Gravitational-wave Asteroseismology with f-modes from Neutron Star Binaries at the Merger Phase

Abstract

Gravitational-wave signals from coalescing binary neutron stars (BNSs) can yield important information about the properties of nuclear-matter equation of state. We investigate a direct link between the frequency of the quadrupolar 2 f-mode oscillation (f 2f ) of nonrotating and rotating neutron stars calculated by a nonlinear hydrodynamics code in the conformally flat approximation and the gravitational-wave frequency associated with the peak amplitude ( f max ) of binary neutron stars from a set of publicly available simulations. We find that f max and f 2f differ by about 1%, on average, across 45 equal-mass systems with different total mass and equations of state. Interestingly, assuming that the gravitational-wave frequency is still approximately equal to twice the orbital frequency f orb near the merger, the result indicates that the condition for tidal resonance ( ∣ m ∣ f orb ) m = 2 = f 2 f is satisfied to high accuracy near the merger. Moreover, the well-established universal relation between f max and the tidal deformability of equal-mass binary systems can now be explained by a similar relation between f 2f and the tidal deformability of isolated neutron stars. For unequal-mass binaries, f max increasingly deviates from f 2f of the two stars as the mass ratio decreases from unity. Therefore, it is possible to relate the gravitational-wave signal at the merger of a BNS system directly to the fundamental oscillation modes and the mass ratio. This work potentially brings gravitational-wave asteroseismology to the late-inspiral and merger phases of BNSs.

Oscillations of highly magnetized non-rotating neutron stars

“…Simulations. Simulations are performed with our code Gmunu [75][76][77][78] . For each of the 12 equilibrium models, we execute Gmunu three times, once for each initial perturbation function.…”

“…A more rigorous approach to compute strongly magnetized equilibrium models is recently presented and demonstrated by an opensource code XNS 3,[64][65][66][67][68][69][70][71][72][73][74] . Moreover, the GRMHD code Gmunu [75][76][77][78] allows us to robustly evolve NSs in dynamical spacetime even with extreme magnetic fields of 10 15−17 G. With these powerful tools in hand, we are now in a much better position to systematically investigate the oscillation modes of magnetized NSs.…”

General-relativistic Radiation Transport Scheme in Gmunu. II. Implementation of Novel Microphysical Library for Neutrino Radiation—Weakhub

“…In Paper I (Cheong et al 2023), we mentioned the formulations of the general-relativistic multifrequency radiation transport module within the General-relativistic multigrid numerical (Gmunu) code (Cheong et al 2020(Cheong et al , 2021(Cheong et al , 2022, which has been widely used in multiple applications (Ng et al 2021;Leung et al 2022;Yip et al 2023). The module is based on the two-moment general-relativistic multifrequency radiative transfer scheme (Thorne 1981;Shibata et al 2011;Cardall et al 2013a).…”