Black holes across the universe are losing enormous amounts of energy, and it could be due to magnetic fields slowing their spin. Understanding this process could help solve the long-standing mystery of how black hole jets are powered.
A new study of the supermassive black hole M87* has shown that the energy near its edge is flowing outward, not falling in. Scientists have long theorised that the magnetic fields of spinning black holes extract energy from their parent objects to power the extraordinary black hole jets they launch across the universe at nearly the speed of light. But the question of how they do so has remained unanswered.
at Princeton University and his colleagues used data from the Event Horizon Telescope – an enormous observatory consisting of linked telescopes all around the world – to examine the energy flow near M87*.
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“The power of the jet coming from M87* is at minimum thought to be 1042 ergs per second, which is 250 million times the sun’s total luminosity – that’s the same power as 3000 TNT bombs, each with the same mass as Earth, detonated every second,” says Chael. “Some measurements are as much as 1000 times that number.”
This extraordinary energy cannot be coming from within the black hole itself, as everything that crosses its event horizon is trapped. That means it must come either from the black hole’s rotation or from the disc of hot plasma falling into the black hole. The EHT data seems to support the former idea, showing energy streaming away from close to M87*’s edge. The researchers postulate this is probably because of magnetic fields threaded through the black hole.
The process is a bit like twirling a spinning top with strings attached to its edges. “The magnetic fields are what is extracting the energy from the spin of the black hole – they’re being wound up and sort of torquing the black hole back down,” says Chael. “Because it’s just the rotational energy that’s being extracted, not the mass, it doesn’t break any laws of physics.”
Black hole jets are instrumental in moving energy from the centres of galaxies to the outskirts, so figuring out how they are powered is key to understanding the evolution of galaxies over cosmic time. However, we can’t know for sure that these jets are powered by the black holes themselves until we have more sensitive images that see closer to the edge of the black hole, Chael says. This could be done by the next-generation Event Horizon Telescope, which would use even more observatories around the world to take better images – and even videos – of supermassive black holes. That project is still in the planning phases.
The Astrophysical Journal
