# General relativistic magnetohydrodynamic simulations of binary neutron star mergers forming a long-lived neutron star

## TL;DR

This paper presents the first detailed general relativistic magnetohydrodynamic simulations of binary neutron star mergers resulting in long-lived neutron star remnants, exploring their structure, magnetic field evolution, matter ejection, and gravitational wave signals.

## Contribution

It introduces a novel simulation approach for long-lived neutron star remnants from binary mergers, expanding understanding beyond the typical black hole formation scenario.

## Key findings

- Long-lived neutron star remnants can form and remain stable for extended periods.
- Magnetic fields are significantly amplified during the merger process.
- Distinct gravitational wave signatures are associated with long-lived neutron star remnants.

## Abstract

Merging binary neutron stars (BNSs) represent the ultimate targets for multimessenger astronomy, being among the most promising sources of gravitational waves (GWs), and, at the same time, likely accompanied by a variety of electromagnetic counterparts across the entire spectrum, possibly including short gamma-ray bursts (SGRBs) and kilonova/macronova transients. Numerical relativity simulations play a central role in the study of these events. In particular, given the importance of magnetic fields, various aspects of this investigation require general relativistic magnetohydrodynamics (GRMHD). So far, most GRMHD simulations focused the attention on BNS mergers leading to the formation of a hypermassive NS, which, in turn, collapses within few tens of ms into a black hole surrounded by an accretion disk. However, recent observations suggest that a significant fraction of these systems could form a long-lived NS remnant, which will either collapse on much longer timescales or remain indefinitely stable. Despite the profound implications for the evolution and the emission properties of the system, a detailed investigation of this alternative evolution channel is still missing. Here, we follow this direction and present a first detailed GRMHD study of BNS mergers forming a long-lived NS. We consider magnetized binaries with different mass ratios and equations of state and analyze the structure of the NS remnants, the rotation profiles, the accretion disks, the evolution and amplification of magnetic fields, and the ejection of matter. Moreover, we discuss the connection with the central engine of SGRBs and provide order-of-magnitude estimates for the kilonova/macronova signal. Finally, we study the GW emission, with particular attention to the post-merger phase.