Neutron star kicks and their relationship to supernovae ejecta mass

Abstract

We propose a simple model to explain the velocity of young neutron stars. We attempt to confirm a relationship between the amount of mass ejected in the formation of the neutron star and the `kick' velocity imparted to the compact remnant resulting from the process. We assume the velocity is given by $v_{\rm kick}=\alpha\,(M_{\rm ejecta} / M_{\rm remnant}) + \beta\,$. To test this simple relationship we use the BPASS (Binary Population and Spectral Synthesis) code to create stellar population models from both single and binary star evolutionary pathways. We then use our Remnant Ejecta and Progenitor Explosion Relationship (REAPER) code to apply different $\alpha$ and $\beta$ values and three different `kick' orientations then record the resulting velocity probability distributions. We find that while a single star population provides a poor fit to the observational data, the binary population provides an excellent fit. Values of $\alpha=70\, {\rm km\,s^{-1}}$ and $\beta=110\,{\rm km\,s^{-1}}$ reproduce the \cite{RN165} observed 2-dimensional velocities and $\alpha=70\, {\rm km\,s^{-1}}$ and $\beta=120\,{\rm km\,s^{-1}}$ reproduce their inferred 3-dimensional velocity distribution for nearby single neutron stars with ages less than 3 Myrs. After testing isotropic, spin-axis aligned and orthogonal to spin-axis `kick' orientations, we find no statistical preference for a `kick' orientation. While ejecta mass cannot be the only factor that determines the velocity of supernovae compact remnants, we suggest it is a significant contributor and that the ejecta based `kick' should replace the Maxwell-Boltzmann velocity distribution currently used in many population synthesis codes.