‘Quiet’ pulsar suddenly reveals three glitches in 15 years of observations

Using more than 15 years of observations from the Parkes radio telescope, astronomers detected three rotational glitches in the pulsar PSR J1637−4642—a young neutron star that had shown no signs of glitches since its discovery. The three glitches included one that changed its rotation rate by nearly 3 parts per million. Their paper was posted to the arXiv preprint server on Aug. 20. It has been accepted for publication in the Astrophysical Journal Letters.

Spinning stars

Neutron stars that rapidly spin, sending out regular pulses of radio signals, are called pulsars. Some pulsars occasionally experience sudden increases in their rotation rate, known as glitches. Younger pulsars tend to exhibit this complex timing behavior often, which is why they are interesting to study.

While the exact cause of glitches remains unknown, astronomers think a glitch is caused by a sudden transfer of angular momentum from the superfluid inside the star to its solid crust.

PSR J1637−4642 is a 41,000-year-old pulsar—extremely young by cosmic standards. It spins once every 154 milliseconds. Despite its young age and relatively high energy output, it remained “quiet” and had never shown glitches during roughly a decade of observations after its discovery.

Three hiccups

In this study, a team of astronomers led by Zhaoyi Wang of Xiamen University examined this young pulsar. They analyzed 15.5 years of data collected with the Murriyang radio telescope between February 2009 and October 2024.

Their analysis revealed three distinct glitches. The first occurred around 2018 and was by far the strongest. The pulsar’s rotation frequency suddenly increased by about 17.54 microhertz, corresponding to a fractional change of roughly 2.7 parts per million.

The second glitch occurred about three years later and was smaller. It changed the rotation frequency by only about 14 nanohertz. A third glitch appeared roughly 2.7 years after the second. It was intermediate in strength, producing a frequency increase of about 179 nanohertz.

“PSR J1637−4642 adds to the growing class of young pulsars that exhibit large glitches after extended intervals of apparent quiescence,” the team writes in the paper.

Prolonged relaxation

After the first glitch, the pulsar did not immediately settle into its new rotation rate. Instead, part of the change gradually relaxed over time. Modeling the aftermath of this glitch suggested that about 1.9% of the neutron star’s moment of inertia is associated with superfluid material in its inner crust. The model also gives a relaxation timescale of roughly 102 days.

The results fit the long-standing idea that glitches are caused by superfluid neutrons inside neutron stars suddenly transferring angular momentum to the crust. The crust then spins up abruptly, producing the sudden increase in rotational frequency observed as a glitch. After the glitch, the pulsar gradually relaxes, and the inner crust and the superfluid move toward a new equilibrium.

“Our results demonstrate that even ‘quiet’ pulsars can harbor significant glitch activity,” the team concludes. The long period of silence before the first glitch could indicate a buildup of stress over many years before the internal changes that caused the spin-up event.

Who’s behind this story?

Shreejaya Karantha

Shreejaya Karantha is a science writer and astronomy communicator based in India, with a focus on astrophysics and the early universe.

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