Interplay between neutrino kicks and hydrodynamic kicks of neutron stars and black holes

Abstract

Neutron stars (NSs) are observed with high space velocities and elliptical orbits in binaries. The magnitude of these effects points to natal kicks that originate from asymmetries during the supernova (SN) explosions. Using a growing set of long-time 3D SN simulations with the Prometheus-Vertex code, we explore the interplay of NS kicks that are induced by asymmetric neutrino emission and by asymmetric mass ejection. Anisotropic neutrino emission can arise from a large-amplitude dipolar convection asymmetry inside the proto-NS (PNS) termed LESA (Lepton-number Emission Self-sustained Asymmetry) and from aspherical accretion downflows around the PNS, which can lead to anisotropic neutrino emission (absorption/scattering) with a neutrino-induced NS kick roughly opposite to (aligned with) the kick by asymmetric mass ejection. In massive progenitors, hydrodynamic kicks can reach up to more than 1300 km s⁻¹, whereas our calculated neutrino kicks reach (55–140) km s⁻¹ (estimated upper bounds of (170–265) km s⁻¹) and only ∼(10–50) km s⁻¹, if LESA is the main cause of asymmetric neutrino emission. Therefore, hydrodynamic NS kicks dominate in explosions of high-mass progenitors, whereas LESA-induced neutrino kicks dominate for NSs born in low-energy SNe of the lowest-mass progenitors, when these explode nearly spherically. Our models suggest that the Crab pulsar with its velocity of ∼160 km s⁻¹, if born in the low-energy explosion of a low-mass, single-star progenitor, should have received a hydrodynamic kick in a considerably asymmetric explosion. Black holes, if formed by the collapse of short-lived PNSs and solely kicked by anisotropic neutrino emission, obtain velocities of only some km s⁻¹.