Neutron Star "Glitches" Are Clue to Mysterious Radio Bursts

A huge clue to understanding the mysterious, fleeting flashes of radio waves known as fast radio bursts (FRBs) came when one went off in our own galaxy. A highly magnetized neutron star, or magnetar, dubbed SGR 1935+2154, emitted an FRB-like burst on April 28, 2020, and suddenly astronomers had an FRB to study in our own backyard.

Since then, astronomers have been waiting for a repeat. In October 2022, they struck rich once again — and this time, they were ready.

Until 2020, almost all known FRBs originated in faraway galaxies. Yet each one relayed more energy in a fraction of a second than the entire Sun emits in a year. Some even did so more than once! For a while, there were as many ideas as to what could generate these bursts as there were FRBs themselves. Now, with the example in our own galaxy, astronomers know that at least some FRBs originate from magnetars — but how do magnetars do it?

What Astronomers Saw

Upon receiving an alert from the Burst Alert System aboard NASA’s Integral space telescope, Chin-Ping Hu (National Changhua University of Education, Taiwan) and colleagues asked two other NASA space telescopes — the Neutron Star Interior Composition Explorer (NICER) and the Nuclear Spectroscopic Telescope Array (NuSTAR) — to turn to the magnetar and start taking observations.

The team watched the neutron star rotate by virtue of a hotspot on its surface, which likely marks one of the poles of the star’s magnetic field. As the hotspot spins in and out of view — once ever 3.2 seconds! —the neutron star’s brightness appears to pulse. NICER was specifically designed to catch changes on such fast timescales.

NuSTAR, on the other hand, provided spectra to go alongside the brightness observations, which helped determine where emission was coming from. The hotspot, for example, is emitting X-rays because it’s so hot, while other X-rays come from charged particles writhing in the neutron star’s powerful magnetic field.  

Within hours, the astronomers watched drastic changes occur on the city-size star (it spans only 20 km, or 12 miles). First, the neutron star glitched — that is, suddenly it started spinning faster. Then, more slowly, the rotation rate decreased over four hours, leading into a loud burst of radio waves (detected on the ground by a radio telescope in Canada known as CHIME and by the Green Bank Telescope in West Virginia). Another four hours later came a second glitch. Hu and colleagues publish this sequence of events in the February 14th Nature. 

During the glitches, the spectra showed the X-rays were largely coming from the hotspot. But leading up to and during the radio burst, in between the glitches, the emission from the magnetically trapped particles strengthened.

First a Wind, then a Burst

Neutron stars are known to glitch when the surface is out of sync with the interior. “An imperfect analogy is a spinning fishbowl with water sloshing inside,” Hu explains. “In this case, imagine if the water in the fishbowl was spinning faster than the glass. The glass then catches up — this is a spin-up glitch.”

Glitches might occur when motions underneath the neutron star’s surface stress the crust, which then ruptures in a starquake. The rupture is most likely to happen near the hotspot.

Even if the neutron star only spins up by a tiny fraction of a second, the energy involved in a starquake is incredible. After all, for a 20-kilometer body rotating every 3.2 seconds, the surface whips around at 42,000 mph; changing that by even a little requires a lot of energy.

The weird thing about SGR 1935’s glitch, though, is that the spin-up dissipated so quickly. Most neutron stars take weeks or months to recover, but the magnetar was back to its regular spin rate within hours.

That makes sense, though, if the glitch marked a starquake, which also released charged particles in a brief blast of wind. That wind would have robbed the star of its spin almost as quickly as it had gained it.

Then, with all of those particles hanging around in an uber-powerful magnetic field — which is far stronger than any we can make on Earth — the conditions were right for an extreme scenario. Particles (specifically, electrons and their antimatter partners, positrons) are born in pairs from the magnetic field's energy, resulting in what Hu calls an “avalanche.”

“One electron makes a pair, and then each child makes several more, and so on for several generations, until there are thousands of progenies per electron,” he says. These electron-positron pairs could ultimately be responsible for the sudden burst of radio emission in a “laser-like process,” Hu adds.

“This observation connects a rare FRB-like burst to a rare double glitch and provides a clear avenue for further investigations of FRB generation,” says FRB expert Laura Spitler (Max Planck Institute for Radio Astronomy, Germany), who wasn't involved in the study. “Whether this applies to all FRBs is still an open question, but observationally it is fantastic progress.”

The 2020 and 2022 bursts are the only truly “loud” bursts of radio waves that have been detected so far from SGR 1935+2154, though milder activity occurs more often. The team plans to continue monitoring the magnetar to catch more bursts in the future, providing additional data to help test the wind/pair creation scenario.