A long time ago, something mysterious emitted a flurry of radio waves to the cosmos. Last September, this powerful pulse collided with a series of radio telescopes located in the west of the Australian outback.
Although this ephemeral bombardment lasted mere milliseconds, the scientists managed to trace the origin of the burst of radio waves: a galaxy some 4000 million light years away, in the constellation Grus, the crane.
Although astronomers have detected hundreds of comic pulses like this in the last decade, this last study is the first time that a single burst is captured in action and, later, its origin is located. In principle, discovering the origin of the so-called rapid radio bursts (FRB) should help scientists to delimit the machinery that powers these extreme explosions.
“Locations are fundamental,” says Keith Bannister of CSIRO, the Australian national research agency, which has reported its discovery in the journal Science. “The following locations should show how diverse the phenomenon we are investigating is, which would help the theorists to unravel what is happening.”
However, for now this new observation has only increased the mystery.
“I do not know if we are closer to resolving what the FRB are, but this brings us closer to a more complete idea,” says Emily Petroff, of the University of Amsterdam, one of the leading experts in rapid radio bursts.
The rapid bursts of radio came to prominence almost a decade ago, when Duncan Lorimer, an astronomer at West Virginia University, detected a radio wave eruption for a second in data compiled by the Parkes Observatory in Australia. Then, some astronomers were skeptical of the possible cosmic origin of the blast.
It was very powerful and seemed to come from somewhere far away, so many astronomers suspected that it was an ordinary signal disguised as an exotic intergalactic phenomenon.
But with the appearance of more bursts, some detected by different telescopes, the astronomers began to try to locate the distant origins of the bursts seriously. The first theories included black holes that evaporated, cosmic cataclysms, dead and dense stars and, yes, intelligent extraterrestrial beings. But the shortness of the bursts made it difficult to capture and study them.
Then, in 2016, astronomers using the Arecibo Observatory in Puerto Rico announced that a burst, called FRB 121102, was constant. Unlike other bursts of its kind, the FRB 121102 has not stopped and in 2017 the scientists finally mapped its origin: a strange and blurry dwarf galaxy about 3000 million light years away.
Currently, one of the main theories about the origin of these bursts involves young and extremely magnetic neutron stars, called magnetars or magnetostars, massive suns corpses that have lived fast and died young. But although telescopes from Australia, Russia, the United States and Canada have already detected hundreds of rapid radio bursts, their origins remain unknown.
That is why Bannister and his colleagues were anxious to use the ASKAP radio telescopes in the search for the origin of these astronomical phenomena.
With 36 radio antennas spread over more than five square kilometers, astronomers could use the slight delays in the arrival of bursts to different antennas to locate their location in the sky. On September 24, 2018, while using special software designed to locate unique bursts, the telescopes detected the burst called FRB 180924.
Follow-up observations with optical telescopes from Hawaii and Chile helped the team identify the source of the bursts: a galaxy about 3600 million light years. More specifically, the burst comes from the outskirts of the galaxy, possibly a spiral galaxy that might look pretty much like a younger Milky Way.
“The galaxy is rather boring,” says Bannister. “Most of the stars in the universe live in galaxies like this one. So it’s not uncommon if you’re a star, but it differs a lot from the origin of the FRB 121102”.
The galaxy is about a thousand times more massive than the strange dwarf galaxy from which FRB 121102 comes and forms stars at a much slower rate. This means that the existence of dead stars, such as magnetars, should not be easy here, because huge, inflated stars that collapse and turn into dense stellar corpses usually inhabit regions where stars form more quickly.
“It’s disconcerting that these galaxies are so different, but I think it tells us that we still have a lot to learn about the hosts of the FRB,” says Petroff. “In a way, I’m relieved that this is not a dwarf galaxy like the one that houses [FRB 121102], so it would be too easy.”
So what can astronomers do? As they get more bursts, the intergalactic cartographers will get down to track these radio flashes to their homes, and then hope to start unraveling the tangled history of fast radio bursts. Bannister and other scientists suspect that, ultimately, they could find more than one cosmic engine capable of boosting one of these bursts.
“I am inclined towards an explanation in which there are several ways to create an FRB,” says Bannister. “Scientists have a certain [incorporated] desire to unify everything … but sometimes nature is smarter than we are.”