The detailed mechanics of how matter falls onto a black hole beyond the event horizon has been revealed in a new paper.
As predicted by Einstein’s theory of gravity, there is a point at which the material stops orbiting the black hole and falls straight down, plummeting beyond the point of no return.
Now, in X-ray data from an active black hole, we’ve finally seen proof that this “collapsing region” exists.
“Einstein’s theory predicted that this final descent would exist, but this is the first time we’ve been able to demonstrate that it happens,” said theoretical physicist Andrew Mummery of the University of Oxford in the United Kingdom.
“Think of it as a river turning into a waterfall – so far we’ve been looking at the river. This is our first view of the falls.”
Matter making its way to a black hole does not follow a straight line. It swirls around, like water swirling, spiraling, inexorably toward a drain. This is not an idle comparison: the comparison is so apt that scientists use swirling water eddies to study the environment around black holes.
Studying black holes themselves is a bit difficult because the warped spacetime around them is so extreme.
But decades ago, Albert Einstein’s theoretical work predicted that at a certain proximity to the black hole matter would no longer be able to follow a stable circular orbit and would fall straight down—like water over the mouth of an analogous drain.
There’s no reason to believe this isn’t the case—matter must somehow cross the event horizon, and Einstein’s theory of gravity has come under complete scrutiny—but what astrophysicists aren’t sure about is whether or not we’ll be in able to detect it.
The work of Mummery and his colleagues had several parts. One of these was the development of numerical simulations and models that image the plunging region to reveal the type of light it emits. They then needed observational evidence that contained the same emission in the subduction region.
The black hole in question is located in a system about 10,000 light-years away called MAXI J1820+070. This system contains a black hole about 8.5 times the mass of the Sun – and a binary companion star from which the black hole ejects material as the pair of objects orbit, giving off bursts that show up as X-ray flickers.
Astronomers have been observing this black hole to better understand its behavior, so researchers have had access to very high-quality data obtained using the NuSTAR and NICER X-ray instruments in low Earth orbit. In particular, they focused on an outbreak that occurred in 2018.
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Previous studies have noted that there is an additional glow detected in observations of this outburst that cannot be fully explained.
A 2020 study speculated that this glow could arise from the innermost stable region of a circular orbit—that is, the dip zone. Mummery and his colleagues studied this glow closely and found that it matched the emission they obtained from their simulations.
This, the researchers say, finally establishes the existence of the plunging region beyond doubt, giving us a new probe into the extreme gravitational regime in the region just beyond the event horizon of a black hole.
“What’s really exciting is that there are a lot of black holes in the galaxy, and now we have a powerful new technique for using them to study the strongest known gravitational fields,” says Mummery.
“We believe this represents an exciting new development in the study of black holes, allowing us to explore this final region around them.
Only then can we fully understand the gravitational force. This latest plunge of plasma occurs at the very edge of a black hole and shows that matter is responding to gravity in its strongest possible form.”
The study was published in Monthly Notices of the Royal Astronomical Society.
