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Pulsars, rapidly rotating neutron stars that sweep beams of radiation across space like cosmic lighthouses, are among the most enigmatic objects in the universe. While their clockwork precision makes them valuable cosmic timekeepers, they occasionally exhibit sudden, unpredictable jumps in their rotational frequency — anomalies known as glitches. These abrupt spin-ups, often followed by subtle but lasting changes in their slowdown rates, have puzzled astrophysicists for decades. The study of these glitches not only deepens our understanding of pulsars but also provides rare insights into the extreme physics governing these ultra-dense remnants of stellar collapse.

Among the many known pulsars, the Crab and Vela pulsars stand out as prime subjects for glitch studies. Their frequent and well-documented glitches make them valuable test cases for the competing theories that attempt to explain these enigmatic events. Understanding the underlying mechanisms requires delving into the bizarre, high-energy environments that define neutron stars—places where matter exists under pressures and densities far beyond anything found on Earth.

What causes pulsar glitches?

One of the leading explanations for pulsar glitches is the starquake model. This theory suggests that as a neutron star gradually loses rotational energy, the accompanying decrease in centrifugal force causes its crust to contract. Over time, stress builds up until the rigid crust can no longer withstand the strain, resulting in a sudden, catastrophic crack—a starquake. This abrupt readjustment changes the star’s moment of inertia, leading to an increase in rotational speed. The starquake model appears to describe the glitches observed in the Crab pulsar quite well, but it struggles to explain the much larger glitches seen in the Vela pulsar. Some studies suggest that if this model were correct, a measurable parameter called the glitch healing factor (Q) should correspond to the ratio of the superfluid core’s moment of inertia to the total moment of inertia.

Another compelling explanation is the vortex unpinning model, which focuses on the peculiar behavior of superfluid matter inside neutron stars. In this model, the neutron star’s interior contains a superfluid component threaded with countless tiny vortices that store angular momentum. These vortices can become pinned to the crust, preventing the superfluid from slowing down in sync with the outer shell of the star. Over time, as the star’s crust spins down, a mismatch in rotational speeds builds up between the crust and the superfluid interior. When the pinned vortices suddenly break free, they transfer their stored angular momentum to the crust, causing a glitch. This theory aligns well with observations of Vela’s large glitches and provides an elegant explanation for their recurrence.

A third hypothesis, thermally driven glitches, proposes that a sudden influx of thermal energy into the inner crust could increase the coupling between the crust and the superfluid interior, leading to a rapid spin-up. This model links glitch magnitude and duration to the neutron star’s temperature, offering another perspective on the connection between a pulsar’s thermal state and its rotational behavior.

Starquakes and strangeons

Studies have explored the connection between glitches and starquakes. Some research suggests that the frequency of starquakes is directly proportional to the critical strain angle of the crust, with the magnitude of each glitch corresponding to this value. However, one study challenges the feasibility of starquakes as a primary cause of glitches, arguing that the required stress for large spin-up events develops too slowly under conventional neutron star models. A different approach applies earthquake mechanics to neutron stars, calculating stress loading and optimal rupture types for both neutron and strangeon star models. These investigations suggest that a hypothetical strangeon star—a dense object composed of exotic strange quark matter—could provide a more robust explanation for observed glitch amplitudes.

Observational data continue to shape our understanding of these cosmic anomalies. Studies of the Crab pulsar have revealed a persistent increase in its slowdown rate after glitches, hinting at a change in the star’s spin-down torque. These shifts cannot be solely attributed to structural readjustments, implying that other factors, such as enhanced magnetic braking, may be at play. Meanwhile, investigations into gamma-ray burst GRB 211211A suggest that precursor flares from neutron stars might be triggered by resonant crust shattering, generating seismic aftershocks and torsional oscillations detectable as quasi-periodic oscillations (QPOs).

Cracks in Dead Stars

Despite progress in unravelling the mechanisms behind neutron star glitches, significant challenges remain. One major obstacle for the starquake model is the slow accumulation of stress needed to generate large glitches. However, alternative models, such as the strangeon star hypothesis, may help bridge the gap between theory and observation. Additionally, the extreme conditions within neutron stars represent a regime of matter physics that cannot be replicated in terrestrial laboratories. This makes pulsars a natural testing ground for exploring the fundamental properties of ultra-dense matter, offering a rare glimpse into a state of matter inaccessible on Earth.

By piecing together observational data, computational models, and fundamental physics, astronomers are steadily refining our understanding of pulsar glitches. Whether through the cracking of a neutron star’s crust, the unpinning of superfluid vortices, or the influence of exotic matter, each new insight into these cosmic glitches brings us one step closer to unlocking the secrets of the universe’s most extreme objects.

The Crab Nebula. (Image Credit: ESA/Herschel/PACS/MESS Key Programme Supernova Remnant Team; NASA, ESA and Allison Loll/Jeff Hester, Arizona State University).