GRMHD Simulations of Neutron-star Mergers with Weak Interactions: r-process Nucleosynthesis and Electromagnetic Signatures of Dynamical Ejecta

Abstract

Fast neutron-rich material ejected dynamically over ≲10 ms during the merger of a binary neutron star (BNS) can give rise to distinctive electromagnetic counterparts to the system’s gravitational-wave emission that serve as a “smoking gun” to distinguish between a BNS and an NS–black hole merger. We present novel ab initio modeling of the kilonova precursor and kilonova afterglow based on 3D general-relativistic magnetohydrodynamic simulations of BNS mergers with nuclear, tabulated, finite-temperature equations of state (EOSs), weak interactions, and approximate neutrino transport. We analyze dynamical mass ejection from 1.35–1.35 M ⊙ binaries, consistent with properties of the first observed BNS merger GW170817, using three nuclear EOSs that span the range of allowed compactness of 1.35 M ⊙-neutron stars. Nuclear reaction network calculations yield a robust second-to-third-peak r-process. We find few ×10−6 M ⊙ of fast (v > 0.6c) ejecta that give rise to broadband synchrotron emission on ∼years timescales, consistent with tentative evidence for excess X-ray/radio emission following GW170817. We find ≈2 × 10−5 M ⊙ of free neutrons that power a kilonova precursor on ≲ hours timescale. A boost in early UV/optical brightness by a factor of a few due to previously neglected relativistic effects, with enhancements up to ≲10 hr post-merger, is promising for future detection with UV/optical telescopes like Swift or ULTRASAT. We find that a recently predicted opacity boost due to highly ionized lanthanides at ≳70,000 K is unlikely to affect the early kilonova based on the obtained ejecta structures. Azimuthal inhomogeneities in dynamical ejecta composition for soft EOSs found here (“lanthanide/actinide pockets”) may have observable consequences for both early kilonova and late-time nebular emission.

Flares, Jets, and Quasiperiodic Outbursts from Neutron Star Merger Remnants

“…While several numerical relativity simulations of magnetized binary neutron star mergers have been performed (e.g., Kiuchi et al 2014Kiuchi et al , 2022Dionysopoulou et al 2015;Palenzuela et al 2015Palenzuela et al , 2022Ciolfi et al 2017;Most et al 2019;Werneck et al 2023;Combi & Siegel 2023), turbulent magnetic field amplification during the collision (Price & Rosswog 2006;Kiuchi et al 2015;Aguilera-Miret et al 2020), which likely dominates the postmerger magnetic field geometry (Aguilera-Miret et al 2022), is not fully captured by current simulations (Kiuchi et al 2018). Although magnetic fields in the inspiral are not likely to strongly affect the orbital dynamics or gravitational-wave (GW) emission (Ioka & Taniguchi 2000;Zhu et al 2020), they could produce electromagnetic (EM) precursors prior to the merger (see, e.g., Hansen & Lyutikov 2001;Tsang et al 2012;Palenzuela et al 2013;Carrasco & Shibata 2020;Most & Philippov 2020, 2022aZhang 2020).…”

“…Despite their importance, the magnetic field topology after merger remains largely unconstrained. While several numerical relativity simulations of magnetized binary neutron star mergers have been performed (e.g., Kiuchi et al 2014;Dionysopoulou et al 2015;Palenzuela et al 2015;Ciolfi et al 2017;Most et al 2019;Palenzuela et al 2022;Werneck et al 2022;Combi & Siegel 2023;Kiuchi et al 2022)…”

Studying Postmerger Outflows from Magnetized-neutrino-cooled Accretion Disks

“…They found that the secondorder scheme overestimates the amount of proton-rich shockheated ejecta. In addition, Combi & Siegel (2023) performed convergence studies on the outflows from a BNS merger simulation and found that the highest resolution resolves the shock-heated plasma more accurately. The postmerger ejecta can be affected in a similar way.…”

“…Specifying the extraction radius can be a challenging job, and it can affect some quantities' measurements, such as velocity. In the literature, a smaller extraction radius ∼200-500 km has been used to measure the dynamical ejecta or the ejecta from magnetized postmerger HMNSs with tracers (see, e.g., Combi &Siegel 2023 andde Haas et al 2022), and each tracer has to satisfy the geodesic or Bernoulli condition to identify the unbound mass. However, the disk wind is a different scenario; the recent studies by Haddadi et al (2022) showed that if the r-process heating source terms are included in the evolution of the postmerger ejecta during a multisecond simulation, the ejecta's velocity needs to be measured at a much larger distance.…”

“…The emissions from the former source are dimmer and long-lasting near-infrared (Li & Paczynski 1998;Metzger & Berger 2012;Grossman et al 2014;Tanaka et al 2014), while the latter produces optically brighter and bluer transients (Grossman et al 2014;Perego et al 2014;Kasen et al 2015). However, depending on the merger's scenario, it is possible to have multiple channels for outflow formation; in more recent studies by Combi & Siegel (2023) it is shown that the decay of the fast free neutrons in the outermost layers of dynamical ejecta can power a UV/optical kilonova precursor on a ≈1 hr timescale. In the case of a BNS merger, the long-lived postmerger hypermassive neutron star (HMNS) is considered as another channel of neutrino and magnetically driven outflow formations (Curtis et al 2023;de Haas et al 2022).…”