NuSTAR Bringing the High Energy Universe into Focus


First Proof that "Plunging Regions" Exist Around Black Holes


An image of NuSTAR overlaid on an artist's illustration of an accreting black hole. This paper provides the first observational proof of material in the so-called "plunging region", between the inner edge of the accretion disk and the event horizon of the black hole. Image credit: NASA/JPL-Caltech

News Release • May 16, 2024

Using X-ray data from NASA's NuSTAR and NICER satellites to test a key prediction of Einstein’s theory of gravity, an international team of astrophysicists have published the first observational proof that a “plunging region” around black holes not only exists, but exerts some of the strongest gravitational forces yet identified in the galaxy.

Unlike in Newton’s theory in gravity, Einstein’s theory finds that particles sufficiently close to a black hole are unable to follow circular orbits. Instead, they rapidly “plunge” toward the black hole at close to the speed of light – giving the plunging region its name. This study used X-ray data to find evidence for the plunging material and understand the forces generated by black holes.

“This is the first look at how plasma, peeled from the outer edge of a star, undergoes its final fall into a black hole,” said Dr. Andrew Mummery of Oxford University, who led the study. “What’s really exciting is that there are many black holes in the Galaxy, and we now have a powerful new technique for using them to study the strongest known gravitational fields.”

“Einstein’s theory predicts that this final plunge should exist, but this is the first time we’ve been able to demonstrate it happening,” Mummery continued. “Think of it like a river turning into a waterfall – hitherto, we have been looking at the river. This is our first sight of the waterfall.”

“This represents an exciting new development in the study of black holes, allowing us to investigate this final area around them. Only then can we fully understand the gravitational force,” Mummery added.  “This final plunge of plasma happens at the very edge of a black hole and shows matter responding to gravity in its strongest possible form.”

Astrophysicists have for some time been trying to understand what happens close to the black hole’s event horizon and do this by studying discs of material orbiting around them. In the final region of spacetime, known as the plunging region, it is impossible for material to stop its final descent into the black hole, making the material effectively doomed.

Debate between astrophysicists has been underway for many decades as to whether the so-called plunging region would be detectable. Mummery's team spent the last couple of years developing models for it and, in the study recently published in Monthly Notices of the Royal Astronomical Society, demonstrate its first confirmed detection using NuSTAR.