Astronomers find a strange pulsar blinking in slow motion

About 2,600 light-years away, a dead star is sending signals from beyond the grave. 

Astronomers recently found the strange beacon, which appears to be a pulsar blinking in slow motion — something that shouldn’t be possible.

“It’s incredibly exciting to discover such a long-period pulsar,” says Yuanming Wang, a postdoctoral researcher at Swinburne University in Australia and the lead author of a paper about the discovery published March 28 in The Astrophysical Journal Letters. “But what’s even more exciting is that the new technique we used is opening the door to finding more hidden objects in the cosmos.”

Scientists now plan to look for more of these oddball objects, which could help to bridge the gap between traditional, well-understood fast-rotating pulsars, and those few that rotate — seemingly impossibly — more sluggishly.

Slow and steady

Pulsars are like high-speed cosmic lighthouses. They are made of the dense cores left over from massive stars that ran out of nuclear fuel and went supernova. Following the explosion, what remains of the star collapses into a six-mile-wide (10 kilometers) neutron star. Because the star’s original angular momentum (spin) is conserved, being squished down into such a small object means its spin speeds up, the same way a figure skater whirls faster when they pull their arms in. Pulsars emit beams of radiation from their poles; these beams then sweep across the universe as they spin. If those beams happen to point toward Earth, we receive a flash of light with each rotation, typically at radio wavelengths.

Pulsars throb with clockwork precision — literally — with the fastest flutterers blinking tens of thousands of times each minute. Even the slowest ones tend to flash at least every 10 seconds.

But in 2022, astronomers found an object emitting radio pulses like a pulsar, but far more slowly: only once every 18 minutes. Since then, scientists have discovered a handful more of these objects, known as long-period transients, with periods that range from minutes to hours.

You’d think these could just be very slow-spinning pulsars, but there’s a problem with that explanation. A pulsar’s energy comes from its spinning motion. Despite their regularity, pulsars are in fact gradually slowing down, and the energy lost in that deceleration is converted into the radio beams they emit. Long-period transients can’t be sluggish pulsars, because they’re already spinning so slowly that the further loss in rotational energy is no longer enough to power beams as strong as the ones we see from them. (The point at which a pulsar rotates too slowly to produce radio beams is called the pulsar death line. Pulsars that fall below this line stop emitting radio beams, so we stop seeing them as pulsars.) 

So, long-period transients can’t be so slow and so bright if they’re caused by the same mechanisms as rotationally powered pulsars. Some might still be neutron stars behaving in unexpected ways. Others could be different objects altogether, such as white dwarfs or binary systems.

A missing link

The recently discovered pulsar, called PSR J0311+1402, may offer new clues. With a 41-second period, it spins much more slowly than typical pulsars, but much faster than long-period transients. And astronomers think this middle-of-the-road object could be a missing link between the two populations.

Scientists spotted the stellar strobe with the Australian Square Kilometre Array Pathfinder (ASKAP) and followed up with other radio observatories to measure its luminosity, period, and polarization, all of which which strongly indicate that it’s a pulsar. But it’s still spinning too slowly to be powered solely by rotation. 

“It doesn’t fit neatly into the leading theory of how pulsars emit radio waves,” Wang says. “If a pulsar spins too slowly, we shouldn’t see any radio emission at all. Yet this object is very slow,” leaving astronomers wondering where the energy powering its beams is coming from. 

Studying this strange beacon and finding more like it could help astronomers learn more about how long-period transients generate their beams.

“I think there should be more objects like this waiting to be discovered,” Wang says. “Right now, we see a gap in rotation periods between known pulsars and long-period transients. Traditional pulsar search methods haven’t typically looked for objects spinning slower than 10 seconds, which means we could have been missing them simply because we weren’t searching for them before!”

Astronomers typically find pulsars using a single large radio telescope, which can’t easily detect flashes less often than about once every 10 seconds because slower signals are fainter and more easily lost in background noise. Long-period transients are usually found using arrays of multiple radio antennas working together — a method called interferometry. This method is less sensitive to fast blinkers, because combining the signals from multiple telescopes takes just long enough that it can blur or even erase rapid changes from the data. That leaves pulsars with medium periods, like PSR J0311+1402, in a blind spot for both methods.

But that’s changing, as this discovery shows. “Now, we’re using a new system with ASKAP called CRACO, which [is] perfect for catching objects in this missing range,” Wang says. It’s the system that found PSR J0311+1402, and could help astronomers figure out where pulsars stop and long-period transients begin, or whether they’re more similar than we believe. “Answering these questions will help us understand the entire neutron star population and what powers their emissions.”

The discovery, and the mystery it underscores, shows how much we have left to learn about the cosmos.

“With the new radio facilities coming online, we might find loads of weird stuff in the sky,” said Michelle Collins, an astronomy lecturer at the University of Surrey in England who was not involved in the study, in a podcast. “We may find that this is the tip of the iceberg, with many more long-period neutron stars waiting to be discovered,” Collins tells Astronomy.