The detailed mechanisms of how matter from beyond the event horizon falls into a black hole have been revealed in a new paper.
As predicted by Einstein’s theory of gravity, there is a point at which material stops circling the black hole and falls straight down, suddenly plunging beyond the point of no return.
Now we have finally seen evidence that this ‘diving region’ exists in X-ray data from an active black hole.
“Einstein’s theory predicted that this definitive jump would exist, but this is the first time we have been able to demonstrate that it happens,” says theoretical physicist Andrew Mummery of the University of Oxford in Britain.
“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 waterfall.”
Matter making its way to a black hole does not follow a straight line. It circles around, like water swirling, spiraling, inexorably toward a drain. This is not an empty comparison: so apt is the comparison that scientists use swirling water eddies to study the environment around black holes.
Studying the black holes themselves is a bit tricky, because the space-time around them is so extreme.
But decades ago, Albert Einstein’s theoretical work predicted that at a certain distance from the black hole, matter would no longer be able to follow a stable circular path and would fall straight down – like water over the edge of that analog disposal.
There’s no reason to think this isn’t the case – matter has to cross the event horizon somehow, and Einstein’s theory of gravity has held up to criticism across the board – but what astrophysicists are unsure about is whether we would or would not do. able to detect it.
The work of Mummery and his colleagues consisted of several parts. One of these was the development of numerical simulations and models that display the descending region to reveal what type of light it emits. Then they needed observational evidence containing the same emissions from the diving area.
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 that has about 8.5 times the mass of the Sun – and a binary star, which the black hole strips of material as the pair of objects orbits Earth, causing bursts that manifest as flickers of x-rays.
Astronomers have been monitoring this black hole to better understand its behavior, allowing the researchers to access very high-quality data obtained using the X-ray NuSTAR and NICER instruments in low Earth orbit. In particular, they focused on an eruption that occurred in 2018.
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Previous studies had noted that observations of this eruption included an additional glow that could not be fully explained.
A 2020 study speculated that this glow could arise from the inner stable circular orbit region, i.e. the dive zone. Mummery and his colleagues studied this glow with particular care and found that it matched the emission they derived from their simulations.
This, the researchers say, definitively confirms the existence of the plummeting region, beyond any doubt, and gives us a new investigation into the extreme gravitational regime in the region immediately outside a black hole’s event horizon.
“What’s really exciting is that there are many black holes in the Milky Way, 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 explore this final area around them.
Only then can we fully understand gravity. This final plasma burst occurs at the very edge of a black hole and shows that matter responds to gravity in its strongest possible form.”
The research has been published in the Monthly notices of the Royal Astronomical Society.