Mysterious radio bursts come from outside our galaxy

The Parkes radio telescope, where the
 observations were made.

Mysterious radio bursts come from outside our galaxy
A huge surge of power in just a few milliseconds and then, nothing.

by John Timmer - July 6 2013, 11:30am PDT

http://goo.gl/PvYH2

Astronomers using a radiotelescope to perform a survey of a broad patch of the sky have spotted a set of unusual events that last for just a handful of milliseconds. The events don't repeat and aren't accompanied by anything obvious at optical X-ray wavelengths. A careful examination of their properties, however, gives reason to believe that they are likely to occur at great distances from our galaxy, suggesting they are the product of cataclysmic occurrences. Based on the four events detected during their survey of a single patch of the sky, the astronomers suggest that thousands of them may be visible from Earth each day—provided we know where to look.

There's really not a lot to say about the Fast Radio Bursts (FRBs) themselves. Four were identified that differ largely in intensity; all of them lasted for less than five milliseconds. If the events were associated with bursts at high-energy wavelengths, then we have hardware in space that should have detected them. Although the radiotelescope used for the survey couldn't provide careful location information, there was nothing out of the ordinary in the area reported at optical wavelengths, either.

If the radio bursts were coming from within our galaxy, then a variety of situations could produce signals with that kind of energy. So, the authors tried to figure out where the bursts are coming from. To do that, they relied on the fact that radiowaves interact with the ionized interstellar gas in a way that alters their spectrum. The more gas they interact with, the larger the effect. Based on the properties of these FRBs, it appears they went through a lot of gas. But their locations indicate that they didn't travel through much of our galaxy's gas, which implies that they must also be affected by some combination of the source galaxy and the intergalactic medium.

If they're that far away, then they must be very energetic. "At cosmological distances, this indicates that they are more luminous than bursts from any known transient radio source," the authors noted, before concluding that this indicates "that the FRBs are likely cataclysmic in nature." But it can't be a rare cataclysm; based on the amount of sky surveyed and the frequency of the FRBs they detected, the authors estimated that as many as 104 might be visible across the entire sky.

So, what's the cataclysm? That's what's hard to explain. The typical radio bursts we've seen are either repetitive (like a pulsar) or accompanied by signs at other wavelengths. These are neither. One option the astronomers considered was a neutron star merger, but those mergers are expected to be very rare and wouldn't produce that much energy. The same problem faces an alternative explanation, which is that it could be the evaporation of a black hole—but it simply doesn't produce enough energy. A supernova with an orbiting neutron star might produce this sort of energy, but those aren't expected to be all that common.

So, for now, the source is a bit of a mystery. Which means astronomers will probably start devising ways to better pin down the locations of these events and do follow-ups with other hardware to try to see precisely where they originate and whether anything interesting is going on in the region. But, as an accompanying perspective notes, it took us decades to pin down the likely source of gamma-ray bursts, so it might not be wise to hope for a quick resolution to the mystery.

Science, 2013. DOI: 10.1126/science.1236789  (About DOIs).