Testing Approximations of Thermal Effects in Neutron Star Merger Simulations

TL;DR

This study evaluates the accuracy of approximate thermal modeling in neutron star merger simulations by comparing simplified methods to detailed equations of state, focusing on gravitational-wave signals and remnant evolution.

Contribution

It provides a systematic comparison between approximate and consistent thermal treatments in neutron star merger simulations, highlighting their impact on observable signals.

Findings

Gravitational-wave frequencies differ by 50-250 Hz depending on thermal treatment.

Remnant collapse delay times are sensitive to thermal modeling.

Matter ejection and black hole formation are affected by thermal approximation.

Abstract

We perform three-dimensional relativistic hydrodynamical calculations of neutron star mergers to assess the reliability of an approximate treatment of thermal effects in such simulations by combining an ideal-gas component with zero-temperature, micro-physical equations of state. To this end we compare the results of simulations that make this approximation to the outcome of models with a consistent treatment of thermal effects in the equation of state. In particular we focus on the implications for observable consequences of merger events like the gravitational-wave signal. It is found that the characteristic gravitational-wave oscillation frequencies of the post-merger remnant differ by about 50 to 250 Hz (corresponding to frequency shifts of 2 to 8 per cent) depending on the equation of state and the choice of the characteristic index of the ideal-gas component. In addition, the delay time to black hole collapse of the merger remnant as well as the amount of matter remaining outside the black hole after its formation are sensitive to the description of thermal effects.