Pulsar PSR J1637−4642, a neutron star that astronomers have monitored for more than 15 years, produced its most powerful glitch on record in 2018 after nearly a decade of complete radio silence, according to a new study by astronomers at Xiamen University in China. The star's rotation frequency jumped by 17.54 microhertz in an instant, and two further glitches followed over the next six years.
The team, whose findings are posted on arXiv, analysed data collected by the Murriyang radio telescope at Australia's Parkes Observatory between February 2009 and October 2024.
A Decade of Silence, Then a Sudden Speed-Up
PSR J1637−4642 is exceptionally young by astronomical standards, at roughly 41,000 years old, and completes one rotation every 154 milliseconds. It stayed quiet for almost 10 years after its discovery before the first and strongest glitch struck in 2018, changing its rotation frequency by 2.7 millionths.
A second, far weaker glitch followed about three years later, roughly a thousand times smaller than the first. A third glitch of intermediate strength came about 2.7 years after that. The difference in scale between the three events spans more than three orders of magnitude.
What Causes a Pulsar Glitch
Pulsars are fast-spinning neutron stars that emit narrow beams of radio waves. As each beam sweeps past Earth with every rotation, astronomers can track the star's spin with extreme precision. Occasionally a pulsar's rotation suddenly speeds up in an event known as a glitch.
The exact cause of glitches is not fully understood, but the leading hypothesis links them to interactions between a pulsar's solid crust and a superfluid interior, an exotic form of matter with no viscosity. When angular momentum built up in this superfluid "reservoir" suddenly transfers to the crust, observers record a jump in frequency.
Only the Biggest Glitch Showed Recovery
Only the powerful 2018 glitch was followed by a noticeable recovery. Rather than locking instantly onto its new value, the frequency declined gradually over about 100 days. The recovery was small, however: just 1.5% of the original jump was undone, while 98.5% of the increase persisted in later observations. No significant recovery was detected after either of the two weaker glitches.
Evidence for a Superfluid Layer
To explain the observations, the researchers applied what is known as the vortex creep model, which describes how superfluid matter gradually hands off accumulated angular momentum to the solid crust. A Bayesian analysis showed that the superfluid reservoir accounts for about 1.87% of the star's total moment of inertia, a figure that matches theoretical estimates for the superfluid layer inside a neutron star's inner crust.
The characteristic relaxation time of around 102 days also allowed the team to estimate the pulsar's internal temperature at about 100 million kelvins, consistent with standard cooling models for an object of this age.
What Happens Next
The researchers said the findings show that pulsars that appear stable for years can quietly build up enormous internal stresses, which are released in powerful glitches. The fact that three events in the same star varied in scale by more than a factor of 1,000 does not fit easily into simple accumulation models and points to a more complex, possibly stochastic process behind glitches.
Continued monitoring of PSR J1637−4642, the team said, should help refine the waiting times between future glitches and improve understanding of how superfluid matter behaves inside neutron stars.
