a black hole, known as the event horizon, will never be seen or heard from in our universe again. But while photons cannot escape from behind the event horizon, the exterior environment around black holes fuel some of the most spectacular light shows in the universe. For instance, the radiant X-ray flares the team observed at I Zwicky 1 are likely fueled by pyrotechnic interactions between the black hole’s twisting magnetic fields and high-energy electrons in its corona, all of which are located outside the event horizon.
Wilkins has suspected for some time that X-ray photons from the far side of a black hole could reverberate around the disk and appear on the observable side that is pointed toward Earth. When the researchers searched for evidence of this trippy effect at I Zwicky 1, they were rewarded with “direct observational evidence for the re-emergence of emission from behind the black hole, bent into our line of sight by strong gravitational light bending,” according to the study.
“As I started analyzing the echoes, I noticed that as the flares faded away, there were additional short echoes of X-rays appearing at different times in different colors,” Wilkins explained. “Studying these more closely, I figured out that these echoes exactly matched our theoretical predictions of how the echoes from the gas that’s behind the black hole would appear.”
“While the signal was just as we expected it, and as it had been predicted by Einstein’s theory of General Relativity, it was exciting to be able to see that signal first-hand in an observation of a black hole,” he added.
I Zwicky 1 was a perfect target for this type of breakthrough because its central black hole is gobbling up gas at an elevated rate, causing it to shine brightly in X-ray light. In addition, its flares were especially radiant and brief, causing a “a strong echo from the disk and a strong signal from the far side of the disk, that we were able to pick out,” Wilkins noted.
Now that these echoes have been detected at I Zwicky 1, scientists are better equipped to search for them around other black holes, which could reveal new insights about the intense environments just outside of these objects.
“We are learning how to use these echoes—both the time the echoes are seen after a flash of X-ray emission, and the slight shifts in the color of the X-rays as they get warped, travelling around the black hole to us—to reconstruct an image of the extreme environment just outside the black hole,” Wilkins said.
“We know that gas falling into a supermassive black hole is able to power some of the brightest light sources we see across the whole Universe, and that the energy output from the supermassive black holes is so high that they played an important role in shaping the formation of the galaxies and the structure of the Universe as we know it today,” he added. “By reconstructing this image of the environment around the black hole, we are learning exactly how black holes power such bright objects and were able to fulfill their role in the formation of galaxies.”
These efforts to unravel the mysteries of black holes, including their influence on the evolution of galaxies, is built on instruments like NuSTAR, XMM-Newton, and the gargantuan Event Horizon Telescope, which released the first-ever image of a black hole in 2019. But a new generation of X-ray observatories will soon greatly exceed the capabilities of these existing missions, such as the ESA-led Advanced Telescope for High-ENergy Astrophysics (Athena), currently due for launch in 2031.
It is “a really exciting time for X-ray astronomy,” said Wilkins, who is part of the international Athena team. “This new telescope will be much larger than any X-ray telescope we have launched before, which will vastly increase the detail we will be able to see around supermassive black holes.”