SUPERMASSIVE black holes at the centre of galaxies are thought to shoot out violent jets of matter and energy, but exactly how they do it has been a mystery. Now, the most realistic computer simulations yet suggest that as black holes spin they coil up the magnetic fields around them so tightly that when these fields uncoil they spew particles hundreds of thousands of light years into space.
Astronomers have observed thousands of jets radiating at radio frequencies from what are called 鈥渞adio-loud鈥� quasars. But they cannot agree on how the jets, thought to be powered by black holes a billion times the mass of the sun, are formed.
According to one leading theory, the jets begin in an accretion disc of ionised gas, or plasma, around the black hole. The plasma鈥檚 charged particles create magnetic fields as the disc rotates, and these push gas and radiation out at the poles of the black hole.
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In a competing theory, the jets begin closer to the black hole, and are caused by its spinning. These black holes are so massive they are thought to drag the surrounding space and time around with them. The blurred region at the edge of the spinning behemoths twists the magnetic fields generated in the accretion disc as the gas in the disc falls towards the black hole. These twisted fields contribute to the jets of matter flying outwards.
And that鈥檚 just what Vladimir Semenov and Sergey Dyadechkin of the Institute of Physics in St Petersburg, Russia, and Brian Punsly of the Boeing Space and Intelligence Systems in Torrance, California, saw in their computer simulations. To simplify the calculations, they modelled the magnetic field lines in the plasma as lengths of string. When the plasma comes as close to the black hole as Saturn is to the sun, 鈥渢hings really start going鈥� and the lines become strongly twisted, Punsly says. Stresses build up in these lines, which are being wound up at nearly the speed of light. Eventually they unwind, violently ejecting matter.
But other researchers say the source of the jets is still up for debate. 鈥淚t鈥檚 been a rather controversial business,鈥� says Roger Blandford, an astrophysicist at Stanford University in California who helped spearhead both competing theories about 30 years ago. He says he has identified seven different scenarios for magnetic fields that could cause the jets. 鈥淚 honestly don鈥檛 know what the answer is.鈥� Bigger computer simulations are likely to reveal the answer in the next few years.
David Meier, an astrophysicist at NASA鈥檚 Jet Propulsion Lab in Pasadena, California, hopes observations will provide telling clues. 鈥淲hat we鈥檇 like to be able to do is measure the black holes鈥� spin because that would be an independent confirmation of this idea,鈥� he says.
It is possible that both theories explain what is going on around black holes. Some astronomers think spinning black holes may cause jets in radio-loud quasars, while accretion-disc jets, which probably don鈥檛 travel as far and therefore are hard to observe, may arise from 鈥渞adio-quiet鈥� quasars.
