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

Abstract

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 onto the neutron star crust to instigate yieldings. Due to the typical frequencies of g-modes being ∼100Hz, 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 g1- and g2-modes may lead to crustal failure and trigger precursor flares.

Constraining equation of state groups from $g$-mode asteroseismology

“…We, therefore, are seeking a scheme to obtain the properties of individual star from the observables without the a priori input of the mass ratio. We illustrate that with g-mode frequency obtained from electromagnetic observations, e.g., the timing of precursors (Kuan et al 2021b), such methodology can be established.…”

Electromagnetic Precursors of Short Gamma-Ray Bursts as Counterparts of Gravitational Waves

“…This can be easily reached by the tidally-induced f-mode oscillation in the last seconds before the merger [65] (see also Figure 3 of Reference [75]). Recent works show that the g-mode can also lead to the breaking of the NS crust [66].…”

“…It has also been proposed that, if the NS is highly magnetized, i.e., being a magnetar with B 10 13 G, the crust failure may trigger a violent reconstruction of magnetic fields, leading to a magnetar-giant-flare-like event (e.g., [40,66]). However, it should be noted that for magnetar giant flares, the crust failure is believed to be caused by the sudden rearrangement of magnetic field [77].…”

“…Besides, there are two more scenarios proposed only for SGRB precursors. During the inspiral phase of the NS-NS/black hole (BH) binary, the magnetospheric interaction of the binary [55][56][57][58][59][60][61][62][63], or the crust crack of the NS [64][65][66] may also generate gamma-ray emissions. As such, precursors of SGRBs may shed light on the physical processes right before or shortly after the merger.…”

Fast Rotating Neutron Stars: Oscillations and Instabilities

“…The different oscillation patterns of a neutron star are characterized by their restoring force, e.g., p(pressure)-modes, g(gravity)-modes, i(Coriolis)-modes, s(shear)-modes or w(wave)modes. The f -mode is the fundamental mode of the p-mode sequence and it is the oscillation mode most likely to be excited in violent processes such as neutron star formation by supernova core collapse ( [20,21]), the pre-merger interaction of neutron stars (see, e.g., [22][23][24][25][26][27][28][29][30]), the early-post-merger oscillations of the final object ( [31][32][33][34][35][36][37][38][39][40][41]). In the case that the merging neutron stars are of relatively small mass and the post-merger object is a fast spinning neutron star, the unstable f -mode oscillations can lead to its spin-down ( [42]).…”