To calculate how far the bursts have come, astronomers use a concept called the dispersion measure. Each burst covers a range of radio frequencies, as if the whole FM band were playing the same song. But electrons in space scatter and delay the radiation, so that higher frequency waves make it across space faster than lower frequency waves. The more space the signal crosses, the bigger the difference, or dispersion measure, between the arrival time of high and low frequencies – and the further the signal has travelled.

Michael Hippke of the Institute for Data Analysis in Neukirchen-Vluyn, Germany, and John Learned at the University of Hawaii in Manoa found that all 10 bursts' dispersion measures are multiples of a single number: 187.5 (see chart). This neat line-up, if taken at face value, would imply five sources for the bursts all at regularly spaced distances from Earth, billions of light-years away. A more likely explanation, Hippke and Lerned say, is that the FRBs all come from somewhere much closer to home, from a group of objects within the Milky Way that naturally emit shorter-frequency radio waves after higher-frequency ones, with a delay that is a multiple of 187.5 (arxiv.org/abs/1503.05245).

They claim there is a 5 in 10,000 probability that the line-up is coincidence. "If the pattern is real," says Learned, "it is very, very hard to explain."

Cosmic objects might, by some natural but unknown process, produce dispersions in regular steps. Small, dense remnant stars called pulsars are known to emit bursts of radio waves, though not in regular arrangements or with as much power as FRBs. But maybe superdense stars are mathematical oddities because of underlying physics we don't understand.

It's also possible that the telescopes are picking up evidence of human technology, like an unmapped spy satellite, masquerading as signals from deep space.

The most tantalising possibility is that the source of the bursts might be a who, not a what. If none of the natural explanations pan out, their paper concludes, "An artificial source (human or non-human) must be considered."

"Beacon from extraterrestrials" has always been on the list of weird possible origins for these bursts. "These have been intriguing as an engineered signal, or evidence of extraterrestrial technology, since the first was discovered," says Jill Tarter, former director of the SETI Institute in California. "I'm intrigued. Stay tuned."

Astronomers have long speculated that a mathematically clever message – broadcasts encoded with pi, or flashes that count out prime numbers, as sent by aliens in the film Contact –could give away aliens' existence. Perhaps extraterrestrial civilisations are flagging us down with basic multiplication.

But a fast radio burst is definitely not the easiest message aliens could send. As Maura McLaughlin of West Virginia University, who was part of the first FRB discovery points out, it takes a lot of energy to make a signal that spreads across lots of frequencies, instead of just a narrow one like a radio station. And if the bursts come from outside the galaxy, they would have to be incredibly energetic to get this far.

If the bursts actually come from inside the Milky Way, they need not be so energetic (just like a nearby flashlight can light up the ground but a distant light does not). Either way, though, it would require a lot of power. In fact, the aliens would have to be from what SETI scientists call a Kardashev Type II civilisation (see "Keeping up with the Kardashevs").

But maybe there's no pattern at all, let alone one that aliens embedded. There are only 10 bursts, and they fit into just five groups. "It's very easy to find patterns when you have small-number statistics," says McLaughlin. "On the other hand, I don't think you can argue with the statistics, so it is odd."