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

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 $10\mathrm{km}{\mathrm{s}}^{-1}$, much lower than theoretical predictions, and another with kicks around $100\mathrm{km}{\mathrm{s}}^{-1}$, 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 $2.3{\mathrm{M}}_{\odot }$ and $4.9{\mathrm{M}}_{\odot }$, corresponding to stars with birth masses of about $10{\mathrm{M}}_{\odot }$ and $15{\mathrm{M}}_{\odot }$. 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.

62 pages, 21 figures