A newfound, slow pulsar shouldn’t emit radio waves — yet it does

Astronomers have added a new species to the neutron star zoo, showcasing the wide diversity among the compact magnetic remains of dead, once-massive stars.

The newfound highly magnetic pulsar has a surprisingly long rotation period, which is challenging the theoretical understanding of these objects, researchers report May 30 in Nature Astronomy. Dubbed PSR J0901-4046, this pulsar sweeps its lighthouse-like radio beam past Earth about every 76 seconds — three times slower than the previous record holder.

While it’s an oddball, some of this newfound pulsar’s characteristics are common among its relatives. That means this object may help astronomers better connect the evolutionary phases among mysterious species in the neutron star menagerie.

Astronomers know of many types of neutron stars. Each one is the compact object left over after a massive star’s explosive death, but their characteristics can vary. A pulsar is a neutron star that astronomers detect at a regular interval thanks to its cosmic alignment: The star’s strong magnetic field produces beams of radio waves emanating from near the star’s poles, and every time one of those beams sweeps across Earth, astronomers can see a radio pulse.

The newfound, slowpoke pulsar sits in our galaxy, roughly 1,300 light-years away. Astrophysicist Manisha Caleb of the University of Sydney in Australia and her colleagues found it in data from the MeerKAT radio telescope outside Carnarvon, South Africa.

Further observations with MeerKAT revealed not only the pulsar’s slow, steady radio beat — a measure of how fast it spins — but also another important detail: The rate at which the spin slows as the pulsar ages. And those two bits of info revealed something odd about this pulsar. According to theory, it should not be emitting radio waves. And yet, it is.

As neutron stars age, they lose energy and spin more slowly. According to calculations, “at some point, they’ve exhausted all their energy, and they cease to emit any sort of emission,” Caleb says. They’ve become dead to detectors.

A pulsar’s rotation period and the slowdown of its spin relates to the strength of its magnetic field, which accelerates subatomic particles streaming from the star and, in turn, generates radio waves. Any neutron stars spinning as slowly as PSR J0901-4046 are in this stellar “graveyard” and shouldn’t produce radio signals.

But “we just keep finding weirder and weirder pulsars that chip away at that understanding,” says astrophysicist Maura McLaughlin of West Virginia University in Morgantown, who wasn’t involved with this work.

The newfound pulsar could be its own unique species of neutron star. But in some ways, it also looks a bit familiar, Caleb says. She and her colleagues calculated the pulsar’s magnetic field from the rate its spin is slowing, and it’s incredibly strong, similar to magnetars (SN: 9/17/02). This hints that PSR J0901-4046 could be what’s known as a “quiescent magnetar,” which is a pulsar with very strong magnetic fields that occasionally emits brilliantly energetic bursts of X-rays or other radiation. “We’re going to need either X-ray emission or [ultraviolet] observations to confirm whether it is indeed a magnetar or a pulsar,” she says.

The discovery team still has additional observations to analyze. “We do have a truckload more data on it,” says astrophysicist Ian Heywood of the University of Oxford. The researchers are looking at how the object’s brightness is changing over time and whether its spin abruptly changes, or “glitches.”

The astronomers also are altering their automated computer programs, which scan the radio data and flag intriguing signals, to look for these longer-duration spin periods — or even weirder and more mysterious neutron star phenomena. “The sweet thing about astronomy, for me, is what’s out there waiting for us to find,” Heywood says.