New Slow Pulsar Shouldn’t Be Emitting Radio Waves, But It Does

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

The new highly magnetic pulsar has a surprisingly long rotation period, which challenges the theoretical understanding of these objects, the researchers report on May 30 at natural 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.

Although it is an eccentric, some of the characteristics of this new pulsar are common to its relatives. This means that this object can help astronomers better connect the evolutionary phases of mysterious species in the menagerie of neutron stars.

Astronomers know of many types of neutron stars. Each is the compact object remaining after the explosive death of a massive star, but their characteristics may vary. A pulsar is a neutron star that astronomers detect at regular intervals thanks to its cosmic alignment: the star’s strong magnetic field produces beams of radio waves emanating from the star’s poles, and each time a these beams sweep the Earth, astronomers can see a radio pulse.

The new slow pulsar is in our galaxy, about 1,300 light-years away. Astrophysicist Manisha Caleb from 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 slow, steady radio beat of the pulsar – a measure of how fast it spins – but also another important detail: how fast the spin slows as the pulsar ages. And these two pieces of information revealed something strange about this pulsar. According to the theory, it should not emit radio waves. And yet, it is.

As neutron stars age, they lose energy and spin more slowly. According to the calculations, “at some point, they have exhausted all their energy and they stop emitting all kinds of emissions,” explains Caleb. They became dead to the detectors.

The period of rotation of a pulsar and the slowing down of its spin are linked to the strength of its magnetic field, which accelerates subatomic particles coming from the star and, in turn, generates radio waves. All neutron stars rotating as slowly as PSR J0901-4046 are in this stellar “graveyard” and should not produce radio signals.

But “we keep finding weirder and weirder pulsars that erode that understanding,” says astrophysicist Maura McLaughlin of the University of West Virginia at Morgantown, who was not involved in this work.

The pulsar found could be its own unique kind of neutron star. But in some ways, it also feels a bit familiar, Caleb says. She and her colleagues calculated the pulsar’s magnetic field from the rate at which its rotation slows, and it’s incredibly strong, similar to magnetars (SN: 09/17/02). This hints that PSR J0901-4046 could be what is called a “resting magnetar”, which is a pulsar with very strong magnetic fields that occasionally emit bursts of X-rays or other brilliantly energetic radiation. “We’re either going to need an X-ray show or [ultraviolet] observations to confirm whether it is indeed a magnetar or a pulsar,” she said.

The discovery team still has additional observations to analyze. “We have a truckload of more data on this,” says astrophysicist Ian Heywood of the University of Oxford. Researchers are studying how the object’s brightness changes over time and whether its rotation changes abruptly, or “glitches.”

Astronomers are also modifying their automated computer programs, which analyze radio data and report intriguing signals, to look for these longer rotation periods — or even weirder, more mysterious neutron star phenomena. “To me, the good thing about astronomy is what we’re waiting to find out,” says Heywood.

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