Revisiting neutron starquakes caused by spin-down

Abstract

Context. Pulsars show a steady decrease in their rotational frequency, occasionally interrupted by sudden spin-ups called glitches, whose physical origin is still a mystery. One suggested explanation for at least the small glitches are starquakes, that is, failures of the solid neutron star crust, in which the progressive reduction in the centrifugal force deforms the star, stressing the solid until it breaks. This produces a spin-up, dissipating energy inside the star. Aims. We explore this suggestion by analyzing a mostly analytical model in order to understand the possible consequences of starquakes, particularly whether they can explain at least the small glitches. Methods. We analyze the deformations and strains produced by the decreasing centrifugal force, modeling the neutron star with a fluid core and a solid crust, each with uniform density and with the core possibly denser than the crust, as a simple approximation to the strong density gradient present in real neutron stars. Results. The deformation of a star with very different densities in the core and crust is qualitatively different from the previously studied case of equal densities. The former more closely resembles the behavior of a fluid star, in which the core-crust interface is a surface of constant gravitational plus centrifugal potential. Conclusions. Regardless of the uncertain breaking strain, the glitch activity in this model is several orders of magnitude smaller than observed, even if only small glitches are considered. For a large breaking strain, suggested by simulations, glitches due to starquakes could be roughly of the correct size but much less frequent than observed glitches. The energy released in each such glitch is much larger than in the standard model of angular momentum transfer from a faster rotating superfluid in the inner crust. On the other hand, we cannot rule out that the heating produced by small starquakes could trigger glitches by allowing neutron superfluid vortices to move. We also confirm that stresses in the neutron star crust can in principle support an ellipticity much larger than some observational upper limits from pulsar timing and continuous gravitational wave searches.

New pulse profile variability associated with the glitch of PSR J0738-4042

“…Crustquakes are invoked to explain smaller Crab-like glitches (Baym & Pines 1971;Rencoret et al 2021) and may play a dominant role in unpinning trigger for larger glitches (Ruderman 1976;Alpar et al 1996) since a single vortex line freed by a quake is able to release a large number of vortices in high density traps (Cheng et al 1988;Warszawski et al 2012). There are three main contributions to strain in the crust that generate a fracturing quake: a combination of gravitational force with secular spin-down (Giliberti et al 2020;Rencoret et al 2021) or crustal magnetic field (Franco et al 2000;Lander et al 2015) or pinning stress due to vortex-nuclei interaction (Ruderman 1976;Anderson et al 1982) and vortex-flux tube entanglement at the base of the crust (Ruderman et al 1998;Sedrakian & Cordes 1999). The spin-down induced stresses act to reduce the centrifugal bulge of the neutron star crust formed when the star was rotating much faster.…”

Pulsar Glitch Activities: The Spin Parameters Approach

“…Therefore, glitch models are built on the neutron star crust quake mechanism or the neutron star interior superfluid vortex distribution and angular momentum transfer within the neutron star Anderson & Itoh 1975;Alpar et al 1984;Zhou et al 2014;Eya et al 2017a. There has been no lack of theories to explain pulsar glitch trigger mechanisms and subsequent post-glitch behavior (see, for example, Baym et al 1969;Baym & Pines 1971;Alpar 1995;Xiao et al 2011;Zhou et al 2014;Lai & Xu 2016;Rencoret et al 2021, and for reviews (Haskell & Melatos 2015;Antonopoulou et al 2022;Zhou et al 2022)). Despite that, there has not been a consensus on the actual origin of glitches in neutron stars.…”

Accumulation of Elastic Strain toward Crustal Fracture in Magnetized Neutron Stars

“…A sudden spin-up in a radio pulsar is produced by the transfer of angular momentum from the superfluid components of the core to the normal crust (Anderson & Itoh 1975;Alpar et al 1984). Crust quakes were also discussed in other models (Franco et al 2000;Giliberti et al 2020;Rencoret et al 2021, for recent studies). The elastic deformation is caused by a decrease in centrifugal force, owing to a secular spin-down, and the crust eventually fractures when the strain exceeds a critical threshold.…”