An unexplained, long-period radio-transient discovery

Natasha Hurley-Walker didn’t set out to find long-period neutron stars. In 2020 she received a Future Fellowship, a multiyear grant from the Australian Research Council designed to give midcareer researchers the financial flexibility to explore new projects. For Hurley-Walker, of the International Centre for Radio Astronomy in Australia, it was a chance to break free of established ideas and the pressure to publish. She decided that she was “going to throw everything at every wall … and see what sticks.”

One of Hurley-Walker’s exploratory projects involved using data from an Australian radio telescope—the Murchison Widefield Array (MWA), seen in figure 1—in a new way. Instead of using the typical data-processing method, which is optimized for making deep images of the sky, she took a shot in the dark and invented a new technique to find transient radio sources, objects that appear and disappear over time. She followed that path even though previous studies had drawn a blank at radio frequencies.

Figure 1.

Hurley-Walker’s undergraduate student Tyrone O’Doherty compared data that were as similar as possible: the same area of the sky, the sky in the same position relative to the horizon, and the same RF bands. The only changes that should be found between images are the uncontrollable variations, such as the background signal from the ionosphere. One observation is subtracted from the other, and an image is made of the result. All the sources that do not change simply disappear, leaving only those that have changed. When Hurley-Walker examined a difference image from one region of the sky, she saw a strong signal of an object that’s now known as GLEAM-X J1627. ¹

Freedom to explore

GLEAM-X J1627 pushed at the boundaries of what should be possible for a pulsar, a type of neutron star. Pulsars are like cosmic lighthouses: The rotating stars shine a beam of radio emission toward Earth with periods that usually fall between 1 ms and 12 s and last for decades. With its period of 18 minutes, the object was much slower than a typical pulsar. GLEAM-X J1627 didn’t persist for a long time, lending credence to the alternative explanation that it was a type of neutron star known as a magnetar, which is powered by strong magnetic fields and emits for only a few months.

Without enough data, though, the researchers couldn’t say for sure. So Hurley-Walker designed a dedicated observing project to search for more long-period pulsars. She and her postdoc Tim Galvin automated their data analysis code to better handle the large amount of data they anticipated collecting from the MWA. With another undergraduate, Csanad Horvath, Hurley-Walker worked to improve their detection techniques.

In August 2022, after only a couple weeks of observing, they found GPM J1839-10, their second strange object. ² Because it had an even longer period of 22 minutes and a radio brightness exceeding other pulsars at similar RF bands, Hurley-Walker wasn’t sure it was even a neutron star. But before she jumped to any conclusions, she went hunting for more data.

Lost in the library

Pulsars are extraordinarily consistent and persistent. If GPM J1839-10 is a pulsar, it should be detectable in archive data. Hurley-Walker reached out to Scott Hyman, an emeritus professor at Sweet Briar College in Virginia, to assist with the archival search. Hyman is no stranger to inexplicable long-period transients, having found his own such object, with a 77-minute period, in 2005. ³ More than a decade later, no theory can fully explain what he found.

Hyman and Hurley-Walker began a search not only in the MWA archives but also in those of other radio telescopes, and they made new observations with an x-ray telescope—the European Space Agency’s XMM-Newton—and an optical telescope. Unlike GLEAM-X J1627, which emitted detectable radio waves for only a few months, GPM J1839-10 showed up in data from as far back as September 1988, at nearly the beginning of digital radio telescope records. Because of the varying interval of time between observations, sometimes only a single pulse was detected and often without great resolution.

Although GPM J1839-10 is bright, it was undiscovered until now because of how astronomers search for pulsars. Pulsars are expected to have short periods, so astronomers look for them by searching for changes on time scales of milliseconds to seconds. With her new method designed to search for long time-scale changes, Hurley-Walker could find objects that changed brightness even if they had longer periods. The technique would not have been possible without modern image processing to analyze the large amounts of data covering a large area of the sky, the configuration of the MWA antennas to provide greater spatial-frequency coverage than other radio telescopes, and the stability of the MWA telescope to allow GPM J1839-10 to stand out against background noise.

Something old, something new

Faced with two ultralong-period pulsars—and a handful of objects with other anomalous properties (see figure 2)—Hurley-Walker wanted to figure out whether the objects were even neutron stars.

Figure 2.

A pulsar produces luminous beams of light via pair production: the generation of electrons and positrons from the rapid rotation of the neutron star and its magnetic field. That same magnetic field expels luminous bursts of radio energy. (To learn more about the physics of neutron stars, see the article by Lars Bildsten and Tod Strohmayer, Physics Today, February 1999, page 40 .) Based on the current understanding of pulsar models, however, GPM J1839-10 is not rotating fast enough for pair production to occur. Maybe a new understanding of pulsars is needed, or maybe the star isn’t a pulsar despite having a regular period over three decades.

Magnetars are similar to pulsars—powerful sources of energy with regular periods—but their energy is provided by a near-constant reconfiguration of their twisted magnetic fields. Although magnetars emit at x-ray wavelengths, XMM-Newton did not detect any such emission from GPM J1839-10. The lack of x-ray emission, and the fact that magnetars stop emitting after a few months, has led Hurley-Walker to believe that GPM J1839-10 isn’t a magnetar either.

Since the paper was released, Hurley-Walker has talked with other astronomers and come up with new theories. One posits that the star is a white dwarf. By coincidence, Ingrid Pelisoli of the University of Warwick published her discovery of a radio-bright pulsating white dwarf binary system⁴ shortly after Hurley-Walker’s paper on GPM J1839-10 came out. They learned of each other’s work only when presenting their results at the same conference. The slow rotation and radio emission are promising evidence of a binary white dwarf, but a white dwarf’s pulsation is partly powered by the accreting material of its companion star.

Without confirmation of another star, current theories can’t explain how such a slow rotator could generate enough electrons to emit radio beams. Follow-up optical observations of GLEAM-X J1627 showed it to be isolated, and GPM J1839-10 has too consistent of a pulse over such a long period of time to be part of a binary system, unless it is perfectly face-on so that no Doppler shift is detectable.

The hunt is on

With no theory to account for all the known properties of the new ultralong-period transients, ⁵ Hurley-Walker is searching for more clues about what the mysterious objects her team found might be. She has applied for time on the Hubble Space Telescope and plans to apply for time on the James Webb Space Telescope to get data on GPM J1839-10. Either optical (Hubble) or IR (Webb) photometry would allow Hurley-Walker to determine the type of star based on how much light is emitted across the spectrum. Unfortunately, it’s difficult to observe so close to the galactic plane. The space-based telescopes are the only instruments with enough resolution to fully resolve such a small source in such a crowded region of the sky.

While she waits for observation time on a space telescope, Hurley-Walker continues to use the MWA to look for more such objects. Only six months elapsed between her two long-period pulsar discoveries. In contrast, the first two observations of another type of unusual radio source, the fast radio burst, were four years apart. “Back of the envelope that means that they’re not fantastically uncommon.” She plans to search at higher galactic latitudes so that the field is less crowded and more telescopes will be able to perform follow-up observations.