Binary compact star mergers

Neutron star mergers

Neutron star mergers will produce multi-messenger transient signals (see the right figure for the overall picture), especially the gravitational wave (GW) radiation, the gamma-ray burst (GRB) and kilonova. Most currents researches on electromagnetci (EM) signal are focused in the post-merger phase. While my primary interests are the EM radiation in the pre-merger phase. Also, there is an increasing interest on the dynamical tides in eccentric neutron star mergers. I'm also studying this topic now.

EM counterpart theories: gamma-ray precursors and fast radio bursts

Due to the GW radiation, the neutron star binary inspirals quickly in the last orbits. We treat the magnetic field of the neutron star as dipoles. The structure of the magnetic field can be very complicated, while we study three representative cases here (see the right figure). The red and blue spots are the neutron stars, while the strings are the magnetic field lines. The EM interaction of the binary will extract the orbital kinetic energy and dissipated in EM radiation. Case 0 works also for NS-BH binaries, and in this case the major energy release process is caused by the electromotive force (EMF) when the unmagnetized companion crosses the magnetic field lines of the magnetized one. This is based on the unipolar inductor model for the Jupiter–Io system. And for the first time, we have provided an analytical model for the cases where two stars are magnetized and derived the energy release through magnetic reconnection or the magnetic field compression and relaxation processes in Case 1 or 2. Syncrtron radiation and thermal radiation are in the gamma-ray band (see the bottom-left figure). Fast radio burst (FRB) can also be generated in this process.

Ref:

Wang and Liu, 2021, Galax, 9, 104. https://ui.adsabs.harvard.edu/abs/2021Galax...9..104W/abstract

Wang et al. 2018, ApJ, https://iopscience.iop.org/article/10.3847/1538-4357/aae531

Wang et al. 2016, ApJL, https://iopscience.iop.org/article/10.3847/2041-8205/822/1/L7

EM counterpart observations: gamma-ray precursors in Fermi/GBM short GRB sample

We performed a stringent search for precursor emission of short gamma-ray bursts from the Fermi/GBM data, and find 16 precursor events with 4.5σ significance (see the figure below for the light curves of eight GRBs). We find that the durations of the main SGRB emission (TGRB) and the precursor emission, as well as the waiting time (Twt) in between, are roughly comparable to each other, with Twt ≈ 2.8TGRB1.2 approximately satisfied for most cases except one significant outlier. We also perform spectral analyses to the precursors and SGRBs, and find that the spectra of precursor emission can be fitted with the blackbody, non-thermal cutoff power law and/or power-law models. We consider several possible models for precursor emission in SGRBs and find that luminosity and spectral shape may be explained by the the shock breakout or the photospheric radiation of a fireball launched after the merger for thermal precursors, or magnetospheric interaction between two neutron stars prior to the merger for nonthermal precursors. For the fireball photospheric model, a matter-dominated jet is preferred and a constraint on the fireball Lorentz factor can be placed as Γ ~ 30. For the magnetospheric interaction model, the jet launching mechanism may be constrained. In particular, those events with Twt/TGRB<<1(e.g., GRB191221802) require the formation of a supramassive or stable neutron star after the merger, with the delay time defined by the timescale for an initially baryon-loaded jet to become magnetically dominated and relativistic.

Ref: Wang et al. 2020, ApJL, https://iopscience.iop.org/article/10.3847/2041-8213/abbfb8

GW: Eccentric neutron star inspiral

We study the tidal effects in eccentric neutron star inspiral. The NS is modeled as a compressible ellipsoid, which can deform nonlinearly due to tidal forces, while the orbit evolution is treated with the post-Newtonian (PN) theory up to 2.5-PN order (see the right video for the dynamical evolution of a typical case). The tidal interaction can accelerate the inspiral, and cause orbital frequency and phase shifts. For eccentric inspirals, the frequency and phase shifts oscillate considerably near pericenter passages, and the oscillating amplitudes increase with eccentricities. As a result, the GW phase is significantly influenced by the tidal effect. At merger, the cumulative GW phase shift can reach more than 10 radians (for typical NS of mass 1.4 M⊙ and radius 11.6 km).

Ref: Wang & Lai, 2020, PRD, https://journals.aps.org/prd/abstract/10.1103/PhysRevD.102.083005

GW: post-merger remnant (magnetar-accretion disk system)

We study the r-mode instability of the post-merger magnetar with a hyper-critical accretion disk. The accretion rate can be as high as solar mass per minutes. R-mode instability can be driven in this system by GW radiation. We study the GW radiation from the magnetar, and provide the GW waveforms in the frequency domain (see the left figure).

Ref: Wang & Dai, 2017, A&A, http://www.aanda.org/10.1051/0004-6361/201629610

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