Prompt Merger Collapse and the Maximum Mass of Neutron Stars

We perform hydrodynamical simulations of neutron-star mergers for a large sample of temperature-dependent nuclear equations of state and determine the threshold mass above which the merger remnant promptly collapses to form a black hole. We find that, depending on the equation of state, the threshold mass is larger than the maximum mass of a nonrotating star in isolation by between 30 and 70 percent. Our simulations also show that the ratio between the threshold mass and maximum mass is tightly correlated with the compactness of the nonrotating maximum-mass configuration. We speculate on how this relation can be used to derive constraints on neutron-star properties from future observations.

Figure 1

Coefficient $k$ [Eq. (1)] as a function of $C_{\max }=G{M}_{\max }/(c^2{R}_{\max })$ (crosses) and $C_{1.6}^{*}=G{M}_{\max }/(c^2{R}_{1.6})$ (circles).

Figure 2

Left panel: Coefficient $k$ [Eq. (1)] versus radius $R_{1.6}$ of $1.6M_{\odot }$ NSs. Right panel: Compactness $C_{\max }$ as a function of the EoS’s stiffness expressed by the ratio of the average density $⟨\rho ⟩=3M_{\max }/(4\pi R_{\max }^3)$ and central energy density ${\rho }_c$.

Figure 3

Dominant GW frequency $f_{\mathrm{peak}}^{\mathrm{stab}}$ of the postmerger phase for different NS EoSs as function of radius $R_{\max }$ (left panel) and $M_{\mathrm{stab}}$ (right panel). For each EoS, $f_{\mathrm{peak}}^{\mathrm{stab}}$ is the frequency $f_{\mathrm{peak}}$ for the most massive dynamically stable binary ($M_{\mathrm{tot}}=M_{\mathrm{stab}}$).