Pulsar wind tori and the spin-kick connection

Abstract

Neutron stars in general have space velocities much higher than their progenitors. This is believed to be the result of momentum kicks imparted during core collapse. However, the nature of the kick remains an open question. Abetted by recent progress in measuring pulsar proper motions and spin orientations, it has become possible to make improved comparison between the pulsar's linear and angular momentum vectors; this places constrains on the kick physics and hence probes the core collapse dynamics. To investigate the spin-kick connection, we improved the Crab pulsar's proper motion measurement using HST astrometry with over 6 years of observations. The updated result indicates the pulsar velocity is off the spin axis by 26° ± 3°. For the spin vector measurements, recent Chandra observations reveal toroidal termination shocks and polar jets in many young pulsar wind nebulae, with the symmetry axes indicating the pulsar spin axes. We developed a procedure for fitting simple Doppler boosted 3-D torus models to the X-ray data. This provides robust and nearly model independent measurements of pulsar spin orientations. All the results suggest correlation between pulsar motions and spin directions. We assemble this evidence for young isolated pulsars and test several scenarios for a birth kick driven by accretion and cooling of the proto-neutron star, where the momentum thrust is proportional to the neutrino cooling luminosity. The simulations include the effects of pulsar acceleration and spin-up. The fit to the pulsar spin and velocity samples suggests a 10% anisotropy in the neutrino flux is required, with a finite pre-kick spin of 10--20 rad s -1 , a kick timescale of 1--3 s and kick-induced spin.