FRB: Mystery Radio Signal From Galaxy Four Billion Light-years Away Discovered by Scientists

A radio signal coming from a galaxy billions of light-years away has been discovered by scientists. What is producing the signal—a fast radio burst, or FRB—is unknown.

FRBs are radio signals that last just a few milliseconds. What causes them is not known, because it has been impossible to work out where they came from. They were first identified decades ago, but they are normally found in data long after the event has happened.

Pinpointing the source of an FRB is a bit like seeing a bright camera flash in a very dark room—and then trying to find the camera in the dark. On top of that, the room is potentially billions of light-years in size.

Scientists have identified dozens of FRBs and most appear to be one-off events. This led them to speculate that they could be produced some cataclysmic event, such as a neutron star collapsing into a black hole.

However, the mystery deepened in 2016 when researchers discovered an FRB that appeared to be repeating. This discovery—known as FRB 121102—meant that, at least in this case, a one-off event could not explain it. Since then, researchers have been able to trace back these radio signals to the star-forming region of a dwarf galaxy three billion light-years away, and are now working to determine the cause. One of the most popular theories is that the bursts are coming from a recently-formed magnetar—a spinning neutron star with an extremely high magnetic field.

But by repeating, FRB 121102 is an anomaly. For the vast majority of FRBs, scientists do not know where they come from or what is causing them.

In a study published in the journal Science, an international team of researchers has announced they have discovered where a single FRB came from—a galaxy over four billion light-years away. The burst, named FRB 18092, was first identified in September, 2018 using the 36-antenna Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope.

After the initial event, the team, led by Keith Bannister of Australia's Commonwealth Science and Industrial Research Organisation (CSIRO), used the tiny differences in the time it took for the light from the signal to reach various telescope antennas to zoom in on the where the FRB came from.

Not only were they able to trace it to the galaxy it came from, they were also able to find its position within the galaxy—in the "galactic suburbs." "It's a very average galaxy," study author Ryan Shannon, from Australia's Swinburne University of Technology, told Newsweek. "The burst comes from the outskirts of it, away from the center of the galaxy."

The galaxy where FRB 18092 came from is very different to that of FRB 121102. "If anything the galaxy has an old stellar population," Shannon said. "This is in contrast to the repeater which has a young stellar population."

The team does not know what celestial event caused the burst, and finding that the
two known FRB locations are very different raises even more questions: "Astronomers providing explanations for fast radio bursts will need to take pause given this discovery," Shannon said. "A lot of the theory of what causes FRBs is built around the first repeater, its host galaxy and environment. The fact that the hosts are so different means that if there is only one source, there would need to be a re-think of the theory. If not....there could be different ways to produce the bursts."

He also said that given there are thousands of FRBs in the sky every day, a single catastrophic event may not be sufficient to explain these mystery signals: "It would need to be a catastrophic event that occurs relatively regularly."

However, he added: "It's hard to rule out magnetars, as they are the only objects we know of that have the power to produce the intense radio pulses we see from FRBs. There's been a second channel proposed to form magnetars—the merger of white dwarf stars. These would be found in the outskirts of old galaxies."

The discovery has been welcomed by astronomers. Jason W. T. Hessels, from the University of Amsterdam, who was not involved in the study, said it was a "seminal result." He told Newsweek: "To understand the physical origin(s) of FRBs, and whether there are multiple types of FRB generators, we absolutely need precision localisations that unambiguously identify the host galaxies of these sources. The fact that this FRB is in a host galaxy that is quite unlike the dwarf galaxy of the first repeater, FRB 121102, indeed suggests that they could be from different types of objects, and that repeating and non-repeating FRBs come from physically distinct source types. However, more precision localisations are needed for us to build up a clear picture of what's going on."

Paul Scholz, from Canada's Dominion Radio Astrophysical Observatory, who was also not involved, told Newsweek: "The second localization presented in [the study] is different from FRB 121102 in almost every way: the source is not repeating, it was found to be in a large galaxy with an old population of stars, and most of the other pieces of evidence that support the young magnetar theory aren't present.

"This means that either 1) this FRB is caused by a different type of source than FRB 121102, or 2) the source that causes both FRBs is the same, but can occur in both dwarf galaxies with young recently-formed stars and massive galaxies with older, evolved stars. (1) would mean that there are multiple phenomena that can generate FRBs and (2) would limit the types of phenomena that could cause FRBs to those that can occur in both young and old environments.

"To disentangle these two possibilities we'll need to build up more host galaxy identifications, so I'm looking forward to further results from ASKAP and other telescopes that can provide these precise localizations."

Shannon said the team hopes to find even more bursts. If they are able to trace more FRBs back to their host galaxies, they will be able to start finding similarities that could help solve the mystery. Should they find out what is causing these radio signals, it may open up a new way for scientists to study the universe, he added.