Evidence of polar and ultralow supernova kicks from the orbits of Be X-ray binaries

TL;DR

This study reveals three distinct supernova kick modes in neutron star formation, including ultra-low and aligned kicks in Be X-ray binaries, challenging existing theories and expanding understanding of supernova dynamics.

Contribution

The paper provides evidence for multiple supernova kick modes, including ultralow and aligned kicks, based on orbital analysis of Be X-ray binaries, highlighting previously unrecognized diversity in neutron star formation.

Findings

Identification of two kick populations with different velocities

Evidence for kicks aligned within 5 degrees of progenitor rotation

Discovery of a third, ultralow velocity kick mode

Abstract

Supernovae, the explosive deaths of massive stars, create heavy elements and form black holes and neutron stars. These compact objects often receive a velocity at formation, a "kick" whose physical origin remains debated. We investigate kicks in Be X-ray binaries, containing a neutron star and a rapidly spinning companion. We identify two distinct populations: one with kicks below $10\,\rm{km}\,\rm{s}^{-1}$, much lower than theoretical predictions, and another with kicks around $100\,\rm{km}\,\rm{s}^{-1}$, that shows evidence for being aligned within 5 degrees of the progenitor's rotation axis. The distribution of progenitor masses for the two populations have medians around $2.3\,\rm{M}_\odot$ and $4.9\,\rm{M}_\odot$, corresponding to stars with birth masses of about $10\,\rm{M}_\odot$ and $15\,\rm{M}_\odot$. The second component matches the low-velocity mode observed in isolated pulsars. Combined with the known high-velocity component, which dominates isolated pulsars, this suggests three distinct kick modes. These results reveal previously unrecognized diversity in neutron-star formation.