New insights on the spin-up of a neutron star during core-collapse

TL;DR

This study investigates how the spiral mode of SASI can influence neutron star spin-up during core-collapse, showing it is a viable but non-systematic process with stochastic variations.

Contribution

The paper demonstrates through 2D simulations that SASI spiral modes can spin up neutron stars, highlighting the importance of initial shock radius and revealing stochastic effects.

Findings

Spiral SASI mode dominates when shock-to-neutron star radius ratio exceeds a critical value.

The spiral mode increases asymmetry and shock expansion, affecting neutron star spin.

SASI timescale is short enough to influence neutron star spin before explosion.

Abstract

The spin of a neutron star at birth may be impacted by the asymmetric character of the explosion of its massive progenitor. During the first second after bounce, the spiral mode of the Standing Accretion Shock Instability (SASI) is able to redistribute angular momentum and spin-up a neutron star born from a non-rotating progenitor. Our aim is to assess the robustness of this process. We perform 2D numerical simulations of a simplified setup in cylindrical geometry to investigate the timescale over which the dynamics is dominated by a spiral or a sloshing mode. We observe that the spiral mode prevails only if the ratio of the initial shock radius to the neutron star radius exceeds a critical value. In that regime, both the degree of asymmetry and the average expansion of the shock induced by the spiral mode increase monotonously with this ratio, exceeding the values obtained when a sloshing mode is artificially imposed. With a timescale of 2-3 SASI oscillations, the dynamics of SASI takes place fast enough to affect the spin of the neutron star before the explosion. The spin periods deduced from the simulations are compared favorably to analytical estimates evaluated from the measured saturation amplitude of the SASI wave. Despite the simplicity of our setup, numerical simulations revealed unexpected stochastic variations, including a reversal of the direction of rotation of the shock. Our results show that the spin up of neutron stars by SASI spiral modes is a viable mechanism even though it is not systematic.