Constraining the p-Mode–g-Mode Tidal Instability with GW170817

Abstract

We analyze the impact of a proposed tidal instability coupling p modes and g modes within neutron stars on GW170817. This nonresonant instability transfers energy from the orbit of the binary to internal modes of the stars, accelerating the gravitational-wave driven inspiral. We model the impact of this instability on the phasing of the gravitational wave signal using three parameters per star: an overall amplitude, a saturation frequency, and a spectral index. Incorporating these additional parameters, we compute the Bayes factor (ln B^(pg)_(!pg)) comparing our p−g model to a standard one. We find that the observed signal is consistent with waveform models that neglect p−g effects, with lnB^(pg)_(!pg) = 0.03^(+0.70)_(−0.58) (maximum a posteriori and 90% credible region). By injecting simulated signals that do not include p−geffects and recovering them with the p−g model, we show that there is a ≃50% probability of obtaining similar lnB^(pg)_(!pg) even when p−g effects are absent. We find that the p−g amplitude for 1.4  M⊙ neutron stars is constrained to less than a few tenths of the theoretical maximum, with maxima a posteriori near one-tenth this maximum and p−g saturation frequency ∼70  Hz. This suggests that there are less than a few hundred excited modes, assuming they all saturate by wave breaking. For comparison, theoretical upper bounds suggest ≲10^3 modes saturate by wave breaking. Thus, the measured constraints only rule out extreme values of the p−g parameters. They also imply that the instability dissipates ≲10^(51)  erg over the entire inspiral, i.e., less than a few percent of the energy radiated as gravitational waves.

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