Precursor flares of short gamma-ray bursts from crust yielding due to tidal resonances in coalescing binaries of rotating, magnetized neutron stars

Abstract

As evidenced by the coincident detections of GW170817 and GRB 170817A, short gamma-ray bursts are likely associated with neutron star-neutron star merger events. Although rare, some bursts display episodes of early emission, with precursor flares being observed up to ∼ 10 seconds prior to the main burst. As the stars inspiral due to gravitational wave emission, the exertion of mutual tidal forces leads to the excitation of stellar oscillation modes, which may come into resonance with the orbital motion. Mode amplitudes increase substantially during a period of resonance as tidal energy is deposited into the star. The neutron star crust experiences shear stress due to the oscillations and, if the resonant amplitudes are large enough, may become over-strained. This over-straining can lead to fractures or quakes which release energy, thereby fueling precursor activity prior to the merger. Using some simple Maclaurin spheroid models, we investigate the influence of magnetic fields and rapid rotation on tidally-forced f -and r-modes, and connect the associated eigenfrequencies with the orbital frequencies corresponding to precursor events seen in, for example, GRB 090510.

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

“…It has been suggested that the crust breaking strain is around 0.1 [74], which corresponds to a critical ellipticity of c ≈ δR/R ≈ 4 × 10 −6 , where R + δR is the elongated NS radius. 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].…”

“…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 (Torres-Forné et al (2018;), the pre-merger interaction of neutron stars (see, e.g., Lai (1994); Kokkotas and Schafer (1995); Fuller and Lai (2011); Gold et al (2012); Chirenti et al (2017); Chaurasia et al (2018); Suvorov and Kokkotas (2020); Kuan et al (2021a,b)), the early-post-merger oscillations of the final object (Shibata and Taniguchi (2006); Bauswein and Janka (2012); Hotokezaka et al (2013); Bernuzzi et al (2014); Bauswein et al (2014); Lehner et al (2016); Bauswein et al (2017); Rezzolla and Takami (2016); Takami et al (2015); De Pietri et al (2018); Breschi et al (2019)). In the case that the merging neutron stars are of relatively small mass and the postmerger object is a fast spinning neutron star, the unstable f-mode oscillations can lead to its spin-down ).…”

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]).…”