Cosmic detectives: clues in an ancient fast radio burst

On 14 May last year, the Parkes radio telescope in New South Wales, Australia picked up a mysterious signal. Lasting only a few milliseconds, it was fast and furious. The intense radio burst was the first ever to be captured by a radio telescope as it transpired – the Parkes telescope just happened to be pointing at the right patch of sky. Reported in the Monthly Notices of the Royal Astronomical Society, the event was a coup for Parkes and PhD student Emily Petroff from Melbourne’s Swinburne University of Technology.

But what exactly was it?

Not much is known about the origins of these fast radio bursts. They have been detected before but only after the fact, when astronomers noticed mysterious blips in their recordings. So when the Parkes telescope picked up the signal in May, telescopes around the world immediately kicked in, straining to detect the explosive bursts at other wavelengths so that we might learn more about the identity of these signals.

But alas – as randomly and abruptly as it had arrived, the fast radio burst disappeared with no trace or afterglow.

In the face of this gaping mystery, the world’s astronomers have come up with a number of hypotheses about the source of the signal. It’s a great case of seeing astronomy in action.

Let’s put on Sherlock Holmes’ deerstalker hat for a minute and study each clue in turn.

Fast

One of the biggest clues is that the signal lasted only a few milliseconds. That means light took that long to cross the radiation-emitting region, putting its size at about 900 km. This points to the source being something relatively small, more the size of a star than a galaxy.    

Radio

Until this discovery we didn’t know if fast radio bursts emitted any other wavelengths of light and, at this stage, it seems they don’t. This is strange as most high-energy events radiate all wavelengths of light.

Burst

These bursts appear and disappear quickly in a single flash. They do not repeat which suggests the intensity of the explosion likely destroyed whatever caused it.  

Distance

This event was bright and easily seen by radio telescopes, but that says nothing about its distance: it could be a faint object close by or an extremely bright but distant one.

Sometimes the cores of massive stars survive the explosion

and are crushed.

So how do we establish distance? As the radio burst travels through space, it is dispersed by the sparse atoms it encounters, much as light travelling through a prism is broken into a rainbow. We can measure how much the radio wave has been dispersed by the time it reaches the telescope, then calculate the number of atoms it would take to cause this degree of dispersion. The density of atoms in space is extremely low – barely one atom per cubic metre (the air you breathe is a trillion trillion times denser). From this latest fast radio burst, it turns out the light had to travel a very long way to account for the degree of dispersion we saw: billions of light-years.

If the fast radio burst came from billions of light-years away, then the event packed as much energy as the Sun releases in a day into an event that lasted for mere milliseconds. This intensity supports the notion that the event was likely to destroy whatever caused it.

Not a glitch

We can rule out the Parkes telescope being faulty. The Arecibo telescope in Puerto Rico detected a fast radio burst in archived data last year. It had similar signal strength to the Parkes fast radio burst, was over in milliseconds and the source was a few billion light-years away. So how do we know the Arecibo telescope wasn’t also faulty? While they’re both radio telescopes, that’s about where similarities between Arecibo and Parkes end. They have different signal receivers and software analysis packages. Most importantly, the Arecibo telescope is half a world away and nestled in the mountains, unlike the Parkes telescope in the Australian outback, which means any interference from local transmitters or reflections from surrounding terrain are different. In essence, there's no way that such similar signals could be produced by faults in such different telescopes.

Common

While they may seem rare, fast radio bursts actually happen much more frequently than we thought. In 2013, PhD student Dan Thornton scanned through a data archive and found four fast radio bursts simply by looking at what had been recorded from a quarter of the sky over a period of just under five minutes. At this rate, we can expect around 10,000 fast radio bursts over the whole sky in a day – the problem is having our telescopes pointing at the right patch of sky to catch one happening in real time.

All together now

Right – so we’re looking for a small region shining with radio waves from a one-off event of incredible violence. My three favourite candidates are:

•      A massive star. Something 10-100 times larger than our Sun, it would explode as it used up all the fuel at its centre and fire a focused jet of energy outwards in what’s called a gamma-ray burst. Radio waves are also produced in the explosion but are more widely spread than the focused beam of gamma rays. So if a gamma-ray burst shoots just past Earth we might detect those widely spread radio waves, but not the gamma rays.
•      Colliding neutron stars. Sometimes the cores of massive stars survive the explosion and are crushed to massive densities, forming an object called a neutron star. Two of these might collide to form a black hole, releasing a huge amount of energy over a small region of space in the process.
•      Super-sized flare from a magnetar – a type of neutron star with a powerful magnetic field. Solar flares from our Sun are generated by a twisting and sudden snapping of its magnetic field. A magnetar might release a flare millions of times more powerful than our Sun that would shine out as a radio burst.

Until we can detect some more of these fast radio bursts, we are limited to educated guesses. But while the night sky might appear calm and unchanging to our eyes, there’s a hidden Universe up there filled with mysterious and startlingly events we’re only beginning to explore. It’s never been a more exciting time to be an astronomer.

More from Alan Duffy:  What's behind the hole in the Sun?