Post-merger Gravitational-wave Signal from Neutron-star Binaries: A New Look at an Old Problem

The spectral properties of the post-merger gravitational-wave signal from a binary of neutron stars encodes a variety of information about the features of the system and of the equation of state describing matter around and above nuclear saturation density. Characterizing the properties of such a signal is an "old" problem, which first emerged when a number of frequencies were shown to be related to the properties of the binary through "quasi-universal" relations. Here we take a new look at this old problem by computing the properties of the signal in terms of the Weyl scalar psi 4. In this way, and using a database of more than 100 simulations, we provide the first evidence for a new instantaneous frequency, f0 psi 4 , associated with the instant of quasi-time-symmetry in the dynamics, and which also follows a quasi-universal relation. We also derive a new quasi-universal relation for the merger frequency fmerh , which provides a description of the data that is 4 times more accurate than previous expressions while requiring fewer fitting coefficients. Finally, consistent with the findings of numerous studies before ours, and using an enlarged ensemble of binary systems, we point out that the l = 2, m = 1 gravitational-wave mode could become comparable with the traditional l = 2, m = 2 mode on sufficiently long timescales, with strain amplitudes in a ratio divide h 21 divide / divide h 22 divide similar to 0.1-1 under generic orientations of the binary, which could be measured by present detectors for signals with a large signal-to-noise ratio or by third-generation detectors for generic signals should no collapse occur.