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

TL;DR

This paper investigates how fallback accretion onto neutron stars during supernovae influences their spin-kick alignment, revealing that fallback tends to misalign the spin and velocity due to angular momentum carried perpendicular to the kick direction.

Contribution

It demonstrates that fallback accretion generally causes spin-kick misalignment in neutron stars, challenging previous assumptions of alignment and providing estimates of fallback mass limits.

Findings

Fallback accretion tends to produce spin-kick misalignment.

Angular momentum from fallback is primarily perpendicular to the kick velocity.

Maximum fallback mass for fast-spinning neutron stars is about 0.01 solar masses.

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 Alfv\'en radius. This imposes a limit of $\mathord{\sim}10^{-2}M_\odot$ of fallback accretion for fast-spinning young neutron stars with periods of $\mathord{\sim}20\,\mathrm{ms}$ and less for longer birth spin periods.