The supermassive black hole at our galaxy's centre enjoys a slow dinner, thanks to a giant magnet.
The Milky Way's black hole, Sagittarius A*, is clearly sucking in hot gas because doing so makes it burp out radio waves that we can detect. But jets of matter streaming outwards slow down its gas consumption. "The black hole is not feeding to its full potential," says Ralph Eatough at the Max Planck Institute for Radio Astronomy in Bonn, Germany.
The jets could be the result of a magnetic field produced by charged ions swirling around in the gas but it wasn't clear whether the field was strong enough. To find out, Eatough and colleagues turned to a nearby spinning star, called a pulsar
This particular pulsar was discovered earlier in the year, just 0.3 light years away from the black hole. Eatough's team realised that the strong bursts of radio waves emitted by the pulsar would be rotated by the black hole's magnetic field, and so could be used to measure the strength of the field.
The rotations suggested that, at the event horizon, the field is hundreds of times stronger than Earth's magnetic field, and strong enough to produce the jets that slow the hole's chomping.
Finding other pulsars nearer to the black hole could teach us even more, says Eatough. These would feel its gravitational pull strongly enough to put Einstein's most famous theory to the test.
Although Einstein's theory of general relativity is hugely successful at explaining the effects of gravity, it seems incompatible with quantum mechanics – so physicists are interested in testing it with ever more precision. Measuring the influence of the Milky Way black hole's gravitational pull on the timing of close-orbiting pulsars could do the trick.
"Because the black hole at the centre of the galaxy is the biggest nearby, this should provide one of the best tests of general relativity in the galaxy," says Eatough.
Journal reference: Nature, DOI: 10.1038/nature12499
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