New constraints on the Bray conservation-of-momentum natal kick model from multiple distinct observations

Abstract

Natal supernova kicks, the linear momentum compact remnants receive during their formation, are an essential part of binary population synthesis (BPS) models. Although these kicks are well-supported by evidence, their underlying distributions and incorporation into BPS models is uncertain. In this work, we investigate the nature of natal kicks using a previously proposed analytical prescription where the strength of the kick is given by $v_\text{k}=\alpha\frac{m_\text{ejecta}}{m_\text{remnant}}+\beta~\text{km s}^{-1}$, for free parameters $\alpha$ and $\beta$. We vary the free parameters over large ranges of possible values, comparing these synthetic populations simultaneously against four constraints: the merger rate of compact binary neutron star (BNS) systems, the period-eccentricity distribution of galactic BNSs, the velocity distribution of single-star pulsars, and the likelihood for low-ejecta mass supernovae to produce low-velocity kicks. We find that different samples of the parameter space satisfy each tests, and only 1 per cent of the models satisfy all four constraints simultaneously. Although we cannot identify a single best kick model, we report $\alpha=115^{+40}_{-55}~\text{km s}^{-1}, \beta=15^{+10}_{-15}~\text{km s}^{-1}$ as the center of the region of the parameter space that fulfils all of our constraints, and expect $\beta\geq0~\text{km s}^{-1}$ as a further constraint. We also suggest further observations that will enable future refinement of the kick model. A sensitive test for the kick model will be the redshift evolution of the BNS merger rate since this is effectively a direct measure of the delay-time distribution for mergers. For our best fitting values, we find that the peak of the BNS merger rate is the present-day.