Introduction
The gravitational wave (GW) detection GW170817 coincident with electromagnetic (EM) counterpartradiation across the EM spectrum and, in particular, the detection of a short gamma ray burst (sGRB) 1.7s following the inferred merger time (event GRB 170817A), provides the best confirmation so far that compact binary mergers, in which at least one of the binary companions is a neutron star are progenitors of sGRBs. We have recently demonstrated this possibility by self-consistent simulations in full general relativistic magnetohydrodynamics (GRMHD) of merging black hole-neutron star (BHNS) binaries, and merging neutron star binaries (NSNS). Depending on the spin priors of the binary companions, the GW170817 inferred masses are in the broad range of $0.86M_\odot-2.26M_\odot$, though the total mass of the system is constrained to be $\sim 2.73M_\odot-3.29 M_\odot$ with $90\%$ credibility. These masses are consistent with astrophysical observations of NSs which, along with the optical counterparts, indicate the presence of matter, and hence strongly suggest the coalescence of a NSNS as the progenitor of GW170817, although it cannot rule out the possibility that one of the binary companions is a stellar mass BH.
Our GRMHD simulations of BHNSs, in which the NS is modeled as an irrotational $\Gamma=2$ polytrope, showed that an incipient jet $-$a collimated, mildly relativistic outflow which is magnetically dominated$-$ may be launched from the highly spinning BH + disk remnant if: a) the NS is endowed with a magnetic field that extends from the stellar interior into the exterior, as in a radio pulsar; b) the tilt angle between the magnetic moment and the total angular momentum of the system is small; and c) the initial BH spin satisfies $a/M_{\rm BH}\gtrsim 0.4$. On the other hand, our GRMHD studies of NSNS mergers, where the NS is again modeled as an irrotational $\Gamma=2$ polytrope, showed that NSNS systems may launch an incipient jet whether or not the seeded poloidal magnetic field is confined to the NS interior as long as the binary undergoes delayed collapse to a BH. The lifetime of the jet [$\Delta t\sim 100(M_{\rm NS}/1.625M_\odot){\rm ms}$] and the outgoing electromagnetic luminosities $\sim$[$ L_{\rm EM}\sim 10^{51}\rm erg/s$] in the above cases turn out to be consistent with short-duration sGRBs.
Due to the limited sensitivity of the second observing run (O2) of Advanced LIGO, and assuming that the progenitor of GW170817 is the merger of a NSNS system, there is no current consensus yet whether the GW170817 remnant is a highly spinning BH + disk or a long-lived supramassive NS (SNS). Here we assume that the GW170817 remnant is a transient HMNS that eventually collapses to a highly spinning BH surrounded by a magnetized accretion disk. The primary motivation for these simulations is to address the question: What is the final spin of the GW170817 remnant black hole?
The final spin of the BH may have a strong impact on the remnant disk and the formation and lifetime of the magnetically sustained jet and the outgoing electromagnetic Poynting luminosity and, therefore, it may explain or give new insight regarding sGRB phenomenology. To address this question, we perform GRMHD simulations of spinning NSNS configurations in a quasicircular orbit that undergo delayed collapse to BH.
We find that following merger the redistribution of angular momentum by magnetic braking due to winding and magnetic turbulence driven by the magnetorotational instability (MRI) in the HMNS remnant, along with the dissipation of angular momentum due to gravitational radiation, induce the formation of a massive, nearly uniformly rotating inner core surrounded by a magnetized, Keplerian, disk-like envelope. In all cases, by $t-t_{\rm mer}\sim 15(M_{\rm NS}/1.625M_\odot)$ ms $-20(M_{\rm NS}/1.625M_\odot)\rm ms$ following the merger, the HMNS collapses to a BH, with a final spin $a/M_{\rm BH} \simeq 0.78$, surrounded by in an accretion disk whose rest mass depends strongly on the initial spin of the NSs. We observe that the larger the initial spin, the heavier the disk. After $\Delta t\sim 3000M-4000M$ $\sim 45(M_{\rm NS}/1.625 M_\odot)$ ms $-60(M_{\rm NS}/1.625 M_\odot)\rm ms$ following merger, a magnetically-driven and sustained incipient jet is launched. The lifetime of the jet [$\Delta t\sim 100(M_{\rm NS}/1.625M_\odot)$ ms $-140(M_{\rm NS}/1.625M_\odot)\rm ms$] and its respective outgoing EM Poynting luminosity [$L_{\rm EM}\sim 10^{51.5\pm 1}\rm erg/s$] turn out to be consistent with typical short-duration sGRBs as well as with the BZ process for launching jets and their associated Poynting luminosities.
Simulations were performed on the Blue Waters supercomputer at UIUC. 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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