Pulsars, the rapidly rotating remnant cores of massive stars that have died in violent explosions, are among the densest objects in the universe apart from black holes. These neutron stars, which emit beams of electromagnetic radiation as they spin, act like cosmic lighthouses, pulsing with remarkable regularity. However, astronomers have observed occasional disruptions in their otherwise steady spin-down—sudden increases in rotational speed known as glitches. Understanding these glitches provides crucial insights into the extreme physics governing neutron stars and the exotic states of matter they contain.
A frictionless liquid
To explain these glitches, scientists consider neutron stars as two-component systems: a rigid outer crust and a superfluid interior core. This superfluid—a state of matter that flows without friction—plays a key role in the rotational behaviour of pulsars. The interaction between these two components influences the star’s ability to store and transfer angular momentum, ultimately leading to observable glitches.
One leading explanation for pulsar glitches is the vortex unpinning model. As a pulsar gradually slows down due to energy loss, angular momentum builds up in the vortices of the superfluid core. These vortices normally migrate outward but can become “pinned” to atomic nuclei in the boundary between the core and the outer crust. Over time, this causes a mismatch in rotation between the core and the crust. When the pinning force is overcome, the vortices suddenly unpin and move outward in a rapid burst, transferring angular momentum to the crust. This results in a sudden increase in the pulsar’s rotation speed—a glitch.
Another theory, the starquake model, attributes glitches to the structural failure of the neutron star’s crust. As the star cools and loses rotational energy, the rigid crust contracts. Eventually, the built-up stress leads to a sudden “quake,” much like tectonic activity on Earth. This crustal adjustment momentarily speeds up the star’s rotation. However, observations of glitches in the Vela pulsar suggest that this model may not fully explain its behaviour.
The Healing Parameter
A crucial piece of evidence for distinguishing between these models is the glitch healing parameter, Q, which measures how quickly the pulsar’s rotation rate returns to its previous trend after a glitch. This parameter is related to the moment of inertia of the superfluid core compared to the total moment of inertia of the star. Observations of Vela pulsar glitches suggest that the starquake model does not match the measured values of Q, reinforcing the idea that vortex unpinning is the primary cause of these events.
Beyond glitches, the presence of superfluid neutrons within neutron stars has broader implications for astrophysics. Superfluidity affects the star’s global seismic oscillations (GSOs), influencing how vibrations propagate through the crust and core. Recent studies indicate that the interaction between superfluid neutrons and the solid lattice of the crust alters the frequencies of certain oscillation modes, such as Alfvén waves, by at least 10%—a significant correction that must be accounted for in theoretical models.
The Future of Pulsar Research
Investigating pulsar glitches not only deepens our understanding of neutron star interiors but also sheds light on the behaviour of superfluid matter under extreme conditions—an area of physics with implications for both astrophysics and condensed matter studies. As telescopes and observational techniques continue to improve, future discoveries may provide even clearer insights into these mysterious celestial objects, helping us unlock the secrets hidden within the densest stars in the universe.
Illustration of a Magnetar. (Image Credit: NASA/GSFC).
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