Scientists are puzzling over two newly discovered stars with mysterious properties. These objects look like neutron stars—the extremely dense stellar cores left behind after massive stars explode in supernovae—but they have other characteristics that astronomers can’t explain. They may even represent a new type of stellar object, unlike anything seen before.
The two stars, one of which was announced this month in the journal Nature, emit long pulses of radio waves about every 20 minutes. This means they spin much more slowly than any known neutron star, and they have other odd traits as well.
“We are all still quite amazed and intrigued and baffled,” says astronomer Natasha Hurley-Walker of Curtin University in Bentley, Western Australia, who led the discovery of both of these surprising sources.
The mystery began in 2021, when Hurley-Walker, and colleagues discovered a slowly repeating radio pulse embedded in archival data from 2018. It shone for three months, but by the time they discovered it in the archives, says Hurley-Walker, its activity had died down, and it had become invisible in the skies.
In a January 2022 paper announcing the discovery, the scientists suggested the object could be a highly magnetic neutron star called a magnetar. But the object, with the unwieldly name GLEAM-X J162759.5-523504.3, sent out a pulse every 18 minutes instead of every few seconds like known magnetars. It also had no X-ray signal associated with the radio emission, which doesn’t fit with magnetar theories. And strangely, the object’s estimated magnetic field was much stronger than previously seen.
So the scientists began looking for other similar objects that were still sending radio pulses. In June 2022 Hurley-Walker and her colleagues began observing the entire visible sky every three nights with the Murchison Widefield Array in Western Australia. They had updated their detection algorithms to quickly process the data and look for pulses of radio waves. Almost immediately, they found something that emitted a radio flash every 21 minutes.
At first glance, the radio burster looked similar to the previously discovered object. But once the team began to dig through archival data for additional pulses from this source, named GPM J1839–10, the object only seemed more perplexing.
Unlike the first star, which was observed pulsing for three months, the signal of J1839-10 was detected over decades. “We were able to line up all those pulses, arriving like clockwork over the course of the 33 years,” says Hurley-Walker.
For now, researchers assume both objects are the same. But why would one flash for three months and the other for 33 years and counting? “I don't know,” says Hurley-Walker. “That's the problem.”
Astronomers know of several celestial sources that spew repeating signals, but nearly all flash in intervals of milliseconds to seconds. Many are different types of neutron stars, such as pulsars and magnetars. These are incredibly dense objects, the size of a city but with more mass than the sun.
While the two new objects have some similarities to neutron stars, they’re not a perfect fit. “We've never seen neutron stars do anything like what [this] source is doing right now,” says astrophysicist Manisha Caleb of the University of Sydney in Australia, a co-author on the most recent discovery paper.
Like most spinning cosmic objects, a neutron star’s magnetic field doesn’t align with its rotation axis. If the star is magnetic enough, radiation can stream from its two magnetic poles, and as it spins, astronomers see a regularly pulsing signal. “That beam sweeps across our line of sight like a lighthouse,” explains Hurley-Walker.
Those pulses of radio waves can reveal crucial information about the objects. How fast the spin changes over the years corresponds to both the strength of the star’s magnetic field and the physical processes responsible for the pulses. As the star slows its spin, some of its rotational energy transfers into more emissions. But the spin of GPM J1839–10 doesn’t change enough to account for the energy of the radio bursts, says Harley-Walker, suggesting something else is contributing to the signal.
The fact that these two strange objects were found in our own galaxy within a year of each other also came as a surprise. Neutron stars aren’t that common, according to theories of their formation, and these objects with their slow spins and radio flares are even more rare.
“We expect so few that there is no way we could have detected two of them in such a small survey in a year’s time,” says Nanda Rea, an astrophysicist at the Institute of Space Sciences in Barcelona, Spain, and a co-author on the recent study.
A different stellar remnant
Perhaps these puzzling pulsars aren’t neutron stars at all, but instead a different type of stellar remnant—one that is much more common, spins slower, and is larger. They could be white dwarfs.
While neutron stars are left behind after giant stars explode, white dwarfs are the leftover cores of more moderately sized stars, like the sun. At their largest, white dwarfs hold about a sun’s worth of mass in an Earth-size sphere instead of a city-size one. And the number of these objects expected in the Milky Way means that finding two highly magnetic ones in a year would make sense, says Rea.
An isolated white dwarf shouldn’t be able to produce the powerful radio emissions the scientists have detected, but perhaps there is more to the story.
Researchers have found a few extreme magnetic white dwarfs in recent years. Astrophysicist Manuel Malheiro of the Aeronautics Institute of Technology in Brazil points out that a highly magnetic white dwarf with a spin period of seven minutes was discovered in 2021. This star may be the result of two white dwarfs merging, making it more massive, with a faster rotation, and a stronger magnetic field than the pre-merger stars.
Malheiro has studied the possibility that highly magnetic white dwarfs might be responsible for some suspected neutron star sources, and he thinks that may be the case with the two new objects. If they’re neutron stars, he says, it’s clear they’re very strange ones. And he suggests researchers “more seriously” consider white dwarf pulsars as the sources responsible for these radio signals.
Solving a cosmic mystery
Last month, Hurley-Walker was at a conference focused on radio-flashing compact objects. After her team presented their discovery, she asked the scientists in the audience if they thought these mysterious sources were neutron stars, white dwarfs, or something else entirely, “and it was split essentially evenly across all of those three groups,” she says.
The puzzle remains for now, but because the scientists found the new object so soon after using their new search algorithms, they hope to find more of these objects in the next few years.
"As you look at the sky in a way that people haven't done before, there's almost always something there to be found,” says astrophysicist Jason Hessels of the University of Amsterdam. He and his colleagues are collaborating with Hurley-Walker to search for more of these objects with the LOFAR radio array spread across the Netherlands and several other European countries.
And while researchers aren’t sure what these two stars are, Hurley-Walker delights in the fact that they seem to have stumbled upon something new. “This is just my dream come true,” she says.