Neutron star kicks – II. Revision and further testing of the conservation of momentum ‘kick’ model

Abstract

In Bray & Eldridge (2016) we proposed a simple neutron star "kick" formula, v kick = α (M ejecta / M remnant )+β to explain the observed 2D velocities of young single neutron stars. Using this kick we found that there is no statistically significant preference for a kick orientation nor for any of the three initial mass function (IMF) slopes tested, and that populations including binary stars reproduced the kick distribution better than single star only populations. However, recent analysis by Janka (2017), prompted us to revisit our basic assumptions and our new analysis has led to revised "best-fit" kick values of α = 100 km s −1 and β = −170 km s −1 . The reduction of β to a negative value is due to using the 2D observed kick velocity distribution rather than the modelled 3D velocity distribution for neutron stars (NS). To further test the validity of the new kick, we have created synthetic populations of runaway star and double neutron star (DNS) binaries at solar metallicity (Z = 0.020) using our best-fit kick. We find our new kick values create runaway star velocities and DNS period distributions in agreement with the comparable observational distributions with only the DNS eccentricities in tension with the observations. From our DNS and BH-BH datasets we estimate a predicted DNS merger rate at solar metallicity of 3,864 +1,570 −2,371 Gpc −3 yr −1 and a BH-BH merger rate of 5 +40 −1 Gpc −3 yr −1 .

Dependence of gravitational wave transient rates on cosmic star formation and metallicity evolution history

“…The current observationally inferred NS-NS merger rate is significantly higher than most extant population synthesis codes predict Mapelli & Giacobbo 2018;Chruslinska et al 2019). However, novel kick schemes or a different metallicity evolution history of the Universe have been shown to push predictions close to the observed constraint, with the highest being that from using the kick of Bray & Eldridge (2018) as we see here. Both models (i) and (ii) are at the lower bound of observed rate while model (iii) is at the upper bound.…”

Double neutron star formation: merger times, systemic velocities, and travel distances

“…Note that we refer to the NS progenitor as a helium star, even though multiple phases of mass transfer may have removed most of the helium envelope prior to core collapse (Tauris et al 2013). The absolute value in Equation 1 is necessary since without it, the kick magnitude would be negative for sufficiently low helium star masses (see discussion in Bray & Eldridge 2018). For reference in Table 1 we provide the kick velocities from this prescription for the He-star masses used throughout this work.…”

“…The top three rows show the effects of Maxwellian kicks with σ k of 15, 50, and 265 km s −1 , while the columns show the models in which M He = 1.5, 2, 3, and 5 M . The bottom row shows the analogous distributions using the single-magnitude SN kick prescription from Bray & Eldridge (2018) with velocities provided in Table 1. Blue, orange, and green colored contours (which have logarithmically separated contour lines) in each panel show the distributions for initial orbital separations of 1, 10, and 100 R .…”

“…The kick velocity applied to newly born 1.4 M NSs with different He-star progenitor masses according to the prescription fromBray & Eldridge (2018) provided in Equation 1.…”

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.…”