Flip-flopping magnetic fields hint at a solution for puzzling fast radio bursts from space

Fast radio bursts — intense, millisecond-long flashes of radio energy from space — have amazed astronomers since they were first observed in 2007. A single burst can radiate as much energy in its short life as the sun does in a few days.

The vast majority of short-lived pulses originate outside our Milky Way galaxy. We don’t know what produces most of them, or how.

In new research published in Sciencewe observed a repetitive fast radio burst for more than a year and found signs that it is surrounded by a strong but highly variable magnetic field.

Our results suggest that the source of this cosmic explosion may be a binary system consisting of a neutron star swirling by winds of dense, magnetized plasma produced by a massive companion star or even a black hole.

A fast-paced radio burst that never stops repeating

The repetitive burst known as FRB was 20190520B discovered in 2022 by astronomers at the 500-meter high Aperture Spherical Radio Telescope (FAST) in China. Repetitive fast radio bursts are rare, but FRB 20190520B is the rarest of them all: it is the only one that never rests and produces radio bursts a few times an hour, sometimes on multiple radio frequencies.

After this intriguing object was first found, astronomers scrambled to follow up on the first sighting using other radio wavelengths.

Read more: More ‘bright’ fast radio bursts revealed, but where do they all come from?

Further investigation showed that FRB 20190520B is located in an extremely dense environment in a dwarf galaxy 3.9 billion light-years away. There are also materials around the FRB source that produce strong sustained radio emissions.

This led to the suggestion that the bursting source is a young neutron star in a complex environment.

Powerful magnetic fields

What else can we learn about this intergalactic firework and its environment? We performed observations of FRB 20190520B using CSIRO’s Parkes Radio Telescope, Murriyang, in New South Wales and the Green Bank Telescope in the United States.

To our surprise, FRB 20190520B turned out to produce strong signals at relatively high radio frequencies. These high-frequency signals turned out to be highly polarized – meaning that the electromagnetic waves “wave” much more strongly in one direction than the other.

We found that the direction of this polarization changes at different frequencies. Measuring how much it changes tells us about the strength of the magnetic field the signal has passed through.

It turns out that this measure of polarization suggests that the environment around FRB 20190520B is highly magnetized. What’s more, the strength of the magnetic field seemed to vary over the 16 months we observed the source — and even reversed completely twice.

This change in direction of the magnetic field around a fast radio burst has never been observed before.

Filling in the picture

What does this tell us about FRB 20190520B? The most popular theories to explain recent observations of repetitive fast radio bursts involve binary systems consisting of a neutron star and another massive star or black hole.

While we cannot yet rule out other hypotheses, our results favor the massive star scenario.

Read more: A brief history: What we know so far about fast radio bursts across the universe

Massive stars are known to have strong stellar winds with organized magnetic fields around them. If the source of the outbursts moved in and out of the stellar wind region as it travels through its orbit, we would expect the direction of the observed magnetic field to reverse.

The time scale of the magnetic field reversal, the measured variability in the apparent field strength, and the dense plasma surrounding the burst source all fit into this picture.

What’s next?

Our observations may provide crucial evidence to support the hypothesis that sources of repetitive fast radio bursts have a massive companion capable of producing highly magnetized plasma.

More importantly, the binary hypothesis gives us a prediction for the future. If correct, the changes in polarization of FRB 20190520B’s radio signals should rise and fall over time.

So we’ll look. Future observations with Murriyang and the Green Bank Telescope will reveal whether FRB 20190520B really is in a binary system – or whether the universe will surprise us again.