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Albert Einstein was right: There is a region at the edge of black holes where matter can no longer stay in orbit and instead falls into it, as his theory of gravity predicted.
Using telescopes that can detect X-rays, a team of astronomers has observed for the first time this region – the so-called “crash region” – in a black hole about 10,000 light-years from Earth. “We ignored this region because we lacked the data,” said researcher Andrew Mummery, lead author of the study, published Thursday in the Royal Astronomical Society’s journal Monthly Notices. “But now that we do it, we can’t explain it any other way.”
It’s not the first time that black holes have helped confirm Einstein’s grand theory, also known as general relativity. The first photo of a black hole, taken in 2019, had previously reinforced the revolutionary physicist’s basic assumption that gravity is just matter bending the fabric of space-time.
Many of Einstein’s other predictions have been proven correct over the years, including gravitational waves and the universal speed limit. “It’s hard to bet against him at this point,” said Mummery, a Leverhulme-Peierls fellow in the physics department at the University of Oxford in the United Kingdom.
“We specifically set out to find this one – that was always the plan. We argued for a long time about whether we would ever find it,” Mummery said. “People said it was impossible, so it’s really exciting to confirm that it’s there.”
NASA/CXC/M. White
In one artist’s illustration, a black hole extracts material from a companion star, forming a disk that rotates around the black hole before falling into it.
The observed black hole is in a system called MAXI J1820+070, which consists of a star smaller than the Sun and the black hole itself, estimated to be 7 to 8 solar masses. The astronomers used NASA’s space-based NuSTAR and NICER telescopes to collect data and understand how hot gas, called plasma, is sucked into the black hole by the star.
NuSTAR is short for Nuclear Spectroscopic Telescope Array, which orbits the Earth, and NICER, officially known as Neutron Star Interior Composition Explorer, is on the International Space Station.
NASA/JPL Caltech
NASA’s space-based NuSTAR telescope, seen here in an artist’s concept, has been used for the first time to detect the “crash region” around a black hole.
“Around these black holes there are large disks of orbiting material (from nearby stars),” Mummery said. “Most of it is stable, meaning it can flow easily. It’s like a river, while the falling area is like the edge of a waterfall – all support is gone and you just fall headfirst. Most of what you can see is the river, but at the very end there is this tiny region that is basically what we found,” he added, noting that while the “river” has been widely observed , however, this is the first evidence of “waterfall.”
Unlike the event horizon, which is closer to the center of the black hole and does not allow anything to escape, including light and radiation, in the “crash region” light can still escape, but the matter is declared doomed by the strong gravitational pull, Mummery.
The results of the study could help astronomers better understand the formation and evolution of black holes. “We can really learn more about them by studying this region because it’s right on the edge and therefore gives us the most information,” Mummery said.
What the study lacks is an actual image of the black hole because it is too small and too far away. But another team of Oxford researchers is working on something even better than a picture: the first film about a black hole. To achieve this, the team must first build a new observatory, the Africa Millimeter Telescope in Namibia, which Mummery expects will be online within a decade. The telescope, which will join the international Event Horizon Telescope collaboration that captured the groundbreaking image of the black hole in 2019, will allow scientists to observe and film large black holes at the center of the Milky Way and beyond.
According to Christopher Reynolds, a professor of astronomy at the University of Maryland, College Park, finding actual evidence of the “crash region” is an important step that will allow scientists to significantly refine models of the behavior of matter around a black hole . “This can be used to measure the rotation rate of the black hole, for example,” said Reynolds, who was not involved in the study.
Dan Wilkins, a researcher at Stanford University in California, calls it an exciting development, pointing out that in 2018 there was an enormously bright burst of light from one of the black holes in our galaxy, coupled with an excess of high-energy X-rays.
“We had hypothesized at the time that this excess came from hot material in the ‘crash region,’ but we did not have a complete theoretical prediction of what this emission would look like,” said Wilkins, who also was not involved in the new study.
This study actually performs that calculation, he added, using Einstein’s theory of gravity to predict what the X-rays emitted from material in the “crash region” would look like around a black hole and comparing them with the data from that bright 2018 burst.
“This will be the most important discovery space in the next decade or so,” Wilkins said, “as we look toward the next generation of X-ray telescopes that will give us more detailed measurements of the innermost regions just outside the event horizon of black holes.”