Theoretical support for the hydrodynamic mechanism of pulsar kicks

Abstract

The collapse of a massive star's core, followed by a neutrino-driven, asymmetric supernova explosion, can naturally lead to pulsar recoils and neutron star kicks. Here, we present a two-dimensional, radiation-hydrodynamic simulation in which core collapse leads to significant acceleration of a fully formed, nascent neutron star via an induced, neutrino-driven explosion. During the explosion, an $\ensuremath{\sim}10%$ anisotropy in the low-mass, high-velocity ejecta leads to recoil of the high-mass neutron star. At the end of our simulation, the neutron star has achieved a velocity of $\ensuremath{\sim}150\text{ }\text{ }\mathrm{km}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$ and is accelerating at $\ensuremath{\sim}350\text{ }\text{ }\mathrm{km}\text{ }{\mathrm{s}}^{\ensuremath{-}2}$, but has yet to reach the ballistic regime. The recoil is due almost entirely to hydrodynamical processes, with anisotropic neutrino emission contributing less than 2% to the overall kick magnitude. Since the observed distribution of neutron star kick velocities peaks at $\ensuremath{\sim}300--400\text{ }\text{ }\mathrm{km}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$, recoil due to anisotropic core-collapse supernovae provides a natural, nonexotic mechanism with which to obtain neutron star kicks.