The enormous black hole at the centre of the Milky Way Galaxy generates a surprisingly strong magnetic field, more than a hundred times stronger than that of Earth, radio astronomers have revealed — using a rare kind of neutron star as a probe. The finding helps to explain why the hole is growing relatively slowly compared with the voracious rate at which one might expect a black hole to feed.

Radio astronomers have been scanning the central region of the Milky Way for years in search of pulsars — neutron stars that have a characteristic, pulsating radio-wave signature. They hope to use these stars to test Einstein’s general theory of relativity. If a pulsar orbits a black hole, subtle changes in its pulsation frequency can reveal how the black hole warps the space that surrounds it, a prediction of Einstein's theory that would thus be put to a stringent test.

Hopes that such a pulsar had been found were high on 24 April when a NASA space telescope caught an X-ray source flaring up in the vicinity of Sagittarius A*, the black hole at the centre of the Galaxy1. Only two days later, news reached experts during a radio astronomy meeting in Bonn, Germany, that another NASA observatory had found that the source pulsated every 3.76 seconds and was a rare type of pulsar called a magnetar2, says Heino Falcke, a radio astronomer at Radboud University in Nijmegen, the Netherlands.

The magnetic field of a magnetar can be 1,000 times stronger than that of an ordinary pulsar, and 100 trillion times stronger than that of Earth. Only some 20 magnetars are known, and this was only the fourth magnetar found that had a detectable radio pulsation.

Falcke and his colleagues began to observe the location of the flare using the 100-metre Effelsberg Radio Telescope near Bonn — one of the world’s largest steerable dishes. Their measurements, published online today in Nature3, confirm earlier estimates that the magnetar is at least 20,000 times farther from Sagittarius A* than Earth is from the Sun.

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That's too far for precise relativistic measurements, says study co-author Ralph Eatough of the Max Planck Institute for Radioastronomy in Bonn. But the team was able to use the star to measure another crucial feature of the black hole: the magnetic field of the stellar material being pulled into the hole’s orbit. The material in this ‘accretion disk’ becomes hot and ionized as it circles the black hole, and produces a magnetic field that slows its descent, turning what might have been a feeding frenzy into a leisurely meal.

The pulsar lies within the reach of the black hole's magnetic field, which twists the pulsar’s radio waves — which would normally wiggle along a plane — into a corkscrew motion, Eatough explains. That rotation effect is different for different wavelengths, a fact that enabled the researchers to estimate the strength of the magnetic field. “The rotation is way higher than anything seen in the Galaxy,” Eatough says, “with the exception of Sagittarius A* itself”, whose radio waves get twisted by its own magnetic field.

“The result nicely ties into the larger picture of why Sagittarius A* is so faint — a puzzle that has stimulated a lot of research over the past 20 years,” says astronomer Stefan Gillessen of the Max Planck Institute for Extra­terrestrial Physics in Garching, Germany. Black holes are supposed to be insatiable devourers of matter and energy, and their accretion disks should radiate brightly as they do so, but many of the supermassive black holes nearby seem to be slow eaters. “The fact that most massive black holes don't shine must mean that they are on a diet,” Gillessen says.

Magnetars are rare among pulsars, so the discovery of a magnetar suggests that many more run-of-the-mill pulsars should exist near Sagittarius A*, hidden behind its hot gas cloud. Although this magnetar is just a bit too distant from the black hole to be used for testing Einstein, “it raises our hopes of finding more pulsars nearer to the black hole”, Eatough says.