From nowhere, they appear as a sudden surge of power in the radio spectrum. Then, a few milliseconds later, they're gone—and as far as we could tell, they never come back. They've picked up the name "fast radio bursts," but nobody's entirely sure of what produces them. Follow-up observations have generally failed to find anything interesting in their direction, and the bursts didn't seem to repeat, leaving everyone who cares about these sorts of things a bit mystified.

One possible explanation for their one-time-only appearance would be that they're the product of a process that destroys the object that creates them. Thus, if they were produced by the collapse of a neutron star into a black hole (to give just one example), there'd be no way for that to happen twice.

But a new study suggests that at least one of them has repeated, which would take cataclysmic explanations off the table. There are enough differences between this burst and previously observed ones, however, to raise the question of whether there might be several processes producing similar surges in radio emissions.

The burst in question (which goes by FRB 121102) was identified using the Arecibo Observatory in Puerto Rico. There was nothing especially unusual about it, but the authors scheduled it for follow-up observations. Some of these turned up nothing. But others turned up several bursts, sometimes within hours of each other. All told, they saw 10 additional bursts (FRB 121102 goes to 11!) over the course of two months. These had different brightnesses, and different intensity peaks showed up at different wavelengths, suggesting a somewhat chaotic process.

Looking through the timing data, the authors found no indication of a periodic process, which makes some sort of orbital behavior unlikely; the quick repetition in some cases also makes an orbital phenomenon unlikely. And, as the authors put it, "Repeat bursts rule out models involving cataclysmic events."

So what is still on the table? Neutron stars with intense magnetic fields, called magnetars, can create giant flares. But none of the ones we know about have produced more than a single flare. The authors suspect it might be a pulsar—a rapidly rotating neutron star—where sporadically, one of the normal pulses is greatly amplified. This sort of behavior has been seen in at least some neutron stars in our galaxy.

Confusing matters further is the question of whether FRB 121102 is a typical fast radio burster. The other ones we know about are more intense by about a factor of 10, and they appear to originate outside our galaxy. Based on the direction it's coming from, however, the authors suggest there's a chance that FRB 121102 comes from an object inside our galaxy. Finally, there's the issue of instrumentation. Most fast radio bursts have been spotted by Australia's Parkes telescope, which isn't as sensitive as Arecibo and wouldn't have identified most of the bursts emitted by FRB 121102.

So, rather than definitively ruling out a destructive cause for fast radio bursts, the best that this paper can do is indicate that at least some of them leave their source intact. Which, given how little we knew about these previously, represents progress.

Nature, 2015. DOI: 10.1038/nature17168 (About DOIs).