Signal from 6 bln light-years away unveils 'missing matter'

Astronomers have for the first time identified the location of a fast radio burst from six billion light-years away, and used it to weigh the cosmic matter through which it travelled, which helped confirm the presence of the elusive 'missing matter'.

In April last year, a fast radio burst (FRB) was detected by the Commonwealth Scientific and Industrial Research Organisation (CSIRO)'s Parkes radio telescope in Australia.
An international alert was triggered to follow it up with other telescopes and within a few hours, a number of telescopes around the world were looking for the signal.

FRBs are mysterious bright radio flashes generally lasting only a few milliseconds. Their origin is still unknown, with a long list of potential phenomena associated with them.
FRBs are very difficult to detect; before this discovery only 16 had been detected.

"In the past FRBs have been found by sifting through data months or even years later. By that time it is too late to do follow up observations," said Evan Keane, Project Scientist at the Square Kilometre Array Organisation.

To remedy this, the team developed its own observing system to detect FRBs within seconds, and to immediately alert other telescopes, when there is still time to search for more evidence in the aftermath of the initial flash.

With the CSIRO's Australian Telescope Compact Array, the team was able detect a radio afterglow that lasted for around 6 days before fading away. This afterglow enabled them to pinpoint the location of the FRB 1,000 times more precisely than for previous events.

The team then used the National Astronomical Observatory of Japan's Subaru optical telescope in Hawaii to look at where the signal came from, and identified an elliptical galaxy some 6 billion light years away.

The optical observation also gave them the redshift measurement (the speed at which the galaxy is moving away from us due to the accelerated expansion of the universe), the first time a distance has been determined for an FRB.

FRBs show a frequency-dependent dispersion, a delay in the radio signal caused by how much material it has gone through. "Until now, the dispersion measure is all we had. By also having a distance we can now measure how dense the material is between the point of origin and Earth, and compare that with the current model of the distribution of matter in the universe" said Simon Johnston, from CSIRO's Astronomy and Space Science division.

"Essentially this lets us weigh the universe, or at least the normal matter it contains," he said.