It came from outer space. That was nothing special: radio telescopes constantly receive signals from astronomical objects as close as our Sun and as far as the fiery edge of the visible Universe. The period of this signal, though, was unusual. Every 2.9 hours, radio waves blasted the strange, spidery antennae of the Murchison Widefield Array in Western Australia for half a minute or so, then cut out.
The Murchison Widefield Array’s unusual design allows it to observe large swathes of the sky at low frequencies. Photo: MWA/Curtin University
For over a decade, no one noticed. Astronomers studying recurring radio signals were focused almost exclusively on pulsars, the remnants of massive stars whose off-kilter magnetic fields launch radio waves into space every time they spin around. These cosmic lighthouses keep time better than an atomic clock. The fastest of them rotates 716 times every second.
The search for fast pulsars pulled a lot of attention away from their less dramatic siblings. They keep better time than slow pulsars and they’re also easier to find. If you observe the sky for 20 minutes, a pulsar with a period of 10 milliseconds will show up 120,000 times in the data. In contrast, the strange 2.9-hour pulse probably wouldn’t even show up once.
Over the last few years, though, radio astronomers discovered exactly 10 of these extremely slow signals. The most recent and longest period is the 2.9-hour signal. A team at Perth, Australia’s Curtin University, led by Natasha Hurley-Walker, announced its discovery at the beginning of December.
There’s only one problem — it shouldn’t exist.
Beyond the death line
When astronomers describe something by how it acts, rather than what it is, that’s a sure sign they don’t know what they’re looking at. Examples include dark matter (“it’s matter, but we can’t see it”), dark energy (“it’s energy, but we can’t detect it”), and fast radio bursts (“well, they emit in radio waves, and they really are very fast”).
Until this month, Long Period Transients (LPTs) fell into that category. The key feature of an LPT is its period (quite long). That’s a problem for our understanding of how periodic radio signals are produced. Pulsars, the rapidly spinning dead stars described above, are the remnants of massive stars that have shed their outer layers in a supernova. But part of their ability to emit radio waves stems from their angular momentum. As they spin, they lose that angular momentum. And when their period becomes too slow, they pop out of existence, never to be seen again.
This is called the pulsar death line: the point beyond which we stop seeing any pulsars. There’s a lot we don’t understand about it, but one thing we do know is that 2.9 hours is so far beyond the death line that it’s practically a zombie. So what was producing these signals? Was it some kind of exotic pulsar? Or something even stranger?
A diagram showing the pulsar death line, which occurs when a pulsar stops spinning quickly enough to produce radio waves. Photo: Swinburne University of Technology
Astronomers get their first clue
The new slow transient discovered by the Curtin team isn’t just the longest-period astrophysical radio signal ever discovered, it’s also the first one astronomers have pinpointed to a specific star. Bafflingly, that star is an M-dwarf, a little red star less than half as hot as the Sun and only a third of its mass. For an emission as strong as this, that’s a bit like arriving at the scene of a murder and finding a toddler holding the knife.
Observations with the powerful radio telescope MeerKAT, an improvement over previous observations, were able to pinpoint the emission to one specific star. Photo: Nature/Hurley-Walker et al. 2024
So what’s going on? There are two leading theories, and both involve dead stars too faint to show up in images. The first theory is that the M-dwarf is in orbit around a pulsar, and something about the orbit and magnetic field setup is allowing it to emit slowly. This could happen if circumstances were exactly right. It’s not very likely.
The second theory is that the M-dwarf orbits a much lighter stellar remnant called a white dwarf. Pulsars are left over from very heavy stars, supported against their own collapse by the pressure of neutrons crammed infinitesimally close together. White dwarfs, in contrast, are the leftover cores from stars like our Sun. They’re lighter and much less dense, supported by electron pressure rather than neutrons. They don’t do much. They just sit there unless another white dwarf comes too close, in which case they explode violently.
In a few cases, though, white dwarfs can steal material from a companion star and accelerate it in their strong magnetic fields until it whips radio emissions into space. We’ve seen this happen twice before. In each case, the period is fast, in line with traditional pulsars. But unlike for pulsars, the emission doesn’t necessarily derive from the rotation speed of the white dwarf. As far as we know, there is no white dwarf death line.
A Hubble image of SIrius A, center, and Sirius B, to the bottom left. Sirius B is a white dwarf. Photo: NASA/ESA/STScI
Strange discoveries lead to new physics
There are a lot of open questions in radio astronomy. We barely know how pulsars produce radio waves. We don’t even know what kind of objects make the mysterious extragalactic signals known as Fast Radio Bursts. As for a white dwarf emitting like a pulsar, all we can say is it has something to do with strong magnetic fields and the presence of a companion star.
But everything in the universe follows the same laws. The underlying mystery here is not what astrophysical object produces any given radio signal but rather what rules govern how matter emits light in extreme conditions. The more we know about those rules, the faster we’ll understand the next strange radio signal that lands on our doorstep. Deciphering long-period transients is a step towards that understanding.
“Up until now, we didn’t expect such objects to exist,” says Fengqiu Adam Dong, a researcher at the National Radio Astronomy Observatory, who studies slow transients. “Some people think that they could solve other mysteries in astronomy like repeating [Fast Radio Bursts].”
So tune your radio to 600 MHz, and listen for a faint little whistle. If you miss it, don’t worry — it will be back in 2.9 hours.
