A study of the agreement between binary neutron star ejecta models derived from numerical relativity simulations

TL;DR

This study compares various neutron star merger ejecta models derived from numerical relativity to assess their agreement and reliability, highlighting significant uncertainties and the unreliability of extrapolations outside calibration ranges.

Contribution

It provides a systematic comparison of ejecta models, evaluates their consistency with neutron star equations of state, and highlights the limitations of current modeling approaches.

Findings

Model differences often exceed assumed errors.

Extrapolation outside calibration ranges is unreliable.

Systematic uncertainties hinder precise interpretation of observations.

Abstract

Neutron star mergers have recently become a tool to study extreme gravity, nucleosynthesis, and the chemical composition of the Universe. To date, there has been one joint gravitational and electromagnetic observation of a binary neutron star merger, GW170817, as well as a solely gravitational observation, GW190425. In order to accurately identify and interpret electromagnetic signals of neutron star mergers, better models of the matter outflows generated by these mergers are required. We compare a series of ejecta models to see where they provide strong constraints on the amount of ejected mass expected from a system, and where systematic uncertainties in current models prevent us from reliably extracting information from observed events. We also examine 2396 neutron star equations of state compatible with GW170817 to see whether a given ejecta mass could be reasonably produced with a neutron star of said equation of state, and whether different ejecta models provide consistent predictions. We find that the difference between models is often comparable to or larger than the error generally assumed for these models, implying better constraints on the models are needed. We also note that the extrapolation of outflow models outside of their calibration window, while commonly needed to analyze gravitational wave events, is extremely unreliable and occasionally leads to completely unphysical results.