PSR J1637−4642 is a neutron star 41,000 years old. It completes about 6.5 rotations per second and has been ticking away in the Milky Way since shortly after the massive star that created it went supernova. Astronomers at the Murriyang radio telescope in Australia began watching it in February 2009. For nearly a decade, it did nothing unusual.
Then, in 2018, it suddenly spun faster.
That event — a sudden, measurable jump in rotation rate — is what astronomers call a glitch. PSR J1637−4642 has now produced three of them, recorded over 15.5 years of monitoring. The most recent was detected around 2023. Each glitch is tiny relative to the star's overall spin, but each one is also a window into physical conditions that cannot be replicated anywhere on Earth.
What a glitch actually is
A pulsar is a neutron star that beams radio waves toward Earth with each rotation, like a lighthouse. Pulsars are born spinning fast and gradually slow down as they radiate energy. That deceleration is predictable enough that astronomers use pulsars as precision clocks — some of the most stable natural timekeepers known.
A glitch breaks that clock, but not by slowing it further. Instead, the pulsar suddenly rotates fractionally faster. The effect is tiny: the largest glitch detected in PSR J1637−4642 increased its spin rate by just 2.7 parts per million. But it arrived as a near-instantaneous event, followed by a gradual partial recovery that in this case played out over roughly 102 days.
The superfluid inside the dead star
The explanation that best fits the observations involves what is happening inside the neutron star — and that interior is one of the most extreme physical environments known.
Neutron stars have a solid outer crust of neutron-rich nuclei. Below that lies the inner crust, where free neutrons behave as a superfluid: a quantum state in which there is no viscosity. In a superfluid, rotation is carried by tiny quantized vortices. As the star's crust gradually spins down due to electromagnetic braking, the superfluid interior does not slow at the same rate. The vortices carry more angular momentum than the crust. When the difference becomes large enough, angular momentum transfers abruptly from the superfluid to the crust, spinning it up.
That is a glitch.
Three events, three different sizes
The three glitches in PSR J1637−4642 were not equal. The first, in 2018, was the largest: a 17.54 microhertz increase in spin frequency. The second, around 2021, was by far the smallest recorded in this star. The third, around 2023, fell between them. The differing magnitudes suggest that the threshold at which angular momentum transfers is not a fixed quantity — the superfluid can release more or less depending on conditions researchers are still working to characterize.
Analyzing the post-glitch relaxation allowed the research team, led by Zhaoyi Wang of Xiamen University, to estimate that approximately 1.9% of the neutron star's total moment of inertia involves the superfluid layer of the inner crust. That fraction directly constrains theoretical models of neutron star interiors — models that attempt to describe how matter behaves at densities impossible to achieve in any laboratory. The findings are reported in a paper accepted for publication in Astrophysical Journal Letters.
Why the quiet years were not empty
The fact that PSR J1637−4642 showed no glitches for its first nine years of observation is not a detail to set aside. It means the star was accumulating the angular momentum deficit — the growing gap between the slowing crust and the superfluid interior — without releasing it. Then it released it three times in six years.
Pulsars that glitch infrequently are harder to study precisely because the data takes longer to build. Fifteen and a half years of monitoring a quiet source turned out to be exactly the observational baseline needed to catch three separate events and analyze them as a group.
The only way to probe the interior of a dead star is to watch how it rotates from thousands of light-years away. Three almost imperceptible speed-ups — each one a fraction of a millionth of a revolution per second — are among the clearest signals currently available about the densest form of matter in the observable universe.