Electromagnetic counterparts of binary neutron star mergers leading to a strongly magnetized long-lived remnant neutron star

TL;DR

This paper investigates electromagnetic signals from binary neutron star mergers with long-lived, highly magnetized neutron star remnants, predicting bright afterglows and kilonovae features that depend on magnetic field amplification.

Contribution

It introduces detailed simulations of electromagnetic counterparts considering magnetic field amplification, providing new insights into observable signatures of long-lived neutron star remnants.

Findings

High luminosity synchrotron afterglows linked to magnetic field amplification.

Bright early kilonovae with rapid optical decline and bright near-infrared emission.

GW170817's features suggest a short-lived remnant or minimal magnetic amplification.

Abstract

We explore the electromagnetic counterparts that will associate with binary neutron star mergers for the case that remnant massive neutron stars survive for $\gtrsim 0.5\,$s after the merger. For this study, we employ the outflow profiles obtained by long-term general-relativistic neutrino-radiation magneto-hydrodynamics simulations with a mean field dynamo effect. We show that a synchrotron afterglow with high luminosity can be associated with the merger event if the magnetic fields of the remnant neutron stars are significantly amplified by the dynamo effect. We also perform a radiative transfer calculation for kilonovae and find that for the highly amplified magnetic field cases, the kilonovae can be bright in the early epoch, while it shows the optical emission rapid declining in a few days and the long-lasting ($\sim 10\,{\rm d}$) emission very bright in the near-infrared wavelength. All these features have not been found in GW170817, indicating that the merger remnant neutron star formed in GW170817 might have collapsed to a black hole within several hundreds ms or magnetic-field amplification might be a minor effect.