Dependence of post-merger properties on the thermal heating efficiency in neutron star mergers

We systematically perform numerical-relativity simulations for equal-mass binary neutron star mergers for the models varying the thermal index $\Gamma_{\rm th}$ with 3 different equations of state (EOSs) of the neutron stars (NSs), which are consistent with current multi-messenger observational data and state-of-the-art theoretical calculations, and 2 different binary total mass ($m_0=2.7\ \text{and}\ 2.9~M_\odot$). By varying the value of $\Gamma_{\rm th}$ within the hybrid EOS framework, we investigate the thermal effects on the merger dynamics, gravitational waves (GWs), and the dynamical mass ejection process. We find that the choice of the constant $\Gamma_{\rm th}$ can change the outcome of the remnant for specific EOSs and $m_0$. We also show that the dynamical ejecta mass is affected by the $\Gamma_{\rm th}$ value in a different way for different EOSs: for a stiff EOS the ejecta mass is high when $\Gamma_{\rm th}$ is small, while for softer EOSs the largest ejecta is achieved when $\Gamma_{\rm th} = 1.3$--$1.4$. While the inspiral motion does not depend on the $\Gamma_{\rm th}$, the post merger phase evolution is highly affected by that. We show that the dominant peak frequency $f_2$ of the post merger GW spectrum monotonically decreases as the $\Gamma_{\rm th}$ increases. We find that the universal relations between NS macroscopic properties and post-merger GW frequencies are subject to non-negligible thermal uncertainties, which can obscure the universal relation between the tidal deformability and $f_2$.