Fast radio bursts: enigmatic and infuriating

The best science stories are mystery stories. Something unexplained occurs, the detectives gather their clues, theories are proposed and shot down. In the end, if all goes well, the mystery is solved – at least until the next time something goes bump in the night.

One of the most perplexing mysteries in astronomy today is the fast radio burst, or FRB. Almost 10 years ago, astronomer Duncan Lorimer at West Virginia University noticed a shockingly bright, incredibly quick signal in data collected by the Parkes radio telescope observatory in New South Wales a few years before. Only a few milliseconds long, the burst was as brilliant as some of the brightest galaxies radio astronomers had ever observed.

Intriguingly, the signal swept across radio frequencies, mimicking the behaviour of bright flashes of radiation from very distant pulsars – ultra-dense stars that emit regular pulses of light. A signal that spreads across frequencies usually indicates that cosmic matter is dispersing the light, in the same way a prism spreads white light into a rainbow.

But while the burst looked a lot like a pulsar blip, it didn’t repeat the way pulsar signals do, and no other telescope detected it. Dubbed the “Lorimer Burst”, it stood for years as a one-off event.

Given its uniqueness, some suggested it must have been some kind of Earth-based interference, or perhaps simply a glitch in the Parkes telescope.

Today, fast radio bursts are no longer anomalies. With a hint of what to look for – very short, bright events – astronomers have scoured data from the Parkes telescope and other radio telescopes around the world. FRBs are now so numerous it’s hard to keep up with their discovery.

Yet FRBs are a study in contradictions. So far, only one source repeats, but at such irregular intervals that astronomers have not been able to determine a pattern. Only two bursts have coincided with emissions in visible or any other kind of light, which is necessary to pinpoint the source of the FRB since the radio telescopes can’t give an exact location.

However, one of those two bursts now appears more likely to be a chance alignment than a true correlation, and the other paints the picture of an explosion with such odd characteristics it is hard to reconcile with any known model.

Careful analysis of different FRB signals has suggested explosions of young stars, or old stars, or even collisions between stars, but none of those fit with an FRB that repeats.

One of the biggest open questions is exactly how far away FRBs are. Every attempt to work out their distance has been inconclusive. Even the pattern of their locations in the sky is odd. If they’re all far beyond our own galaxy, we would expect them to appear at random places in the sky.

If they’re all in our galaxy, we should see them mostly along the plane of the Milky Way, where most of the stars are. In actuality, we’ve found them to lie somewhat more often above or below the plane of the galaxy, not randomly like distant sources, and not in the plane like close ones. But with only 20 or so seen so far, it is hard to draw a conclusion.

Thanks to FRBs, we are now looking at the universe in a new way, redesigning our observation strategies and scouring the data for super-short-duration events. Just as every new observing wavelength we try or instrumental technique we develop opens a new window to the universe, this new frontier may allow us to see an entire zoo of cosmic events that were happening all along, unseen. It wouldn’t be surprising to find that FRBs represent a diverse family of cosmic explosions rather than one kind of thing.

The key to solving this mystery will be to catch an FRB in the act and, at the same time, see its fingerprints on a signal detected with another kind of light, thus allowing us to see the galaxy it came from.

Astronomers are already designing surveys that watch for FRBs with radio telescopes and scour the sky with optical, infrared, or gamma ray telescopes around the world simultaneously. Once we have a handful of real-time FRBs along with their host galaxies, we will start to close this case and, more likely than not, open several exciting new ones.

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