Common belief states that if the sun turned into a black hole tomorrow we’d all be sucked in. But according to astronomer Andy Fabian at the University of Cambridge, “The Earth wouldn’t really notice the difference - it would keep happily orbiting.”

This is 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 “accretion disc” around a black hole, which in turn raises the question of how black holes manage to slurp this matter in. Astronomers think they have figured out exactly what depletes the disc’s angular momentum.

Astrophysicists have believed that the most likely explanation must be magnetic fields in the black hole’s accretion disc. Such fields would create turbulence and friction in the matter, slowing the particles down and robbing them of angular momentum. Magnetism could also drive winds along field lines, spewing material - and angular momentum - out of the system.

There were other theories for how angular momentum could escape. Now astrophysicists think they have ruled out the alternatives. A team studied a jet of matter streaming out from a black hole’s accretion disc, recorded with NASA’s 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 degrees Celcius. That contradicts the idea that thermal heating could generate this outflow, as the matter would have to be at 50 billion degrees Celsius (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.

But astrophysicist Axel Brandenburg of the Nordic Institute of Theoretical Physics in Copenhagen, Denmark, is more cautious. “They have shown that the magnetic field scenario does not contradict the observations, but there’s still room for other new explanations,” he says.

Lead researcher Miller accepts that criticism, but remains confident. “We eliminated the two most plausible alternatives, and they didn’t just fail by a few per cent, they failed dramatically,” says Miller. “The magnetic force is all that is left that is viable.”

Links of Interest

Cambridge scientists follow doomed matter on a ride around a Black Hole