Flares, jets and quasi-periodic outbursts from neutron star merger remnants

TL;DR

This study uses advanced simulations to show that magnetized hypermassive neutron stars can produce periodic electromagnetic flares and outflows before collapsing into black holes, potentially explaining some short gamma-ray burst precursors.

Contribution

It demonstrates that buoyant instabilities in magnetized neutron star remnants can generate quasi-periodic electromagnetic emissions and outflows prior to black hole formation, a novel insight into early post-merger phenomena.

Findings

Periodic electromagnetic flares occur shortly after merger.

Outflows feature quasi-periodic kilohertz substructure.

Electromagnetic substructure is dominated by MHD shearing processes.

Abstract

Using numerical relativity simulations with a subgrid dynamo prescription to generate strong initial magnetic fields, we investigate the possibility of launching a jet-like outflow from the hypermassive neutron star (HMNS) during the early stages of the merger, prior to the remnants collapse to a black hole. We demonstrate that buoyant instabilities in the strongly magnetized HMNS can lead to a periodic emission of powerful electromagnetic flares shortly after the merger. These are followed by a collimated mildly relativistic outflow. Both types of outflows feature quasi-periodic kilohertz substructure. These early-time outflows may power precursors to short-duration gamma-ray bursts (SGRB) or in some cases the entire SGRB. While the overall temporal power spectrum we find broadly agrees with the one recently reported for quasi-periodic oscillations in the SGRB GRB910711, our simulations suggest that the periodic electromagnetic substructure is dominated by magnetohydrodynamic shearing processes rather than correlating with the corresponding post-merger gravitational wave signal.