On the treatment of thermal effects in the equation of state on neutron star merger remnants

TL;DR

This study uses long-term numerical simulations of neutron star mergers to analyze how different thermal treatments in the equation of state influence gravitational wave signals and the stability of the hypermassive neutron star remnant.

Contribution

It compares fully tabulated finite-temperature EOS with hybrid models, revealing significant differences in post-merger dynamics and gravitational wave spectra, especially at late times.

Findings

Thermal modeling affects gravitational wave frequency evolution.

Convective patterns persist beyond 100 ms post-merger.

Inertial modes are excited and detectable by future GW observatories.

Abstract

We present results from long-term, numerical-relativity simulations of binary neutron star mergers modeled using both, fully tabulated, finite-temperature, equations of state and their corresponding hybrid representations. The simulations extend up to 150 ms which allows us to assess the role of the treatment of finite-temperature effects on the dynamics of the hypermassive neutron star remnant. Our study focuses on the analysis of the spectra of the post-merger gravitational-wave signals and on how these are affected by the treatment of thermal effects in the two EOS representations. Our simulations highlight distinct differences in the GW frequency evolution related to the thermal modeling of the EOS, demonstrating that deviations from established quasi-universal relations become significant at late post-merger phases. Furthermore, we investigate the stability of the HMNS against convection. Employing both the Ledoux criterion, necessary condition for the development of convective instabilities, and the Solberg-H{\o}iland criterion, a generalized criterion for axisymmetric perturbations based on a combined analysis of the Brunt-V\"ais\"al\"a frequency and of the epicyclic frequency, we show that differential rotation and thermal stratification in the HMNS give rise to local (yet sustained) convective patterns that persist beyond 100 ms after merger. Those convective patterns, while substantially different between tabulated and hybrid EOS treatments, trigger the the excitation of inertial modes with frequencies smaller than those attained by the fundamental quadrupolar mode, and are potentially within reach of third-generation GW detectors. The late-time excitation of inertial modes, previously reported in studies based on hybrid EOS, is fully supported by the tabulated, finite-temperature EOS simulations presented here, which account for thermal effects in a more consistent way.