ANGULAR MOMENTUM TRANSFER IN VELA-LIKE PULSAR GLITCHES

Abstract

The angular momentum transfer associated to Vela-like glitches has never been calculated {\em directly} within a realistic scenario for the storage and release of superfluid vorticity; therefore, the explanation of giant glitches in terms of vortices has not yet been tested against observations. We present the first physically reasonable model, both at the microscopic and macroscopic level (spherical geometry, n=1 polytropic density profile, density-dependent pinning forces compatible with vortex rigidity), to determine where in the star the vorticity is pinned, how much of it, and for how long. For standard neutron star parameters ($M=1.4 M_{\odot}, R_s=10$ km, $\dot{\Omega}=\dot{\Omega}_{\rm Vela}=-10^{-10}$ Hz s$^{-1}$), we find that maximum pinning forces of order $f_m\approx10^{15}$ dyn cm$^{-1}$ can accumulate $\Delta L_{\rm gl}\approx10^{40}$ erg s of superfluid angular momentum, and release it to the crust at intervals $\Delta t_{\rm gl}\approx3$ years. This estimate of $\Delta L_{\rm gl}$ is one order of magnitude smaller than what implied indirectly by current models for post-glitch recovery, where the core and inner-crust vortices are taken as physically disconnected; yet, it successfully yields the magnitudes observed in recent Vela glitches for {\em both} jump parameters, $\Delta\Omega_{\rm gl}$ and $\Delta\dot{\Omega}_{\rm gl}$, provided one assumes that only a small fraction ($<10%$) of the total star vorticity is coupled to the crust on the short timescale of a glitch. This is reasonable in our approach, where no layer of normal matter exists between the core and the inner-crust, as indicated by existing microscopic calculation. The new scenario presented here is nonetheless compatible with current post-glitch models.