Long-term impact of the magnetic-field strength on the evolution and electromagnetic emission by neutron-star merger remnants

TL;DR

This study uses advanced numerical simulations to explore how magnetic-field strength influences the long-term evolution and electromagnetic emissions of neutron-star merger remnants, revealing phenomena like magnetic flares and potential gamma-ray burst signatures.

Contribution

It introduces improved simulation methods incorporating gravitational-wave reaction and higher-order MHD, enabling detailed long-term modeling of neutron-star merger remnants with high accuracy.

Findings

Magnetic fields can weaken differential rotation in remnants.

Magnetic flares driven by Parker instability occur in outer layers.

Remnants may produce observable signatures in short gamma-ray bursts.

Abstract

Numerical simulations are essential to understand the complex physics accompanying the merger of binary systems of neutron stars. However, these simulations become computationally challenging when they have to model the merger remnants on timescales over which secular phenomena, such as the launching of magnetically driven outflows, develop. To tackle these challenges, we have recently developed a hybrid approach that combines, via a hand-off transition, a fully general-relativistic code (FIL) with a more efficient code making use of the conformally flat approximation (BHAC+). We here report important additional developments of BHAC+ consisting of the inclusion of gravitational-wave radiation-reaction contributions and of higher-order formulations of the equations of general-relativistic magnetohydrodynamics. Both improvements have allowed us to explore BNS merger remnants with high accuracy and over timescales that would have been computationally prohibitive otherwise. More specifically, we have investigated the impact of the magnetic-field strength on the long-term (i.e., $\sim 200\,{\rm ms}$) and high-resolution (i.e., $150\,{\rm m}$) evolutions of the "magnetar" resulting from the merger of two neutron stars with a realistic equation of state. In this way, and for sufficiently large magnetic fields, we observe the weakening or suppression of differential rotation and the generation of magnetic flares in the outer layers of the remnant. These flares, driven mostly by the Parker instability, are responsible for intense and collimated Poynting flux outbursts and mass ejections. This novel phenomenology offers the possibility of seeking corresponding signatures from the observations of short gamma-ray bursts and hence revealing the existence of a long-lived strongly magnetized remnant.