Spinning Black Holes Fire off Violent Jets
Violent jets of matter and energy that shoot out from some black holes originate in their spin, suggest the most realistic simulations of these torrents yet.
Thousands of jets — which radiate at radio wavelengths — have been observed spewing from active galaxies. These galaxies are believed to have black holes at their centres and are called “radio-loud” quasars. The jets are thought to be powered by black holes with masses of a billion Suns. But astronomers cannot agree on how the jets form.
In one leading theory, the jets begin in an accretion disk of ionised gas, or plasma, around the black hole. The plasma’s charged particles create magnetic fields as the disk rotates, and those fields push gas and radiation out at the poles of the black hole.
In a competing theory, the jets begin closer to the black hole — and are caused by the wild spinning of the black hole itself. Black holes are so massive that if they rotate, they are thought to drag the surrounding space and time along with them. This blurred region at the edge of the spinning behemoths also twists the magnetic fields generated in the accretion disk as the gas in the disk falls toward a black hole.
It is this confused region of twisting which is believed to spew out the jets.
“The spinning twists up the magnetic field like a barber pole and the plasma is lifted out along the rotational axis,” says David Meier, an astrophysicist at NASA’s Jet Propulsion Lab in Pasadena, California, who has worked on previous simulations.
Now three researchers in Russia and the US have seen this in their new simulations. To simplify the calculations, they modelled the magnetic field lines in the plasma as strings, which begin in the outer disk.
When the plasma comes as close to the black hole as Saturn does to the Sun, “things really start going” and the lines become very twisted, says Brian Punsly, an astrophysicist at Boeing Space and Intelligence Systems in Colorado, US, and one of the team.
But other researchers say the jets’ source is still up for debate. “It’s been a rather controversial business,” says Roger Blandford, an astrophysicist at Stanford University, US, who helped spearhead both competing theories about 30 years ago.
He calls the new work “interesting” but says he has identified seven different scenarios for magnetic fields – some of which cross into the black hole itself – that could cause the jets. “I honestly don’t know what the answer is,” he told New Scientist.
The reality may be that both theories explain what is going on around black holes. Some astronomers think spinning black holes may cause jets in “radio-loud” quasars, and that accretion-disk jets – which probably do not travel as far and so are hard to observe – may arise from “radio-quiet” quasars.
“But I don’t think the community as a whole has really agreed on which sources are doing which, or whether some of them in fact may do both at different times,” says Meier.