The detailed mechanics of how matter falls onto a black hole from beyond the event horizon have been revealed in new work.
As Einstein’s theory of gravity predicted, there is a point at which material stops orbiting the black hole and falls straight down, plummeting past the point of no return.
Now, in X-ray data from an active black hole, we’ve finally seen evidence that this “submergence region” exists.
“Einstein’s theory predicted that this ultimate dip would exist, but this is the first time we’ve been able to demonstrate that it happens,” says theoretical physicist Andrew Mummery of the University of Oxford in the UK.
“Think of it as a river turning into a waterfall – until now we’ve been looking at a river. This is our first view of the falls.”
Matter traveling towards a black hole does not travel in a straight line. It swirls around like swirling water, spiraling inexorably toward the drain. This is not an empty comparison: it is so apt that scientists use water vortices to study the environment around black holes.
Studying black holes themselves is a bit tricky because the distorted space-time around them is so extreme.
But a few decades ago, the theoretical work of Albert Einstein predicted that at a certain proximity to a black hole, matter would no longer be able to move in a stable circular orbit and would fall straight down – like water through a ledge of a similar drain.
There’s no reason to believe it isn’t—matter must somehow cross the event horizon, and Einstein’s theory of gravity has stood up to rigorous scrutiny—but astrophysicists weren’t sure whether we’d be able to detect it.
The work of Mummery and his colleagues had several parts. One was the development of digital simulations and models depicting the submersible area to reveal the light it emits. After that, they needed observational evidence that contained the same drop in emission area.
The black hole in question is located in the MAXI J1820+070 system, which is about 10,000 light-years away. This system contains a black hole with a mass of about 8.5 times the mass of the Sun and a binary companion star from which the black hole strips matter as the pair of objects orbit, fueled by bursts that show up as flickering X-rays .
Astronomers have been observing this black hole to better understand its behavior, so the researchers were able to access very high-quality data obtained with the NuSTAR and NICER X-ray instruments in low Earth orbit. In particular, they focused on the explosion that occurred in 2018.
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Previous studies have noted that observations of this flare revealed an additional glow that could not be fully explained.
A 2020 study suggested that this glow may be coming from the innermost stable circular region of the orbit—that is, the plunge zone. Mamery and his colleagues studied this glow particularly closely and found that it matched the emission they obtained from the simulations.
This, the researchers say, definitively establishes the existence of a subduction region, without a doubt, giving us a new probe for the extreme gravitational regime in the region just beyond the black hole’s event horizon.
“What’s really exciting is that there are a lot of black holes in the galaxy, and now we have a powerful new technique to use them to study the strongest known gravitational fields,” says Mummery.
“We think this is an exciting new development in the study of black holes that allows us to explore this last region around them.
Only then will we be able to fully understand the force of gravity. This last drop of plasma occurs at the very edge of the black hole and shows that matter is responding to gravity in the strongest possible way.”
The study was published in Monthly Notices of the Royal Astronomical Society.