A Radio Signal In Space Has Been Repeating Every 21 Minutes for 30 Years And Scientists Don't Know Why

Astronomers have discovered a mysterious radio source in our galaxy that has been pulsing every 21 minutes for several decades, creating a long-term pattern that has never been seen in space before, reports a new study. 

The signals probably come from some kind of exotic dead star located about 15,000 light years from Earth, but there’s currently no explanation that can account for all of their perplexing properties. The discovery of the pulses hints that there may be a hidden population of radio sources that repeat over surprisingly long timescales.

Space is filled with a dazzling variety of short-lived bursts of light known as “transients” that sometimes glow in radio light. While some radio transients erupt only once, never to be seen again, others light up in predictable repeating patterns. Scientists have managed to trace some transients back to compact stellar corpses, including pulsars and magnetars, though many radio signals continue to defy explanation.

Now, researchers led by Natasha Hurley-Walker, an astrophysicist at the International Centre for Radio Astronomy Research at Curtin University in Australia, have discovered a bizarre repeating transient called GPM J1839−10 that behaves like nothing else we’ve ever seen in the skies. 

After discovering the transient in 2022, Hurley-Walker and her colleagues were astonished to find that it showed up in astronomical observations dating back to at least 1988. The team said the “long-lived activity of GPM J1839–10 is extremely puzzling” and that its unusual 21-minute period sits “at the very limit of any classical theoretical model,” according to a study published on Wednesday in Nature.

“Intriguingly, the persistence of GPM J1839–10 over three decades indicates that there may be many more long-lived sources, some of which may await discovery in existing archives,” Hurley-Walker and her colleagues noted in the study. “Sources similarly active for decades, but with smaller pulse windows, would yield better constraints on [the period] and thus even stronger tests of models of coherent radio emission from astrophysical sources.”

The researchers first glimpsed GPM J1839–10 during a survey of Milky Way transients with the Murchison Widefield Array, a sensitive radio instrument located in the Wajarri Yamaji Country of outback Western Australia. 

The transient immediately stood out on account of its unprecedented period duration. Whereas most radio repeaters flash on a timescale of seconds to a few minutes, GPM J1839–10 enters an active phase every 21-minutes that lasts anywhere from 30 to 300 seconds. Moreover, the signal showed up in radio observations captured over the past 35 years, in contrast to a similar long-period radio transient that was visible for only a few months.

Scientists have linked many radio transients to pulsars and magnetars, which are types of dead stars that spin rapidly and emit radiant bursts of light. GPM J1839–10 has some of the trappings of a magnetar outburst, but its slow spin should have brought it into a so-called “death valley” state that effectively switches off bright flashes in these weird objects. Likewise, pulsars rotate at a much faster clip than the 21-minute period, leaving GPM J1839–10 with no obvious analogs in the known universe. 

“This new source has neither a short enough period to be explicable by canonical radio pulsar emission nor a short enough activity window for its radio emission to resemble a typical magnetar outburst,” the team said.

The researchers speculate that the signals could be made by another type of dead star, called a white dwarf, that is much larger than pulsars and magnetars. A white dwarf with an extraordinarily strong magnetic field might be able to make the observed patterns, though they noted in the study that “it is perhaps surprising that no close-by highly magnetic white dwarfs have been observed to produce such emission.”

To solve the mystery of GPM J1839–10, Hurley-Walker and her colleagues plan to continue observing the transient, while also poring over archival images from recent decades to hunt for similar sources in the Milky Way.

The detection of GPM J1839–10 “confirms that ultra-long-period radio sources are not extremely rare and there will be many opportunities in upcoming galactic-plane surveys to find further examples,” the researchers concluded.