Natal Kicks of Stellar-Mass Black Holes by Asymmetric Mass Ejection in Fallback Supernovae

TL;DR

This paper proposes that black hole natal kicks result from asymmetric supernova ejecta, similar to neutron stars, and can be amplified by fallback accretion, challenging previous assumptions about their origins.

Contribution

It introduces a model linking black hole kicks to supernova asymmetries and fallback, explaining observed velocities without relying on neutrino emission anisotropies.

Findings

Black hole kicks can be explained by asymmetric supernova ejecta.

Fallback accretion can increase black hole momentum post-formation.

Black hole kick velocities are comparable to neutron star kicks despite mass differences.

Abstract

Integrating trajectories of low-mass X-ray binaries containing black holes within the Galactic potential, Repetto, Davies & Sigurdsson recently showed that the large distances of some systems above the Galactic plane can only be explained if black holes receive appreciable natal kicks. Surprisingly, they found that the distribution of black hole kick velocities (rather than that of the momenta) should be similar to that of neutron stars. Here I argue that this result can be understood if neutron star and black hole kicks are a consequence of large-scale asymmetries created in the supernova ejecta by the explosion mechanism. The corresponding anisotropic gravitational attraction of the asymmetrically expelled matter does not only accelerate new-born neutron stars by the "gravitational tug-boat mechanism". It can also lead to delayed black-hole formation by asymmetric fallback of the slowest parts of the initial ejecta onto the transiently existing neutron star, in course of which the momentum of the black hole can grow with the fallback mass. Black hole kick velocities will therefore not be reduced by the ratio of neutron star to black hole mass as would be expected for kicks caused by anisotropic neutrino emission of the nascent neutron star.