A Radio Burst From Three Billion Years After the Big Bang

On March 4, 2024, a flash of radio waves lasting a fraction of a second reached South Africa's MeerKAT telescope. It had been travelling for more than 10 billion years. It left its source when the universe was only about three billion years old.

The burst, designated FRB 20240304B, is now the most distant fast radio burst ever detected. It more than doubles the previous distance record, set by a detection in 2023. The work is published in Science under the title "A fast radio burst at redshift 2, three billion years after the Big Bang."

From oddity to instrument

The first fast radio burst was identified in 2007, in archival data from Australia's Parkes telescope, and for years the field argued about whether the signals were even astrophysical. They are. Thousands have now been catalogued, a small fraction of them repeat, and a growing number have been traced to specific host galaxies. What has been scarce is distance. Most well-localized bursts come from the comparatively nearby universe, because a faint host galaxy at great distance is extremely hard to find and harder to measure.

That is why this detection is a step change rather than an incremental record. Doubling the distance means the burst's signal crossed a much longer stretch of cosmic history than any localized burst before it, and it shows the localize-then-follow-up pipeline works at the edge of what telescopes can currently do.

Two telescopes, two jobs

Fast radio bursts are millisecond flashes of radio emission, among the most energetic short events in the sky. Finding one is only half the work. To know how far away it is, astronomers need its host galaxy and that galaxy's redshift.

MeerKAT, through the MeerTRAP project, detected the burst and pinned down its position. The host turned out to be invisible to the largest ground-based telescopes. NASA's James Webb Space Telescope found it: a faint, 28th-magnitude dwarf galaxy at the burst's location. Webb's Near-Infrared Spectrograph then measured its redshift at about 2.15 (NASA gives 2.148).

The research was led by Dr. Manisha Caleb and Dr. Themiya Nanayakkara of the University of Sydney, with co-authors including Dr. Laura Driessen, doctoral student Kavya Shaji, and Prof. Ben Stappers of the University of Manchester, principal investigator of MeerTRAP.

"This is an extraordinary glimpse into the distant universe," Caleb said.

A small, young home

The host galaxy matters as much as the distance. "The galaxy hosting this burst is surprisingly small, metal-poor and undergoing a very active episode of star formation," Driessen said.

That profile fits one of the leading explanations for fast radio bursts: young magnetars, highly magnetized neutron stars formed recently in the deaths of massive stars. A galaxy actively forming stars produces a steady supply of them. It fits less comfortably with scenarios that rely on mergers of older neutron stars, which tend to occur long after star formation has faded.

One burst does not settle the origin of the whole class. Many repeating and non-repeating bursts have been traced to different kinds of hosts. But a burst this early, from a galaxy this young, adds weight to the idea that at least some come from newborn magnetars.

Why a distant burst is a useful tool

A fast radio burst is not only an event. It is a probe. As the signal crosses intergalactic space, its different frequencies are delayed by the electrons it passes through. Measuring that delay tells astronomers how much ionized matter sits along the path, gas that is otherwise extremely hard to see.

The farther the burst, the longer the column of the universe it samples. FRB 20240304B's signal crossed most of cosmic history, carrying a record of the material between galaxies along the way. Each burst like it adds a data point for mapping that diffuse matter and for studying how galaxies and their stars evolved.

More to come

The team estimates MeerKAT may be able to detect and localize several bursts per year at redshifts greater than 1. "In principle, sufficiently powerful bursts could be detectable from the very early universe," Shaji said.

"The next step is to push this frontier further and see how close we can get to the first generations of stars," Stappers said.

The read

For years, fast radio bursts were a mystery in search of a source. This detection shows they are becoming instruments. A radio array in the Karoo spotted a flash; a space telescope a million and a half kilometres away identified a galaxy too faint for the biggest ground telescopes; and together they turned a millisecond event into a measurement spanning ten billion years. The record will likely fall again. The method is what is new: a radio flash plus an infrared spectrum can now place an event in the young universe.