Introduction
The coincident detection of gravitational waves (GWs) with counterparts across the electromagnetic (EM) spectrum from GW170817 triggered the beginning of multimessenger astronomy. This single multimesssenger event provides: i) the first direct evidence that compact binary mergers where at least one of the companions is a neutron star can be the progenitors of the central engine that powers short gamma-ray bursts (sGRBs). This conclusion was anticipated and confirmed by self-consistent simulations in full general relativistic, magnetohydrodynamics (GRMHD) of merging binary neutron stars (NSNSs), and binary black hole-neutron stars (BHNSs); ii) tight constraints on the equation of state (EOS) at supranuclear densities; iii) limits to the maximum mass of neutron stars; iv) evidence of ejecta masses of $\approx$ 0.01 - 0.05M with velocities $\approx$ 0.1 - 0.3 c. This ejecta is roughly consistent with the estimated r-process production rate required to explain the Milky Way r-process abundances; and v) an independent measure for the expansion of the Universe. GW170817 also demonstrated that to understand multimessenger observations and, in particular, to understand the physics of matter under extreme conditions, it is crucial to compare them to predictions from theoretical modeling. Due to the complexity of the underlying physical phenomena, such modeling is largely numerical in nature.
As a crucial step in solidifying the role of NSNSs as multimessenger sources, we survey NSNS configurations that undergo either delayed or prompt collapse and treat different representative EOSs and initial geometries of their magnetic field to probe their impact on jet launching and the dynamical ejection of matter. In particular, the NSs are modeled using a piecewise polytropic representation of the nuclear SLy and H4 EOSs. For comparison, we also consider the binaries in which the stars are modeled using a simple polytropic EOS with $\Gamma$ = 2. For the magnetic field, we endow the stars initially with a purely poloidal magnetic field that either extends from the interior of the NSs into its exterior, as in pulsars, or that is confined to the stellar interior. We also evolve unmagnetized configurations to assess the impact of the magnetic field on the ejecta. As NSNS mergers tend to create very baryon-loaded environments, we consider binaries whose merger outcome is a short-$(1 \lesssim \tau_{HMNS} \lesssim 5ms)$, medium-$(5 \lesssim \tau_{HMNS} \lesssim 20ms)$ or long-lived $(\tau_{HMNS} \gtrsim$ 20ms) hypermassive neutron star (HMNS). These choices will allow us to probe the impact of light vs. heavy environments on the physical properties of the incipient jet. Here $\tau_{HMNS}$ is the lifetime of the HMNS.
We find that incipient jets only emerge from binary remnants that undergo delayed collapse, regardless of the EOS. The lifetime of the jet [$\Delta t \sim$ 92-150 ms] and its corresponding outgoing EM Poynting luminosity [$ L_{\rm EM}\sim 10^{52 \pm 1}\rm erg/s$] are consistent with the lifetime of the sGRB central engine, as well as with the Blandford-Znajek (BZ) mechanism for launching jets and their associated Poynting luminosities. Our results can be summarized as follows: i) the closer the total mass of the binary is to the threshold value for prompt collapse, the shorter is the time delay between the GW peak amplitude (our definition of the moment of coalescence) and the jet launching time; ii) the jet launching time strongly depends on the initial geometry of the seed magnetic field; iii) the dynamical ejection of matter following merger strongly depends on the initial magnetic field geometry; iv) using the GW match function, we find that the imprints of the magnetic field on the gravitational radiation can be observed by current based-ground detectors (aLIGO/Virgo/KAGRA) only if the GW event occurs within at a distance $\lesssim$ 6.0 Mpc. If the GW event occurs within a distance $\lesssim$ 50Mpc, these imprints can be observed only with next generation of GW observatories, such as the Einstein Telescope or Cosmic Explorer, with a sigma-to-noise ratio SNR $\gtrsim$ 30.
Simulations were performed on Blue Waters, NASA High-End Computing and XSEDE supercomputer facilities. The Illinois GRMHD code, which implements the BSSN formulation of GR with moving-box adaptive mesh refinement, was used for all simulations.
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