Are we poised to understand fast radio bursts?

• Fast radio bursts last only a fraction of a second, but can release about as much energy as the sun does in a year. What are they? Astronomers aren’t sure.
• Now it seems a nearby magnetar – a type of neutron star, or remnant of an exploded star – might be a source of fast radio bursts within our own Milky Way galaxy.
• “Glitches” in the magnetar’s spin rate could be responsible for these powerful cosmic signals, according to new work by astronomers.

NASA JPL-Caltech posted this original article on February 14, 2024. Edits by EarthSky.

Fast radio bursts in the Milky Way

What’s causing mysterious bursts of radio waves from deep space? Astronomers may be a step closer to providing one answer to that question. Two NASA X-ray telescopes recently observed one such event – known as a fast radio burst – mere minutes before and after it occurred. This unprecedented view sets scientists on a path to better understand these extreme radio events.

While they only last for a fraction of a second, fast radio bursts can release about as much energy as the sun does in a year. Their light also forms a laserlike beam, setting them apart from more chaotic cosmic explosions.

Because the bursts are so brief, it’s often hard to pinpoint where they come from. Prior to 2020, the bursts that scientists had traced were from sources originating outside our own galaxy. Thus, they were too far away for astronomers to see what created them. Then a fast radio burst erupted in Earth’s home galaxy. It originated from an extremely dense object called a magnetar, the collapsed remains of an exploded star.

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A fast radio burst in our galaxy

In October 2022, the same magnetar – called SGR 1935+2154 – produced another fast radio burst. This time, NASA’s Neutron Star Interior Composition Explorer (NICER) on the International Space Station and Nuclear Spectroscopic Telescope Array (NuSTAR) in low Earth orbit studied it in detail. The telescopes observed the magnetar for hours. They caught a glimpse of what happened on the surface of the source object and in its immediate surroundings, before and after the fast radio burst.

The peer-reviewed journal Nature published a study on February 14, 2024, describing the results. It’s an example of how NASA telescopes can work together to observe and follow up on short-lived events in the cosmos.

Glitches in a magnetar

The burst occurred between two “glitches,” when the magnetar suddenly started spinning faster. SGR 1935+2154 is about 12 miles (20 km) across and spinning about 3.2 times per second. That means its surface was moving at about 7,000 mph (11,000 kph). Slowing it down or speeding it up would require a significant amount of energy. So the study’s authors were surprised to see that in between glitches, the magnetar slowed down to less than its pre-glitch speed in just nine hours. That’s a deceleration about 100 times more than scientists had previously observed in a magnetar.

Chin-Ping Hu is an astrophysicist at National Changhua University of Education in Taiwan and the lead author of the new study. Hu said:

Typically, when glitches happen, it takes the magnetar weeks or months to get back to its normal speed. So clearly things are happening with these objects on much shorter time scales than we previously thought, and that might be related to how fast radio bursts are generated.

Spin cycle

When trying to piece together exactly how magnetars produce fast radio bursts, scientists have a lot of variables to consider.

For example, magnetars (which are a type of neutron star) are so dense that a teaspoon of their material would weigh about a billion tons on Earth. Such a high density also means a strong gravitational pull: A marshmallow falling onto a typical neutron star would impact with the force of an early atomic bomb.

The strong gravity means the surface of a magnetar is a volatile place, regularly releasing bursts of X-rays and higher-energy light. Before the fast radio burst that occurred in 2022, the magnetar started releasing eruptions of X-rays and gamma rays (even more energetic wavelengths of light) that high-energy telescopes caught in their peripheral vision. This increase in activity prompted mission operators to point NICER and NuSTAR directly at the magnetar.

Co-author Zorawar Wadiasingh, a research scientist at the University of Maryland, College Park and NASA’s Goddard Space Flight Center, said:

All those X-ray bursts that happened before this glitch would have had, in principle, enough energy to create a fast radio burst, but they didn’t. So it seems like something changed during the slowdown period, creating the right set of conditions.

Triggering fast radio bursts

What else might have happened with SGR 1935+2154 to produce a fast radio burst? One factor might be that the exterior of a magnetar is solid. So, the high density crushes the interior into a state called a superfluid. Occasionally, the two can get out of sync, like water sloshing around inside a spinning fishbowl. When this happens, the fluid can deliver energy to the crust. The researchers think this is likely what caused both glitches that bookended the fast radio burst.

If the initial glitch caused a crack in the magnetar’s surface, it might have released material from the star’s interior into space like a volcanic eruption. Losing mass causes spinning objects to slow down, so the researchers think this could explain the magnetar’s rapid deceleration.

But having observed only one of these events in real time, the team still can’t say for sure which of these factors (or others, such as the magnetar’s powerful magnetic field) might lead to the production of a fast radio burst. Some might not be connected to the burst at all.

George Younes, a researcher at Goddard and a member of the NICER science team specializing in magnetars, said:

We’ve unquestionably observed something important for our understanding of fast radio bursts. But I think we still need a lot more data to complete the mystery.

Bottom line: Scientists used two X-ray telescopes to observe the before and after of a magnetar that produced a fast radio burst. The observations give them more insight to these mysterious deep space signals.