Kicks and Induced Spins of Neutron Stars at Birth

Abstract

Using simulations of non-rotating supernova progenitors, we explore the kicks imparted to and the spins induced in the compact objects birthed in core collapse. We find that the recoil due to neutrino emissions can be a factor affecting core recoil, comparable to and at times larger than the corresponding kick due to matter recoil. This result would necessitate a revision of the general model of the origin of pulsar proper motions. In addition, we find that the sign of the net neutrino momentum can be opposite to the sign of the corresponding matter recoil. As a result, at times the pulsar recoil and ejecta can be in the same direction. Moreover, our results suggest that the duration of the dipole in the neutrino emissions can be shorter than the duration of the radiation of the neutron-star binding energy. This allows a larger dipole asymmetry to arise, but for a shorter time, resulting in kicks in the observed pulsar range. Furthermore, we find that the spin induced by the aspherical accretion of matter can leave the residues of collapse with spin periods comparable to those inferred for radio pulsars and that there seems to be a slight anti-correlation between the direction of the induced spin and the net kick direction. This could explain such a correlation among observed radio pulsars. Finally, we find that the kicks imparted to black holes are due to the neutrino recoil alone, resulting in birth kicks ≤100 km s −1 most of the time.

The Impact of Spin-kick Alignment on the Inferred Velocity Distribution of Isolated Pulsars

“…There is a growing body of observational evidence that spins and velocities of pulsars are preferentially aligned (Johnston et al 2005;Wang et al 2006;Postnov & Kuranov 2008;Noutsos et al 2013;Yao et al 2021). Meanwhile, Janka et al (2022) proposed that supernova fallback would naturally explain the alignment of the spin axis with the kick direction though the neutrino-induced kick models of Coleman & Burrows (2022) show a weaker antialignment. If pulsar spins are indeed aligned with their kicks, then the detectability of the pulsar, which depends on the location of the observer relative to the spin axis, implies that the radial component of the velocity distribution of detectable pulsars follows a different distribution than the components perpendicular to the line of sight.…”