Source Found For Mysterious Repeating Radio Signal – And It’s Still Not Aliens

For the first time, astronomers have found a likely explanation for a type of repeating radio signal first identified two years ago but now found popping up in many places. In a new case, the emissions are coming from the direction of a red dwarf star, believed to be in orbit around a white dwarf, and their interactions are thought to be responsible. Nevertheless, there’s still a great deal we don’t know about the source of this signal, let alone all the somewhat similar ones we have recently found.

When radio astronomy was invented, it only gave us the capacity to see much detail in tiny areas of the sky, so naturally, we looked mostly at the places we expected to find something interesting. New instruments have recently started to offer us a much wider view, and scans of large portions of the sky have revealed a host of new kinds of signals we didn’t expect and often can’t explain.

One of these, now a class known as long-period radio transients, was first reported in 2022, based on archival data from the Murchison Widefield Array (MWA), whose radio surveys have revealed much unexpected strangeness in the radio sky. It showed a radio signal lasting 30-60 seconds, repeating every 18.2 minutes, and definitely not fitting into any known class of objects. 

Professor Natasha Hurley-Walker of the Curtin node of the International Centre for Radio Astronomy Research identified the repeating nature of a transient signal her PhD student Tyrone O’Doherty had found. She kept hunting and has been responsible for several more discoveries since, which deepened the mystery rather than solving it. All long-period radio transients show strong polarization, indicative of a powerful magnetic field, repeat too slowly to be pulsars, and unlike repeating Fast Radio Bursts, come from inside our galaxy. Despite what some want to believe, the spectrum is far too broad to be aliens.

Meanwhile, other teams of astronomers have got in on the act, but until now, all the repeating signals have been impossible to match to a source. In one case, that’s because the source’s location could not be identified precisely, but more often the area of origin is too crowded with potential suspects. “When you look toward them, there are so many stars lying in the way that it’s like 2001: A Space Odyssey. ‘My god, it’s full of stars!’,” Hurley-Walker said in a statement. 

Now, Hurley-Walker has changed that, finding an example known as GLEAM-X J0704−37. Besides being the slowest-repeating example of this kind of signal we have yet seen – once every 2.9 hours – GLEAM-X J0704−37 also comes from an area well outside the galactic plane where confounding objects are scarce. Follow-up observations reveal an M3 red dwarf star, about 32 percent of the Sun’s mass, at the right spot.

As the most common type of star in the galaxy, we’ve studied plenty of red dwarfs and, Hurley-Walker told IFLScience, “they should not have the energy or magnetic field to produce this all by itself. There’s no physical theory that could cause it to do this on its own.”

There are, however, signs the red dwarf is in orbit around something we can’t see. Although Hurley-Walker told us she and colleagues are using larger telescopes for confirmation, they’re quite confident a compact object has the red dwarf locked in orbit, probably one lasting 2.9 hours. For a host of reasons, a neutron star appears unlikely, and some of these apply more strongly still to a black hole, so that leaves a white dwarf as the logical candidate. 

White dwarfs on their own are also unlikely to produce such strong radio emissions, repeating or not. Hurley-Walker told IFLScience: “It takes two to tango.” The most likely explanation is that material is being drawn off the red dwarf by the white dwarf’s gravity and is causing the radio emissions, but the details remain elusive, and Hurley-Walker acknowledged that even what has been stated is tentative.

^{An artist’s impression of AR Scorpii, which produces very different signals from GLEAM-X J0704−37, but one that may have a similar composition.}

Red dwarfs are enormously common, and white dwarfs are hardly rare. The galaxy must be full of pairs locked together. Even though long-period radio transients seem to be surprisingly common for such a new discovery, we still have to wonder why we haven’t seen more. In particular, will any red and white dwarf in an orbit this tight produce radio signals like this, or is there something special about one or other member of the system to create the signals?

“People who work on M dwarfs and white dwarfs are not usually radio astronomers,” Hurley-Walker told IFLScience, since these types of stars emit so little in this part of the spectrum. To solve the mystery will take both sorts of expertise, but, she noted, “the overlaps are no one.”

That will have to change now, and Hurley-Walker recently had a two-hour call with specialists in both color dwarfs from around the world trying to work out what they would expect to see under various scenarios. “It’s a feature of modern science that it has become very specialized,” Hurley-Walker noted. Occasionally something like this happens and forces people to collaborate beyond their usual specialties. 

As one aspect of future research focuses on finding out what exactly is going on in the GLEAM-X J0704−37 system, the other will be to try to establish whether all long-period radio transients have similar causes. Of the objects currently placed in this class, the shortest has a period of seven minutes, which Hurley-Walker told IFLScience might just plausibly be a very slow pulsar. Alternatively, there might be two new phenomena with different causes that happen to look quite similar. A larger sample size would help, and co-author Csanád Horváth, who did the processing to make this discovery, is starting a PhD to search. 

The discovery of GLEAM-X J0704−37 and its explanation are open access in The Astrophysical Journal Letters.

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