Brynmor Haskell

Models of pulsar glitches

Radio pulsars provide us with some of the most stable clocks in the universe. Nevertheless several pulsars exhibit sudden spin-up events, known as glitches. More than forty years after their first discovery, the exact origin of these phenomena is still open to debate. It is generally thought that they an observational manifestation of a superfluid component in the stellar interior and provide an insight into the dynamics of matter at extreme densities. In recent years there have been several advances on both the theoretical and observational side, that have provided significant steps forward in our understanding of neutron star interior dynamics and possible glitch mechanisms. In this article we review the main glitch models that have been proposed and discuss our understanding, in the light of current observations.

On the universality of I-Love-Q relations in magnetized neutron stars

Recently, general relations among the quadrupole moment (Q), the moment of inertia (I) and the tidal deformability (Love number) of a neutron star were shown to exist. They are nearly independent of the nuclear matter equation of state and would be of great aid in extracting parameters from observed gravitational waves and in testing general relativity. These relations, however, do not account for strong magnetic fields. We consider this problem by studying the effect of a strong magnetic field on slowly rotating relativistic neutron stars and show that, for simple magnetic field configurations that are purely poloidal or purely toroidal, the relation between Q and I is again nearly universal. However, different magnetic field geometries lead to different I–Q relations, and, in the case of a more realistic twisted-torus magnetic field configuration, the relation depends significantly on the equation of state, losing its universality. I–Love–Q relations must thus be used with very great care, since universality is lost for stars with long spin periods, i.e. P ≳ 10u2009s, and strong magnetic fields, i.e. B ≳ 1012u2009G.

Glitch recoveries in radio-pulsars and magnetars

Pulsar glitches are sudden increases in the spin frequency of an otherwise steadily spinning down neutron star. These events are thought to represent a direct probe of the dynamics of the superfluid interior of the star. However glitches can differ significantly from one another, not only in size and frequency, but also in the post-glitch response of the star. Some appear as simple steps in frequency, while others also display an increase in spin-down rate after the glitch. Others still show several exponentially relaxing components in the post-glitch recovery. We show that if glitches are indeed due to large-scale unpinning of superfluid vortices, the different regions in which this occurs and respective time-scales on which they recouple can lead to the various observed signatures. Furthermore, we show that this framework naturally accounts for the peculiar relaxations of glitches in Anomalous X-ray Pulsars.

Observational constraints on neutron star crust–core coupling during glitches

We demonstrate that observations of glitches in the Vela pulsar can be used to investigate the strength of the crust-core coupling in a neutron star, and suggest that recovery from the glitch is dominated by torque exerted by the re-coupling of superfluid components of the core that were decoupled from the crust during the glitch. Assuming that the recoupling is mediated by mutual friction between the superfluid neutrons and the charged components of the core, we use the observed magnitudes and timescales of the shortest timescale components of the recoveries from two recent glitches in the Vela pulsar to infer the fraction of the core that is coupled to the crust during the glitch, and hence spun up by the glitch event. Within the framework of a two-fluid hydrodynamic model of glitches, we analyze whether crustal neutrons alone are sufficient to drive the glitch activity observed in the Vela pulsar. We use two sets of neutron star equations of state (EOSs), both of which span crust and core consistently and cover a range of the slope of the symmetry energy at saturation density

Mesoscopic pinning forces in neutron star crusts

30 70%

Pinned vortex hopping in a neutron star crust

of the moment of inertia of the core is coupled to the crust during the glitch, though for softer EOSs

Hydrodynamic simulations of pulsar glitch recovery

Lapprox 30

The effect of superfluid hydrodynamics on pulsar glitch sizes and waiting times

MeV as little as

Evidence for a Minimum Ellipticity in Millisecond Pulsars

5%

Gravitational Waves from Rapidly Rotating Neutron Stars

could be coupled. No EOS is able to reproduce the observed glitch activity with crust neutrons alone, but extending the region where superfluid vortices are strongly pinned into the core by densities as little as 0.016fm