Multi-messenger Emission from Tidal Waves in Neutron Star Oceans

TL;DR

This paper explores how tidal waves in neutron star oceans, excited during binary inspirals or encounters, could produce detectable electromagnetic signals that serve as early warnings for mergers and provide insights into neutron star properties.

Contribution

It introduces the concept of tidally resonant ocean waves as a new electromagnetic precursor to neutron star mergers and analyzes their potential detectability and astrophysical significance.

Findings

Neutron star oceans can sustain tidal waves with frequencies between 0.01-20 Hz.

Tidal resonances may produce electromagnetic flares detectable out to 100 Mpc.

Flares could serve as early warning signals for neutron star mergers.

Abstract

Neutron stars in astrophysical binary systems represent exciting sources for multi-messenger astrophysics. A potential source of electromagnetic transients from compact binary systems is the neutron star ocean, the external fluid layer encasing a neutron star. We present a groundwork study into tidal waves in neutron star oceans and their consequences. Specifically, we investigate how oscillation modes in neutron star oceans can be tidally excited during compact binary inspirals and parabolic encounters. We find that neutron star oceans can sustain tidal waves with frequencies between $0.01-20$ Hz. Our results suggest that tidally resonant neutron star ocean waves may serve as a never-before studied source of precursor electromagnetic emission prior to neutron star-black hole and binary neutron star mergers. If accompanied by electromagnetic flares, tidally resonant neutron star ocean waves, whose energy budget can reach $10^{46}$ erg, may serve as early warning signs ($\gtrsim 1$ minute before merger) for compact binary mergers. Similarly, excited ocean tidal waves will coincide with neutron star parabolic encounters. Depending on the neutron star ocean model and a flare emission scenario, tidally resonant ocean flares may be detectable by Fermi and NuSTAR out to $\gtrsim 100$ Mpc with detection rates as high as $\sim 7$ yr$^{-1}$ for binary neutron stars and $\sim0.6$ yr$^{-1}$ for neutron star-black hole binaries. Observations of emission from neutron star ocean tidal waves along with gravitational waves will provide insight into the equation of state at the neutron star surface, the composition of neutron star oceans and crusts, and neutron star geophysics.