It’s the ultimate alternative energy source. Some of the mysterious radio signals flashing far outside our galaxy may be powered by cosmic batteries. Black holes and neutron stars may hook up into a kind of circuit just before they collide, producing ultra-short, ultra-bright blasts of radio waves.

Astronomers first detected a fast radio burst (FRB) in 2007 – a big flash that released more energy in a few milliseconds than the sun does in a month. Since then, we have found 10 more. Their origins remained a mystery, with theories ranging from star flares to messages from ET.

Now a study suggests that some of the bursts come from black holes in orbit with neutron stars, the dense cores of collapsed stars. When these duos spiral towards each other, their close approach creates a sort of battery, which creates the power that sends these strange bursts into space.


Light bulb moment

In this scenario, the black hole passes within its companion neutron star’s magnetic field, and the two link up form a circuit. The motion from both objects’ orbits and spins generates an electric current, which flows along the magnetic field lines that stretch between them.

“If you hold a disconnected light bulb and wave a magnet around, you can turn on the light,” says Janna Levin of Columbia University in New York. “Waving magnetic fields create electricity. The black hole battery works similarly.”

The neutron star’s “waving” magnetic field generates electricity, and plasma carries electric current along the field lines. As the charged plasma particles travel along the field lines, the area lights up in radio waves.

As the black hole and neutron star spiral closer, the battery’s power grows. In the few milliseconds just before the merger, the “bulb” suddenly lights up super-bright. From far away, the radio waves from that final countdown appear as a sudden burst.

Cosmic fingerprint

Unlike other FRBs, these would have a distinct fingerprint – a preview of dimmer radio waves followed by two peaks in brightness – that will allow scientists to distinguish them from other bursts in the future.

Battery-powered bursts can’t be the only kind out there, though, because not enough of these binary systems exist. “But I do think it tells us that there are many, many possible scenarios for FRBs,” says Maura McLaughlin of West Virginia University in Morgantown, who was part of the first FRB discovery.

Battery-powered FRBs could also help prove Einstein right. His theory of general relativity predicts that when these two massive objects merge, the powerful collision sends shock waves through space-time. No one has ever seen these so-called gravitational waves. But new instruments like LIGO should be able to spot the ones emanating from a collision between a neutron star and a black hole. If astronomers can pick up the pair’s final FRB broadcast, it should help confirm the existence of gravitational waves and unravel their origins.

“It’s probably safe to say that as many as 5000 FRBs could be discovered over the next five years,” says Chiara Mingarelli of the California Institute of Technology in Pasadena. With any luck, some of those will be battery-powered, and come nicely packaged with confirmation of general relativity.

Journal article: The Astrophysical Journal Letters, DOI: arXiv:1511.02870

Image credit: Dana Berry/NASA