Multimessenger Binary Mergers Containing Neutron Stars: Gravitational Waves, Jets, and γ-Ray Bursts

Abstract

Neutron stars (NSs) are extraordinary not only because they are the densest form of matter in the visible Universe but also because they can generate magnetic fields ten orders of magnitude larger than those currently constructed on earth. The combination of extreme gravity with the enormous electromagnetic (EM) fields gives rise to spectacular phenomena like those observed on August 2017 with the merger of a binary neutron star system, an event that generated a gravitational wave (GW) signal, a short γ-ray burst (sGRB), and a kilonova. This event serves as the highlight so far of the era of multimessenger astronomy. In this review, we present the current state of our theoretical understanding of compact binary mergers containing NSs as gleaned from the latest general relativistic magnetohydrodynamic simulations. Such mergers can lead to events like the one on August 2017, GW170817, and its EM counterparts, GRB 170817 and AT 2017gfo. In addition to exploring the GW emission from binary black hole-neutron star and neutron star-neutron star mergers, we also focus on their counterpart EM signals. In particular, we are interested in identifying the conditions under which a relativistic jet can be launched following these mergers. Such a jet is an essential feature of most sGRB models and provides the main conduit of energy from the central object to the outer radiation regions. Jet properties, including their lifetimes and Poynting luminosities, the effects of the initial magnetic field geometries and spins of the coalescing NSs, as well as their governing equation of state, are discussed. Lastly, we present our current understanding of how the Blandford-Znajek mechanism arises from merger remnants as the trigger for launching jets, if, when and how a horizon is necessary for this mechanism, and the possibility that it can turn on in magnetized neutron ergostars, which contain ergoregions, but no horizons.

The Challenges Ahead for Multimessenger Analyses of Gravitational Waves and Kilonova: A Case Study on GW190425

“…Second, information on the viewing angle is potentially obtainable from the observation of a relativistic jet, which can improve parameter estimation on systems with asymmetric outflow geometries (see, e.g., Heinzel et al 2021). Currently, the relativistic jet that is launched in some mergers (see, e.g., Ruiz et al 2021) is not taken into account in the model presented here. Neither is the interaction between the jet and the dynamical and disk wind ejecta, which can alter the kilonova light curve (Klion et al 2021;Nativi et al 2021;Nicholl et al 2021).…”

“…Our results suggest that this constraining power will improve when numerical models of merger outflows, telescope sensitivity, and GW detectors progress (see, e.g., Chornock et al 2019;Cowperthwaite et al 2019;Palmese et al 2019). Currently, significant effort is underway to add more sophisticated treatments of microphysical processes to numerical models, as well as increase the coverage of the parameter space studied with these models (Baiotti & Rezzolla 2017;Shibata & Hotokezaka 2019;Ciolfi 2020;Dietrich et al 2021;Ruiz et al 2021). The Vera Rubin Observatory, an optical, wide-field telescope expected to begin operations in 2021 (Ivezić et al 2019), will have a large impact on taking high-accuracy EM data and detecting GW events at larger distances, while telescopes such as ZTF (Dekany et al 2020) and (near-future) dedicated GW follow-up telescopes such as BlackGEM (Bloemen et al 2016) and GOTO (Gompertz et al 2020) will increase the number of detected EM counterparts.…”

Establishing the Non-Primordial Origin of Black Hole-Neutron Star Mergers

“…Recently, LVC has reported first identified BH-NS binary GW events GW200105 and GW200115, with component masses of (8.9 +1.2 −1.5 M , 1.9 +0.3 −0.2 M ) and (5.7 +1.8 −2.1 M , 1.5 +0.7 −0.3 M ), respectively [79]. In addition to the BH-BH and NS-NS mergers, BH-NS events constitute another major class of mergers and carry significant implications for multimessenger observations [80]. While the detected events are consistent with stellar evolution formation channels [81], speculations about possible PBH origin, considering that NSs are mis-identified solar-mass BHs and detected events correspond to unequal mass PBH-PBH mergers, have already been put forth [82].…”

Models of binary neutron star remnants with tabulated equations of state

“…However, the loss of angular momentum, due to GW emission, as well as dissipative effects (e.g. shear viscosity, magnetic breaking and effective viscosity due to the development of the magneto-rotational instability, see Shibata & Hotokezaka (2019); Ciolfi (2020); Sarin & Lasky (2020); Ruiz et al (2021) for recent reviews and also Radice 2020) will ultimately lead to a delayed collapse to a black hole. A remnant with mass 𝑀 max,rot < 𝑀 < 𝑀 max , where 𝑀 max , which is the maximum mass of a nonrotating star, will be long-lived, spinning down on the timescale of electromagnetic emission, before reaching the axisymmetric instability limit.…”