鈥淓VERYONE thinks that if the sun turned into a black hole tomorrow we鈥檇 all be sucked in, but that just wouldn鈥檛 happen,鈥� says astronomer Andy Fabian at the University of Cambridge. 鈥淭he Earth wouldn鈥檛 really notice the difference 鈥� it would keep happily orbiting.鈥�
That鈥檚 because unless something stripped the Earth of its angular momentum, it would continue in its path exactly as before. The same is true of matter in the 鈥渁ccretion disc鈥� around a black hole, which raises the puzzle of how black holes manage to slurp this matter in. Now astronomers think they have figured out exactly what depletes the disc鈥檚 angular momentum.
For thirty years, astrophysicists have believed that the most likely explanation must be magnetic fields in the black hole鈥檚 accretion disc. Such fields would create turbulence and friction in the matter, slowing the particles down and robbing them of angular momentum, says Jon Miller of the University of Michigan in Ann Arbor. Magnetism could also drive winds along field lines, spewing material 鈥� and angular momentum 鈥� out of the system.
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However, there were other theories for how angular momentum could escape. Now Miller, Fabian and colleagues think they have ruled out the alternatives. The team studied a jet of matter streaming out from a black hole鈥檚 accretion disc, recorded with NASA鈥檚 orbiting Chandra X-ray Observatory. They found that matter was flowing out at 100 kilometres per second and at a temperature of about 1 million 掳C. That contradicts the idea that thermal heating could generate this outflow, as the matter would have to be at 50 billion 掳C (Nature, vol 441, p 953).
The team also found that outflow was highly ionised, a state in which the accretion disc could not absorb enough ultraviolet radiation from surrounding space to generate the winds. That left magnetic fields as the only plausible source of the winds. 鈥淚t was a process of elimination,鈥� says Fabian.
But astrophysicist Axel Brandenburg of the Nordic Institute of Theoretical Physics in Copenhagen, Denmark, is more cautious. 鈥淭hey have shown that the magnetic field scenario does not contradict the observations, but there鈥檚 still room for other new explanations,鈥� he says.
Lead researcher Miller accepts that criticism, but remains confident. 鈥淲e eliminated the two most plausible alternatives, and they didn鈥檛 just fail by a few per cent, they failed dramatically,鈥� says Miller. 鈥淭he magnetic force is all that is left that is viable.鈥�
