Out of tune (Image: NASA's Goddard Space Flight Center)

The wobbles in energy produced as a black hole consumes a star have been detected from further away than ever before.

Because they are so regular, their discoverer likes to think of the wobbles as a kind of sound. “We are able to metaphorically hear a star being devoured by a supermassive black hole,” says Rubens Reis of the University of Michigan in Ann Arbor, “4.8mHz would be a very low D-sharp, slightly out of tune.”

The wobbles, previously only detected in much closer black holes, allow the black hole feeding process to be compared across vast distances, which in turn acts as a test of Einstein’s general relativity, the leading theory of gravity, on cosmic scales.


As the black hole is supermassive, the detection also confirms that black holes feed on star stuff in the same way, whether the black hole is the mass of 10 suns or 10 million suns. “For me that is a rather beautiful thing,” says Reis.

Star shredder

Supermassive black hole Swift J1644+57 was spotted on 28 March last year when NASA’s Swift telescope detected several bursts of gamma rays from a previously quiet part of the universe. The gamma rays were the first sign of the previously dormant black hole starting up again and devouring a star that wandered too close.

It looked so intriguing, Reis and his colleagues followed it up with observations from the European Space Agency’s more powerful XMM-Newton space telescope. That allowed the researchers to detect slight wobbles, known as quasi-periodic oscillations or QPOs, in the amount of energy emitted as the black hole shreds the star.

Astronomers have seen similar wobbles before, but mostly in black holes with masses of only a few suns within our own galaxy. This one is supermassive, with the mass of 10 million suns, and lies 4.5 billion light years away. By measuring this wobble, astronomers can work out how close something could orbit the black hole without getting sucked in. From this they know the black hole’s mass and how fast it is rotating.

Evolutionary clues

Black hole spin is thought to change over time as the black hole swallows stars and becomes more massive. By finding more of these wobbles in disks around distant black holes – whose light reaches us from the early universe – astronomers hope to learn more about how black holes have changed over the lifetime of the universe.

“It looks like this fortuitous observation not only confirms a long-predicted set of behaviours for the disruption and consumption of stars by black holes, but it might lead the way to more directly probing their spin and mass,” says Caleb Scharf, an astrophysicist at Columbia University, New York.

A black hole’s mass and spin can be plugged into Einstein’s equations of general relativity to describe the gravity of a black hole. Some think general relativity might vary over extremely large scales: the properties of distant black holes provide a way to test this.

What’s more, “listening” to supermassive black holes wobbling could even tell us about the one at the centre of our own galaxy, which isn’t itself observable in this way.

Wobbles can only be seen during the period when a black hole is chowing down on the remains of a star or other matter – our supermassive black hole is not currently feeding.

“We do not have much information on the immediate region a few million kilometers around our own supermassive black hole,” says Reis.

Journal reference: Science, 10.1126/science.1223940