The detailed mechanisms of how matter falls onto a black hole from outside the event horizon have been revealed in a new paper.
As predicted in Einstein’s theory of gravity, there is a point at which matter stops orbiting the black hole and falls straight down, plummeting steeply past the point of no return.
Now we’ve finally seen evidence in X-ray data from an active black hole that this “crash region” exists.
“Einstein’s theory predicted that this final crash would happen, but this is the first time we have shown that it actually happens,” says theoretical physicist Andrew Mummery of the University of Oxford in the United Kingdom.
“Imagine it as a river turning into a waterfall – so far we’ve been looking at the river. This is our first view of the waterfall.”
Matter moving toward a black hole does not follow a straight line. It circles around like swirling water spiraling inexorably toward a drain. This is no idle comparison: so apt is the comparison that scientists use swirling vortices of water to study the environments of black holes.
Studying black holes themselves is somewhat difficult because the space-time around them is so extremely distorted.
But decades ago, Albert Einstein predicted in his theoretical work that matter at a certain proximity to the black hole would no longer be able to follow a stable circular orbit and would fall straight down – like water over the edge this analog drain.
There’s no reason to think this isn’t the case – matter must somehow cross the event horizon, and Einstein’s theory of gravity has withstood extensive scrutiny – but astrophysicists weren’t sure whether we would or wouldn’t be able to to recognize it.
The work of Mummery and his colleagues consisted of several parts. One of these was to develop numerical simulations and models that represent the crash area to reveal the type of light it emits. They then needed observational evidence that contained the same emission from the crash region.
The black hole in question is located in a system called MAXI J1820+070 about 10,000 light-years away. This system contains a black hole with about 8.5 times the mass of the Sun – and a binary companion star from which the black hole extracts material as the object pair orbits, producing bursts that manifest as X-ray flickers.
Astronomers have been observing this black hole to better understand its behavior, allowing researchers to access very high-quality data obtained by the NuSTAR and NICER X-ray instruments in low Earth orbit. They particularly focused on an outbreak in 2018.
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Previous studies had noted that observations of this eruption detected an additional glow that could not be fully explained.
A 2020 study speculated that this glow could arise from the innermost stable circular orbit region – i.e. the crash zone. Mummery and his colleagues examined this glow with particular care and found that it matched the emission they had derived from their simulations.
This, the researchers say, finally proves the existence of the crash region beyond question and gives us a new probe of the extreme gravity regime in the region just outside a black hole’s event horizon.
“What’s really exciting is that there are lots of black holes in the galaxy and we now have a powerful new technique to use 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 study this final region around them.”
Only then can we fully understand the force of gravity. This final plasma fall occurs at the very edge of a black hole and shows matter responding to gravity in its strongest possible form.
The research was published in Monthly Notices of the Royal Astronomical Society.