Natal kicks of compact objects

TL;DR

This paper surveys the origin and consequences of natal kicks imparted to NSs and BHs at birth, connecting kick mechanisms from hydrodynamic asymmetries, neutrino emission, and electromagnetic rockets to observational constraints. It highlights how modern 3D SN simulations reproduce substantial NS kicks (often $\gtrsim$ a few $\times 10^2$ km s$^{-1}$, with possible $>10^3$ km s$^{-1}$ in some cases) while ECSNe and AICs yield small hydrodynamic kicks, and how BH kicks arise primarily through fallback and explosion geometry. Direct, indirect, and population-synthesis observations collectively imply a complex, possibly multimodal kick distribution for NSs, and a more constrained, often smaller kick regime for BHs, with significant implications for binary evolution, GW event rates, and microlensing observations. The work underscores open questions about the detailed shape of the kick distributions, the role of progenitor channels, and the interplay between spin, orbital dynamics, and natal kicks, pointing to substantial gains from upcoming surveys and facilities.

Abstract

When compact objects - neutron stars and black holes - are formed in a supernova explosion, they may receive a high velocity at formation, which may reach or even exceed 1000 km s-1 for neutron stars and hundreds of km s-1 for black holes. The origin of the velocity kick is intimately related to supernova physics. A better understanding of kick properties from astronomical observations will shed light on the unsolved problems of these explosions, such as the exact conditions leading to exotic electron capture and ultra-stripped supernovae. Kick velocities are profoundly important in several areas of astrophysics. Being a result of supernova explosions, the kick velocity distribution must be explained in the framework of the supernova mechanism. The kick magnitudes and directions influence many topics related to binary systems, including the rate of compact object coalescences observable through gravitational waves. Moreover, knowledge of the kick velocity distribution is significant in predicting future observational results and their interpretation. For example, it is expected that the Roman space telescope will discover many microlensing events related to neutron stars and black holes; accurate estimates of the number of observable microlensing events require precise kinematic properties of these compact objects.