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), which determines the kicks of NSs born from stars near the low-mass end of SN progenitors. In more massive progenitors aspherical accretion downflows around the PNS can also lead to anisotropic neutrino emission (absorption) with a neutrino-induced NS kick roughly opposite to (aligned with) the kick by asymmetric mass ejection. We estimate upper bounds for the final neutrino kicks of 150–260kms−1, whereas the hydrodynamic kicks can reach up to more than 1300kms−1. Therefore the hydrodynamic kicks dominate for NSs from explosions of higher-mass progenitors, whereas the neutrino kicks dominate in the case of NSs from the lowest-mass progenitors. Our models suggest that the Crab pulsar as a representative of the latter category could have received its velocity of ∼160kms−1 by a neutrino kick due to the LESA asymmetry. Such neutrino kicks of 100–200kms−1 define a nearly ubiquitous floor value, which may shed new light on the origin of pulsars in globular clusters. Black holes, if formed by the collapse of short-lived PNSs and solely kicked by anisotropic neutrino emission, obtain velocities of only some kms−1.