The pulsar, PSR J1637−4642, is just 41,000 years old by cosmic standards and spins once every 154 milliseconds. Despite its youth and high energy output, it had been considered “quiet” — a label astronomers now say was premature. A team led by Zhaoyi Wang of Xiamen University reported the findings in a paper posted to the arXiv preprint server on August 20 and accepted for publication in the Astrophysical Journal Letters.
Three glitches, three different sizes
Wang’s team analyzed data collected between February 2009 and October 2024 at the Parkes (Murriyang) radio telescope. Their analysis surfaced three separate glitches — sudden increases in the pulsar’s rotation frequency that stand apart from the slow, steady spin-down most pulsars undergo.
The first glitch, which occurred around 2018, was by far the strongest. The rotation frequency jumped by roughly 17.54 microhertz, corresponding to a fractional change of about 2.7 parts per million. For a pulsar whose timing had been stable for years, that is a substantial kick.
The second glitch arrived roughly three years later and was much smaller, changing the rotation frequency by only about 14 nanohertz. A third glitch followed about 2.7 years after the second, with an intermediate strength of roughly 179 nanohertz. The three events spanned a window of just over five years after more than a decade of apparent stillness.
“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.
What a pulsar glitch actually is
Pulsars are rapidly spinning neutron stars that beam radio waves outward like a lighthouse. Their rotation is normally predictable enough to rival atomic clocks, but some — particularly young ones — occasionally hiccup. These hiccups are called glitches, and they remain only partially understood.
The leading explanation involves the star’s interior. Neutron stars are thought to contain a superfluid of neutrons in their inner crust, decoupled from the solid outer crust that observers effectively track through radio pulses. Over time, the superfluid spins faster than the crust as the crust slowly brakes. When the difference becomes large enough, angular momentum is suddenly transferred from the superfluid to the crust, and the crust spins up. That abrupt spin-up is the glitch.
Afterward, the system relaxes toward a new equilibrium, and the observed rotation rate gradually settles. This relaxation phase is what the team modeled in detail for the 2018 glitch.
Clues from the relaxation phase
After the first glitch, PSR J1637−4642 did not simply snap to a new rotation rate. Part of the frequency change relaxed gradually over time. By fitting this relaxation, the researchers estimated that about 1.9% of the neutron star’s moment of inertia is tied to superfluid material in its inner crust — the reservoir that powers glitches.
The model also produced a relaxation timescale of roughly 102 days. That number matters because it constrains the properties of the superfluid and the coupling between it and the crust, offering a rare peek inside an object only about 20 kilometers across.
The results fit the long-standing superfluid-transfer picture of glitch origin. They also suggest that the long silence before 2018 was not an absence of activity but a buildup of internal stress — angular momentum accumulating in the superfluid until something gave way.
What Happens Next
The discovery reshapes how astronomers think about so-called quiet pulsars. A decade of glitch-free timing does not guarantee a glitch-free interior; it may simply mean the stress has not yet crossed the threshold for release. That has practical consequences for pulsar timing array experiments, which rely on nanosecond-level stability to detect gravitational waves from supermassive black hole binaries. A glitch in a monitored pulsar can mimic or mask a gravitational-wave signal, so understanding which stars are truly quiet and which are merely biding time is essential.
Continued monitoring of PSR J1637−4642 with Murriyang and other telescopes will test whether the post-2018 glitch cadence of roughly three years holds. If a fourth glitch appears on schedule, the pulsar will become one of the cleanest laboratories for studying neutron star interiors — and a reminder that “quiet” in astronomy often just means “not yet looked at long enough.”
— Priya Nair, science desk, AXO News