A curious radio signal in space was spotted pulsing every 18 minutes and 18 seconds over a period of three months before it disappeared from view, revealing “an unusual periodicity” that has not been observed previously, reports a new study.
Though the source of the signals is still unknown, the scientists who discovered it think those clockwork pulses may be the rotational signature of an exotic dead star, perhaps a white dwarf or a highly magnetized “magnetar,” located some 4,000 light-years away. But no matter what it is, the discovery will reveal “further insight into the evolutionary extremes surrounding the life and death of massive stars,” according to the study, which was published Wednesday in Nature.
“The fact that it is repeating so regularly (the same rate to within one ten thousandth of a second over the three months it was visible) means it is quite likely to be a rotating object,” said Natasha Hurley-Walker, a radio astronomer at the International Centre for Radio Astronomy Research (ICRAR) who led the study, in an email.
“Orbital periods of 18 minutes are possible,” she continued, but “there are no models that produce such bright radio emission from two objects in orbit with each other, with such precision, and any that would produce any kind of radio waves would also produce X-ray emission, which we don't see.”
In addition to presenting a fascinating cosmic puzzle about its origin, the unprecedented object could also help scientists understand other mysterious phenomena, such as fast radio bursts, a class of radio emission that has so far defied explanation.
The story of how the team first detected this weird radio source is as interesting as the signal itself. As part of an undergraduate project, Hurley-Walker had her students comb through a large sky survey called GaLactic and Extragalactic All-sky MWA eXtended (GLEAM-X) captured by the Murchison Widefield Array, one of the most sensitive radio telescopes on Earth.
In a twist that Hurley-Walker likened to the cold open of the Netflix film Don’t Look Up, study co-author Tyrone O'Doherty, who is now a graduate student at ICRAR, noticed the radio source in GLEAM-X observations taken in March 2018 but did not see them in images taken a few months later in May.
The team eventually tracked down 71 pulses in total, each lasting about 30 to 60 seconds and displaying that weird characteristic 18:18-minute period. The radio source, which is known as GLEAM-X J162759.5-523504.3, was visible between January and March 2018, but the researchers haven't been able to pick it up in observations taken before or after that timespan.
“We did do some follow-up observations in 2021, but we haven't re-detected the object, so it appears to be inactive at the moment,” Hurley-Walker said. “But hopefully there are other examples active in the Milky Way, and searches I'm planning this year will find them.”
Though the object was only visible to telescopes for a short time, its bizarre characteristics offer some tantalizing clues about its nature. Its radio pulses display strong linear polarization, which hints at the presence of ordered magnetic fields. It seems too bright to be a planet, and its light profile suggests it’s too compact to be a normal star.
By process of elimination, Hurley-Walker and her colleagues think the object is either a white dwarf or a magnetar, both of which are the leftover remains of stars that have burned out. White dwarfs are the corpses of low-to-medium-mass stars, such as the Sun, while magnetars are the highly magnetic corpses of stars that are about 10 to 25 times as massive as the Sun.
In either case, GLEAM-X J162759.5-523504.3 would be unlike anything seen before. Hurley-Walker notes that the only other white dwarf known to produce similar pulses is 10 times closer to Earth yet 100 times fainter than the newly discovered object, raising the question of why these two objects would display such drastically different radio brightness. Meanwhile, all known magnetars rotate at a quick clip of under 10 seconds, not even close to the 18-minute period seen from GLEAM-X J162759.5-523504.3.
“Theorists predict that there exist ‘ultra-long period magnetars’ (magnetars rotating slowly, and therefore invisibly) scattered through the Milky Way, and what we could be seeing here is one becoming active,” Hurley-Walker noted. “This wasn't exactly expected, but it's at least theoretically possible, and matches our observations very well.”
Either explanation could shed new light on the life cycles of stars, the mysteries of the radio universe, and other unresolved questions. For instance, some scientists think that ultra-long period magnetars might produce fast-radio bursts, which also sometimes repeat in regular rhythms. Spotting an ultra-long period magnetar in the wild could help resolve the longstanding enigmas of fast radio bursts, which is a major goal for astronomers.
To build on the discovery, Hurley-Walker plans to use the MWA to constantly monitor the Milky Way in real time so that she and her colleagues can capture other weird radio transients, and immediately conduct powerful follow-up observations.
“That's my goal for the next few years: find another one (or this one switching back on again), and use every telescope I can to figure out what it really is!” Hurley-Walker said. She also noted that scientists around the world are currently constructing the most sensitive radio telescope on Earth, called the Square Kilometer Array (SKA), which will offer an unprecedented view of the universe at radio wavelengths once it is operational in the late 2020s.
“I hope to work on data from the SKA as it starts to roll in, and with all that sensitivity, I'm sure we'll find even more new discoveries that help us understand the universe,” Hurley-Walker concluded. “And certainly this experience has taught me that it's worth trying out looking at the sky in entirely new ways—you never know what you might find!”