For the first time, scientists have tracked the source of a “fast radio burst” – a fleeting explosion of radio waves which, in this case, came from a galaxy six billion light-years away.
The cause of the big flash, only the seventeenth ever detected, remains a puzzle, but spotting a host galaxy is a key moment in the study of such bursts.
Fast radio bursts last only milliseconds but in that moment, whatever makes them blasts as much energy into space – in the form of radio waves – as our Sun emits in days or even weeks.
To follow this particular signal home, an international team did rapid detective work with multiple telescopes and ultimately snapped an image of the source galaxy in visible light.
Lead author Evan Keane had set up an alert system to trigger this flurry of activity, by running live data from the Parkes Radio Telescope in Australia directly into a supercomputer.
“The goal was to reduce the lag from the thing hitting the dish, to us knowing that it hit the dish, from months – to nothing,” he told the BBC.
Sure enough, when one of these mysterious bursts hit Parkes’ famous 64m dish on April 18 2015, alarm bells rang and emails quickly circled the planet.
Two hours later, the six 22m dishes of the Australian Telescope Compact Array, a 400km drive from Parkes, were already homing in on the culpable corner of the sky.
They caught an afterglow of the flash, which took six days to fade. It was much fainter than the burst itself but allowed the team to zoom in on the source of the burst with 1,000 times more precision than ever before.
Knowing exactly where to look, they next went hunting in optical light using the Subaru Telescope in Hawaii, run by the National Astronomical Observatory of Japan.
“This is not what we expected,” said co-author Dr Simon Johnston, the head of astrophysics at CSIRO which runs the Australian telescopes.
If the host galaxy is old, then the flash was more likely caused by a merger of dead stars than, for example, a supernova blowing up.
“It might mean that the FRB results from, say, two neutron stars colliding rather than anything to do with recent star birth.
“Our discovery opens the way to working out what makes these bursts.”
Crucially, having an optical snap of the galaxy lets astronomers calculate how far away it is, based on its redshift – the stretching of light by the expansion of the Universe.
And with that distance chalked up, the team could do some cosmology.
Within an FRB, Dr Keane explained, the short wavelengths arrive before the longer ones.
“This is because the signal crashes into the particles and dust in between… and they cause a tiny delay. But if you travel six billion light-years, the delay accumulates.
“From that we can say ok, if it got a second’s worth of delay on its journey, that means it passed through this many particles.”
So he and his team, for the first time, used a far-flung radio signal to calculate the density of the intervening swathe of cosmos.
“It’s essentially weighing the Universe.”
Their cosmic weigh-in had a happy result for our current model. It matches the quantity of stuff we expect from the 5% of the Universe that should, in fact, be matter.
The other 95% (25% dark matter and 70% dark energy – we think) is famously mysterious. But in fact, only half of the matter that ought to be in the knowable 5% has ever been detected using telescopes.
The remainder has been referred to as “missing”.
“All of the matter causes a delay in the FRB signal – both the half that we had seen, and the half we hadn’t,” said Dr Keane.
“So we measured this delay, and if you work out how much matter must be there to cause it – it’s right. The missing matter is missing no more.”