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 byTo 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 α and β 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 α = 70 km s −1 and β = 110 km s −1 reproduce the Hobbs et al. (2005) observed 2-dimensional velocities and α = 70 km s −1 and β = 120 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.

Neutron Star Kicks by the Gravitational Tug-boat Mechanism in Asymmetric Supernova Explosions: Progenitor and Explosion Dependence

“…Though this suggestion seems to work well, no detailed physical explanation was provided in Bray & Eldridge (2016). In the following we will provide corresponding arguments and will, based on our current understanding of neutrino-driven SN explosions and the gravitational tug-boat mechanism for NS acceleration, derive simple scaling relations for the NS kick velocity, v NS , as function of basic parameters that are linked to the ejecta mass of the SN, the mass-ejection asymmetry, and the energy of the explosion.…”

“…Rankin 2015;Noutsos et al 2013;Kaplan et al 2008;Ng & Romani 2007;Lai et al 2001), but see Bray & Eldridge (2016), who find no statistical evidence of a preferred kick orientation comparing NS velocity distributions from observations and from their stellar population models for both single and binary star evolutionary pathways.…”

“…Voss & Tauris 2003;Tauris et al 2016). In theoretical models for population synthesis a widely used approach to implement the effects of NS natal kicks is based on single-component or multi-component Maxwell-Boltzmann velocity distributions (see, e.g., Voss & Tauris 2003;Dominik et al 2015), which was questioned recently by Bray & Eldridge (2016). Instead, the latter authors proposed a simple relationship for the NS kick velocity defined as a linear function of the ratio of the SN ejecta mass to the remnant mass.…”

“…Recently Bray & Eldridge (2016) applied such a connection of NS kicks and SN explosion asymmetries, following their inspiration and motivated by the interpretation of observational and modeling results of Cassiopeia A. Correspondingly, they suggested a direct relationship between the velocity of the compact remnant and the ratio of SN ejecta mass to NS mass (this assumption was also made by Beniamini et al 2016).…”

Gravitational waves from supernova mass loss and natal kicks in close binaries

“…The evolution of binary massive stars and the formation of compact binaries are still highly uncertain, though observations of populations such as X-ray binaries, Galactic binary pulsars, and compact binary mergers are continually providing constraints on binary-evolution models. The BPS method provides a framework for interpreting the population properties of compact binaries and has been used for these binary populations (e.g., Belczyński & Bulik 1999;Hurley et al 2002;O'Shaughnessy et al 2005;Osłowski et al 2011;Dominik et al 2012;Fragos et al 2013;Tzanavaris et al 2013;Beniamini et al 2016;Bray & Eldridge 2016;Barrett et al 2018;Bray & Eldridge 2018;Kruckow et al 2018;Taylor & Gerosa 2018;Vigna-Gómez et al 2018). As our understanding of uncertain binary-evolution processes advances, updated models can be implemented into BPS codes.…”