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Black-hole-neutron-star (BHNS) binary mergers can form disks in which magnetorotational instability (MRI)-induced turbulence may drive accretion onto the remnant BH, supporting relativistic jets and providing the engine for a short-hard gamma-ray burst (SGRB). Our earlier study of magnetized BHNSs showed that NS tidal disruption winds the magnetic field into a toroidal configuration, with poloidal fields so weak that capturing MRI with full-disk simulations would require ∼108 CPU hours. In that study we imposed equatorial symmetry, suppressing poloidal magnetic fields that might be generated from plasma crossing the orbital plane. Here we show that initial conditions that break this symmetry (i.e., tilted poloidal magnetic fields in the NS) generate much stronger poloidal fields in the disk, indicating that asymmetric initial conditions may be necessary for establishing BHNS mergers as SGRB progenitors via fully general relativistic magnetohydrodynamic simulations. We demonstrate that BHNS mergers may form an SGRB engine under the right conditions by seeding the remnant disk from an unmagnetized BHNS simulation with purely poloidal fields dynamically unimportant initially, but strong enough to resolve MRI. Magnetic turbulence occurs in the disk, driving accretion and supporting Poynting-dominated jet outflows sufficient to power an SGRB. [ABSTRACT FROM AUTHOR]

The inspirai and merger of a binary neutron star (NSNS) can lead to the formation of a hypermassive neutron star (HMNS). As the HMNS loses thermal pressure due to neutrino cooling and/or centrifugal support due to gravitational wave emission, and/or magnetic breaking of differential rotation, it will collapse to a black hole. To assess the importance of shock-induced thermal pressure and cooling, we adopt an idealized equation of state and perform NSNS simulations in full general relativity through late inspiral, merger, and HMNS formation, accounting for cooling. We show that thermal pressure contributes significandy to the support of the HMNS against collapse and that thermal cooling accelerates its "delayed" collapse. Our simulations demonstrate explicitly that cooling can induce the catastrophic collapse of a hot hypermassive neutron star formed following the merger of binary neutron stars. Thus, cooling physics is important to include in NSNS merger calculations to accurately determine the lifetime of the HMNS remnant and to extract information about the neutron star equation of state, cooling mechanisms, bar instabilities and B-fields from the gravitational waves emitted during the transient phase prior to black hole formation. [ABSTRACT FROM AUTHOR]

As a neutron star (NS) is tidally disrupted by a black hole (BH) companion at the end of a black-hole-neutron-star (BHNS) binary inspirai, its magnetic fields will be stretched and amplified. If sufficiently strong, these magnetic fields may impact the gravitational waveforms, merger evolution and mass of the remnant disk. Formation of highly-collimated magnetic field lines in the disk + spinning BH remnant may launch relativistic jets, providing the engine for a short-hard GRB. We analyze this scenario through fully general relativistic, magnetohydrodynamic BHNS simulations from inspirai through merger and disk formation. Different initial magnetic field configurations and strengths are chosen for the NS interior for both nonspinning and moderately spinning (aBH/MBH = 0.75) BHs aligned with the orbital angular momentum. Only strong interior (Bmax ∼ 1017 G) initial magnetic fields in the NS significantly influence merger dynamics, enhancing the remnant disk mass by 100% and 40% in the nonspinning and spinning BH cases, respectively. However, detecting the imprint of even a strong magnetic field may be challenging for Advanced LIGO. Though there is no evidence of mass outflows or magnetic field collimation during the preliminary simulations we have performed, higher resolution, coupled with longer disk evolutions and different initial magnetic field configurations, may be required to definitively assess the possibility of BHNS binaries as short-hard gamma-ray burst progenitors. [ABSTRACT FROM AUTHOR]