General-relativistic treatment of tidal g-mode resonances in coalescing binaries of neutron stars - II. As triggers for precursor flares of short gamma-ray bursts

In some short gamma-ray bursts, precursor flares occurring - seconds prior to the main episode have been observed. These flares may then be associated with the last few cycles of the inspiral when the orbital frequency is a few hundred Hz. During these final cycles, tidal forces can resonantly excite quasi-normal modes in the inspiralling stars, leading to a rapid increase in their amplitude. It has been shown that these modes can exert sufficiently strong strains on to the neutron star crust to instigate yieldings. Due to the typical frequencies of g- modes being similar to 100 Hz, their resonances with the orbital frequency match the precursor timings and warrant further investigation. Adopting realistic equations of state and solving the general-relativistic pulsation equations, we study g-mode resonances in coalescing quasi-circular binaries, where we consider various stellar rotation rates, degrees of stratification, and magnetic field structures. We show that for some combination of stellar parameters, the resonantly excited g(1) and g(2) modes may lead to crustal failure and trigger precursor flares.

Automated summary

The study investigates the causes of precursor flares in merging quasi-circular binaries and finds that tidal forces resonantly excite quasi-normal modes in stars, leading to a rapid increase in amplitude and ultimately triggering crustal failure.