THERMALLY ACTIVATED POST-GLITCH RESPONSE OF THE NEUTRON STAR INNER CRUST AND CORE. I. THEORY

Abstract

Pinning of superfluid vortices is predicted to prevail throughout much of a neutron star. Based on the idea of Alpar et al., I develop a description of the coupling between the solid and liquid components of a neutron star through {\em thermally-activated vortex slippage}, and calculate the the response to a spin glitch. The treatment begins with a derivation of the vortex velocity from the vorticity equations of motion. The activation energy for vortex slippage is obtained from a detailed study of the mechanics and energetics of vortex motion. I show that the "linear creep" regime introduced by Alpar et al. and invoked in fits to post-glitch response is not realized for physically reasonable parameters, a conclusion that strongly constrains the physics of post-glitch response through thermal activation. Moreover, a regime of "superweak pinning", crucial to the theory of Alpar et al. and its extensions, is probably precluded by thermal fluctuations. The theory given here has a robust conclusion that can be tested by observations: {\em for a glitch in spin rate of magnitude $\Delta\nu$, pinning introduces a delay in the post-glitch response time}. The delay time is $t_d=7 (t_{sd}/10^4\mbox{yr})((\Delta\nu/\nu)/10^{-6})$ d where $t_{sd}$ is the spin-down age; $t_d$ is typically weeks for the Vela pulsar and months in older pulsars, and is independent of the details of vortex pinning. Post-glitch response through thermal activation cannot occur more quickly than this timescale. Quicker components of post-glitch response as have been observed in some pulsars, notably, the Vela pulsar, cannot be due to thermally-activated vortex motion but must represent a different process, such as drag on vortices in regions where there is no pinning. I also derive the mutual friction force for a pinned superfluid at finite temperature for use in other studies of neutron star hydrodynamics.