A strange radio signal pulsing from a spot 15,000 light-years away could point to an unconfirmed type of star.

Called GPM J1839-10, the peculiar cosmic object has been caught flashing out radio waves every 22 minutes. That's incredibly slow, compared to other sources of pulsing radio waves. Moreover, a deep dive into archival information reveals we've been recording its slow pulse for over 30 years. It just managed to escape our notice until now.

According to a team led by astrophysicist Natasha Hurley-Walker of the Curtin University node of the International Centre for Radio Astronomy Research (ICRAR) in Australia, the most likely explanation is that the source is a magnetar with a really slow spin.

If so, it would fly in the face of our understanding of these wild star corpses, which means something weird has to be going on.

"This remarkable object challenges our understanding of neutron stars and magnetars, which are some of the most exotic and extreme objects in the Universe," Hurley-Walker says.

An artist's impression of the Murchison Widelfield Array observing GPM J1839-10. (ICRAR)

The discovery was made after a similar object was discovered in the Milky Way three years ago in archival data, as published in a 2022 paper.

Called GLEAM-X J162759.5−523504.3, it was recorded emitting radio waves for around a minute, every 18 minutes, although it went quiet in 2018 and hasn't been heard from since. Based on the way the light was twisted, it seemed to be coming to us through a highly magnetized environment.

Hurley-Walker and her team wanted to see if they could find other objects with similar behavior, so they took observations of the southern sky using the Murchison Widefield Array in Australia to conduct a survey. They found an object that emits five-minute bursts of radio light every 22 minutes.

They turned other telescopes to its coordinates, and scoured archival data of the location. The new observations allowed them to characterize the radio emission in detail, showing the same twist suggestive of alteration by a magnetic field. And the archival data showed that GPM J1839-10 has been caught pulsing since at least 1988.

"I was five years old when our telescopes first recorded pulses from this object, but no one noticed it, and it stayed hidden in the data for 33 years," Hurley-Walker says.

"They missed it because they hadn't expected to find anything like it."

Infographic of all the observatories that observed GPM J1839-10. (SARAO; Daniel López/IAC; Marianne Annereau; NCRA; CSIRO/Dragonfly Media; AUI/NRAO; ESA)

The profile of the light is very similar to pulses from magnetars. These are a type of neutron star – that's the remnant collapsed core of a massive star that has gone supernova after burning through its fusion fuel – with an extremely powerful magnetic field.

But the strength of that magnetic field correlates with the magnetar's spin period. The magnetic field strength has to be above a certain threshold, called the death line, in order to generate powerful radio emissions. There's a good reason no one expected to find anything like GPM J1839-10.

"The object we've discovered is spinning way too slowly to produce radio waves – it's below the death line," Hurley-Walker explains.

"Assuming it's a magnetar, it shouldn't be possible for this object to produce radio waves. But we're seeing them. And we're not just talking about a little blip of radio emission. Every 22 minutes, it emits a five-minute pulse of radio wavelength energy, and it's been doing that for at least 33 years. Whatever mechanism is behind this is extraordinary."

The research has been published in Nature.