What causes fast radio bursts?

The question

In 2007 Duncan Lorimer and his student David Narkevic, going through archival data from the Parkes radio telescope in Australia, found a single burst of radio waves recorded six years earlier. It lasted less than five thousandths of a second and was extraordinarily bright. Its signal showed that it had travelled through an enormous amount of space: it came from far outside our galaxy.

Since then thousands of such fast radio bursts have been recorded. Over the whole sky, several thousand are estimated to arrive every day. In a few milliseconds, one burst releases as much energy as the Sun emits over days. What can do that, so briefly, and so often?

This question is marked as partly answered: at least some bursts now have a known source. But it isn't clear whether that source explains all of them.

What we know

The bursts are real and come from other galaxies. For years they were doubted: only a handful were known, and in 2015 a similar signal at Parkes turned out to come from a microwave oven in the observatory kitchen, opened before it had finished. But the true bursts kept appearing at other telescopes, and since 2017 dozens have been traced to specific galaxies, some billions of light years away.
Some sources repeat. In 2016 a source was found that flashes again and again, which ruled out explanations based on one-time catastrophes for at least that source. Most known bursts, however, have been seen only once.
In April 2020, two instruments recorded a burst from a magnetar in our own galaxy — a young neutron star with an extremely strong magnetic field. It was weaker than the bursts from distant galaxies, but similar enough to show that magnetars can produce them.
The Canadian CHIME telescope has transformed the field: its first catalogue in 2021 alone contained more than 500 bursts, and their number keeps growing.
The bursts have become a tool. As they pass through space, the radio waves are slightly delayed by the gas they cross. In 2020 this was used to weigh the thin gas between galaxies, finding ordinary matter that had been "missing" from the cosmic inventory.

Main hypotheses

Magnetars

The leading explanation: young, highly magnetised neutron stars produce bursts through flares in their magnetosphere or shocks in the surrounding gas. The 2020 burst from our galaxy strongly supports this, at least for some sources.

Two or more populations

Repeating and non-repeating bursts differ in duration and frequency structure. They may come from different kinds of objects, or from the same objects at different stages of life.

Older and more unusual sources

In 2022 a repeating source was traced to a globular cluster in the galaxy M81, a cluster of very old stars where young magnetars shouldn't exist. Such sources may be magnetars born in mergers of old stars, or something else entirely: interacting binary stars, pulsars in unusual surroundings, collisions with neutron stars.

Where understanding ends

We know that magnetars can make fast radio bursts. We don't know whether they make all of them. The burst from our galaxy was thousands of times weaker than typical bursts from distant galaxies, and it isn't clear whether ordinary magnetars can reach those energies or whether rarer, more extreme objects are needed.

The mechanism itself is also unclear. To produce such a bright, short signal, enormous numbers of particles must radiate in step, coherently, like a laser. Where exactly this happens — close to the star in its magnetic field or far away in shock waves — and how it works are still debated.

And the old question remains: why do some sources repeat while most, as far as we can see, don't? It is not yet known whether the non-repeating ones are simply quieter repeaters observed for too short a time, or a different phenomenon.

What would settle it

Many more bursts with an exact position in their galaxies, showing what kinds of environments they come from. New and planned radio telescopes, such as CHIME's outrigger stations and the Square Kilometre Array, are designed for this.
A burst observed at the same time as a flash in X-rays or other radiation from a distant source, linking it to a specific type of object.
A model of the emission mechanism that predicts the observed properties of bursts and passes tests on new data.