title: Exploring thermal effects in neutron star mergers
Creators: name: Blacker, Sebastian
Publisher: name: UNSPECIFIED
Year: 2024

description: In this work, we perform neutron star merger simulations with equation of state (EoS) models containing nucleonic, hyperonic or deconfined quark matter. In particular, we focus on the different behavior of hyperonic and deconfined quark matter phases at finite temperatures compared to purely nucleonic neutron star material and explore the impact on the resulting gravitational-wave (GW) signal. Both hyperons and deconfined quarks are hypothesized to be present at high densities in neutron stars. However, current observations and theoretical calculations are inconclusive. Therefore, the true composition of neutron star cores remains unknown. To investigate the impact of hyperons on neutron star mergers, we perform the first comprehensive study with several different available hyperonic EoSs. At finite temperatures, more hyperons are produced, therefore lowering the degeneracy pressure of nucleons. We find that this reduced pressure in hot hyperonic material results in a characteristic increase of the dominant postmerger GW frequency up to about 150 Hz compared to purely nucleonic matter. This frequency shift can be directly linked to the presence of hyperons. Although the effect is weak, it could serve as an observational indication of hyperons in the future. Our finding is particularly relevant as mass-radius relations of cold hyperonic stars may be indistinguishable from nucleonic stars. When employing models with a transition to deconfined quark matter, so-called hybrid models, we find that thermal effects can significantly influence the structure of the neutron star merger remnant. For the hybrid EoS sample we consider, this can be attributed to the shift of the quark-hadron phase boundaries at finite temperatures, which strongly reduces the pressure compared to cold matter. We explicitly show that if this change of phase boundaries is not taken into account, the dominant gravitational-wave frequencies can be underestimated by several hundred Hz. Additionally, we devise an effective description to incorporate the effects of shifting phase boundaries to supplement cold EoSs with a Maxwell-type phase transition, i.e. two distinct phases connected by a flat coexistence region. We validate our model by comparing results to our fully temperature-dependent hybrid EoS sample and find very good agreement in the postmerger GW frequencies. We then explore the impact varying phase boundaries have for a fixed, cold EoS. Our findings suggest that for hybrid models, the shape of the phase boundaries can have a strong impact on merger observables such as GW frequencies and the prompt collapse of the remnant to a black hole. We also show explicitly that it is even possible for deconfined quark matter to only occur in finite temperature systems like merger remnants but not in cold, isolated stars. This demonstrates that postmerger GWs contain important information on the underlying EoS complementary to observations from individual NSs. Future detections of GWs from binary NS mergers hence have the potential to shed light on the presence of hyperons and deconfined quark matter in neutron star material and provide information on the phase diagram of quantum chromodynamics in addition to constraints from terrestrial experiments.