Universality of the turbulent magnetic field in hypermassive neutron stars produced by binary mergers

TL;DR

This study demonstrates that the magnetic field configuration in hypermassive neutron stars formed by binary mergers quickly becomes independent of initial conditions, due to small-scale dynamo processes, with implications for understanding magnetic amplification.

Contribution

The paper provides high-resolution simulations showing the rapid loss of initial magnetic topology and the universality of magnetic field strength in merger remnants.

Findings

Initial magnetic topology is quickly forgotten within milliseconds.

Final magnetic energy distribution is largely independent of initial configuration.

Magnetic field reaches around 10^{16} G, driven by small-scale dynamo action.

Abstract

The detection of a binary neutron star merger in 2017 through both gravitational waves and electromagnetic emission opened a new era of multimessenger astronomy. The understanding of the magnetic field amplification triggered by the Kelvin-Helmholtz instability during the merger is still a numerically unresolved problem because of the relevant small scales involved. One of the uncertainties comes from the simplifications usually assumed in the initial magnetic topology of merging neutron stars. We perform high-resolution, convergent large-eddy simulations of binary neutron star mergers, following the newly formed remnant for up to $30$ milliseconds. Here we specifically focus on the comparison between simulations with different initial magnetic configurations, going beyond the widespread-used aligned dipole confined within each star. The results obtained show that the initial topology is quickly forgotten, in a timescale of few miliseconds after the merger. Moreover, at the end of the simulations, the average intensity ($B\sim 10^{16}$ G) and the spectral distribution of magnetic energy over spatial scales barely depend on the initial configuration. This is expected due to the small-scale efficient dynamo involved, and thus it holds as long as: (i) the initial large-scale magnetic field is not unrealistically high (as often imposed in mergers studies); (ii) the turbulent instability is numerically (at least partially) resolved, so that the amplified magnetic energy is distributed across a wide range of scales and becomes orders-of-magnitude larger than the initial one.