Fallback onto Kicked Neutron Stars and its Effect on Spin-Kick Alignment

Abstract

Fallback in core-collapse supernova explosions is potentially of significant importance for the birth spins of neutron stars and black holes. It has recently been pointed out that the angular momentum imparted onto a compact remnant by fallback material is subtly intertwined with its kick because fallback onto a moving neutron star or black hole will preferentially come for a conical region around its direction of travel. We show that contrary to earlier expectations such one-sided fallback accretion onto a neutron star will tend to produce spin-kick misalignment. Since the baroclinic driving term in the vorticity equation is perpendicular to the nearly radial pressure gradient, convective eddies in the progenitor as well as Rayleigh-Taylor plumes growing during the explosion primarily carry angular momentum perpendicular to the radial direction. Fallback material from the accretion volume of a moving neutron star therefore carries substantial angular momentum perpendicular to the kick velocity. We estimate the seed angular momentum fluctuations from convective motions in core-collapse supernova progenitors and argue that accreted fallback material will almost invariably be accreted with the maximum permissible specific angular momentum for reaching the Alfvén radius. This imposes a limit of $\sim 10^{-2}{M}_{\odot }$ of fallback accretion for fast-spinning young neutron stars with periods of $\sim 20\mathrm{ms}$ and less for longer birth spin periods.

9 pages, 4 figures, accepted for publication in MNRAS. Fixed typo and added two references

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