Outflows from accretion disks formed in neutron star mergers: effect of black hole spin

TL;DR

This study examines how black hole spin affects mass ejection from accretion disks post-neutron star merger, revealing increased ejecta and potential observational signatures like blue kilonova components.

Contribution

It introduces two-dimensional hydrodynamic simulations incorporating black hole spin effects on disk outflows, highlighting the impact on ejecta mass and composition.

Findings

Spinning black holes lead to more mass ejection from accretion disks.

Higher black hole spin increases the electron fraction of outflows.

A small amount of Lanthanide-free material can produce observable blue kilonova features.

Abstract

The accretion disk that forms after a neutron star merger is a source of neutron-rich ejecta. The ejected material contributes to a radioactively-powered electromagnetic transient, with properties that depend sensitively on the composition of the outflow. Here we investigate how the spin of the black hole remnant influences mass ejection on the thermal and viscous timescales. We carry out two-dimensional, time-dependent hydrodynamic simulations of merger remnant accretion disks including viscous angular momentum transport and approximate neutrino self-irradiation. The gravity of the spinning black hole is included via a pseudo-Newtonian potential. We find that a disk around a spinning black hole ejects more mass, up to a factor of several, relative to the non-spinning case. The enhanced mass loss is due to energy release by accretion occurring deeper in the gravitational potential, raising the disk temperature and hence the rate of viscous heating in regions where neutrino cooling is ineffective. The mean electron fraction of the outflow increases moderately with BH spin due to a highly-irradiated (though not neutrino-driven) wind component. While the bulk of the ejecta is still very neutron-rich, thus generating heavy r-process elements, the leading edge of the wind contains a small amount of Lanthanide-free material. This component can give rise to a ~1 day blue optical `bump' in a kilonova light curve, even in the case of prompt BH formation, which may facilitate its detection.