Pulsar spin-velocity alignment from single and binary neutron star progenitors

Abstract

The role of binary progenitors of neutron stars in the apparent distribution of space velocities and spin-velocity alignment observed in young pulsars is studied. A Monte-Carlo synthesis of pulsar population from single and binary stars with different assumptions about the NS natal kick model (direction distribution, amplitude, and kick reduction in binary progenitors which experienced mass exchange due to Roche lobe overflow with initial masses on the main sequence from the range 8-11 $M_{\odot }$) is performed. The calculated spin-velocity alignment distributions are compared with observational data obtained from radio polarization measurements. The observed space velocity of pulsars is found to be mostly shaped by the natal kick velocity form and its amplitude; the fraction of binaries is not important here for reasonably large kicks. The distribution of kick direction relative to the spin axis during the formation of a NS is found to affect strongly the spin-velocity correlation of pulsars. Comparison with observed pulsar spin-velocity angles favours a sizeable fraction of binary progenitors and the kick-spin angle $\sim 5-20^{\circ }$. The form of the initial binary mass ratio distribution does not affect our results.

20 pages, 8 figures; Submitted to MNRAS

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