Gravitational waves and nonaxisymmetric oscillation modes in mergers of compact object binaries

TL;DR

This study uses general-relativistic simulations to analyze nonaxisymmetric oscillation modes in the post-merger phase of binary compact object mergers, identifying specific gravitational wave frequency triplets linked to fluid oscillations.

Contribution

It provides the first detailed analysis connecting nonlinear fluid oscillation modes with gravitational wave spectral features in merger remnants.

Findings

Identified a triplet of frequencies in gravitational wave spectrum associated with m=2 and quasiradial modes.

Linked specific gravitational wave peaks to nonlinear mode coupling in merger remnants.

Demonstrated potential for gravitational-wave asteroseismology to constrain high-density matter equations of state.

Abstract

We study the excitation of nonaxisymmetric modes in the post-merger phase of binary compact object mergers and the associated gravitational wave emission. Our analysis is based on general-relativistic simulations, in the spatial conformal flatness approximation, using smoothed-particle-hydrodynamics for the evolution of matter, and we use a set of equal and unequal mass models, described by two nonzero-temperature hadronic equations of state and by one strange star equation of state. Through Fourier transforms of the evolution of matter variables, we can identify a number of oscillation modes, as well as several nonlinear components (combination frequencies). We focus on the dominant m=2 mode, which forms a triplet with two nonlinear components that are the result of coupling to the quasiradial mode. A corresponding triplet of frequencies is identified in the gravitational wave spectrum, when the individual masses of the compact objects are in the most likely range of 1.2 to 1.35 $M_\odot$. We can thus associate, through direct analysis of the dynamics of the fluid, a specific frequency peak in the gravitational wave spectrum with the nonlinear component resulting from the difference between the m=2 mode and the quasiradial mode. Once such observations become available, both the m=2 and quasiradial mode frequencies could be extracted, allowing for the application of gravitational-wave asteroseismology to the post-merger remnant and leading to tight constraints on the equation of state of high-density matter.