Thermal aspects of neutron star mergers

TL;DR

This paper investigates the thermal physics and equilibrium conditions in neutron star mergers, highlighting their impact on the merger dynamics, equation of state, and composition, which are crucial for accurate modeling of signals.

Contribution

It analyzes how different equilibrium notions affect merger simulations, emphasizing the importance of temperature effects and neutrino interactions in modeling neutron star matter.

Findings

Finite temperature effects soften the equation of state in some regions.

Composition changes influence processes like bulk viscosity.

Determining relevant equilibrium conditions is complex due to neutrino absorption.

Abstract

In order to extract maximal information from neutron-star merger signals, both gravitational and electromagnetic, we need to ensure that our theoretical models/numerical simulations faithfully represent the extreme physics involved. This involves a range of issues, with the finite temperature effects regulating many of the relevant phenomena. As a step towards understanding these issues, we explore the conditions for $\beta$-equilibrium in neutron star matter for the densities and temperatures reached in a binary neutron star merger. Using the results from our out-of-equilibrium merger simulation, we consider how different notions of equilibrium may affect the merger dynamics, raising issues that arise when attempting to account for these conditions in future simulations. These issues are both computational and conceptual. We show that the effects lead to, in our case, a softening of the equation of state in some density regions, and to composition changes that affect processes that rely on deviation from equilibrium, such as bulk viscosity, both in terms of the magnitude and the equilibration timescales inherent to the relevant set of reactions. We also demonstrate that it is difficult to determine exactly which equilibrium conditions are relevant in which regions of the matter due to the dependence on neutrino absorption, further complicating the calculation of the reactions that work to restore the matter to equilibrium.