Mesoscopic pinning forces in neutron star crusts

Abstract

The crust of a neutron star is thought to be comprised of a lattice of nuclei immersed in a sea of free electrons and neutrons. As the neutrons are superfluid their angular momentum is carried by an array of quantized vortices. These vortices can pin to the nuclear lattice and prevent the neutron superfluid from spinning down, allowing it to store angular momentum which can then be released catastrophically, giving rise to a pulsar glitch. A crucial ingredient for this model is the maximum pinning force that the lattice can exert on the vortices, as this allows us to estimate the angular momentum that can be exchanged during a glitch. In this paper we perform, for the first time, a detailed and quantitative calculation of the pinning force per unit length acting on a vortex immersed in the crust and resulting from the mesoscopic vortexlattice interaction. We consider realistic vortex tensions, allow for displacement of the nuclei and average over all possible orientation of the crystal with respect to the vortex. We find that, as expected, the mesoscopic pinning force becomes weaker for longer vortices and is generally much smaller than previous estimates, based on vortices aligned with the crystal. Nevertheless the forces we obtain still have maximum values of order f pin ≈ 10 15 dyn/cm, which would still allow for enough angular momentum to be stored in the crust to explain large Vela glitches, if part of the star is decoupled during the event.

Microscopic Calculations of Vortex-Nucleus Interaction in the Neutron Star Crust

“…Therefore hydrodynamical description can be used only to estimate the asymptotic behavior of the vortex-nucleus interaction (∝ 1/r 3 ). It is worth mentioning that the extracted force is at least one order of magnitude larger than those predicted in a recent phenomenological analysis [11]. The repulsive force is compatible with the socalled interstitial pinning, where vortices are trapped at positions that maximize the overall separation from the nearest nuclei [12].…”

Post-glitch exponential relaxation of radio pulsars and magnetars in terms of vortex creep across flux tubes

“…Post-glitch relaxation occurs through reestablishment of this thermal creep conditions for the vortices depinned during a glitch (Alpar et al 1984a). Vortex line pinning to and creep against atomic nuclei in the crustal lattice picture has been confirmed by recent realistic numerical computations (Seveso et al 2016;.…”

Constraints on pulsar masses from the maximum observed glitch

“…In this paper we show for the first time how the maximum glitch amplitude recorded in a given pulsar can robustly constrain its mass when coupled to state of the art calculations of the pinning force between superfluid vortices and ions in the crust 24 . We analyse a physically consistent scenario for the reservoir of angular momentum 25 and propose a method to bracket the mass values using observational data of the maximum event.…”