Effects of Black-Hole Spin

Fixing the mass ratio q = 3 and the initial orbital period, we vary the initial spin parameter of the black hole from ã = -0.5 to 0.75. We find that for spins -0.5, 0.0, and 0.75, the binary inspiral phase lasts for 3.25, 4.5, and 6.5 orbits, respectively. As the initial BH spin parameter ã increases, the total initial angular momentum increases, requiring more gravitational-wave cycles to emit angular momentum and bring the BH and NS close enough to merge. Also, as the spin increases, NS tidal disruption becomes more pronounced, resulting in long tidal tails that eventually form disks with rest mass ≈ 4% and ≈ 15% the rest mass of the neutron star, for ã = 0.00 and ã = 0.75, respectively. Thus BHs with higher spin would likely lead to even more massive disks. Such disks around rotating BHs are good candidates for GRB central engines.

Fig. 1-1 Initial Frame

Fig. 1-2 Final Frame

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Mass Ratio Comparisons

Fixing the initial spin parameter of the black hole ã = 0, we varied the mass ratio from q = 1 to 5. For the cases simulated, a high mass ratio leads to tidal disruption after a higher number of orbits. For sufficiently high mass ratio, the NS is swallowed by the BH prior to tidal disruption (we do not simulate such a case). The equal mass q = 1 case is special, and probably does not occur in nature. After tidal disruption, the NS matter curls around the BH forming a hot, low-density spiral that winds around the apparent horizon and smashes into the tidal tail, generating a large amount of shock heating. A fraction of the heated NS matter in the tail losses angular momentum and falls into the BH. The rest deforms into an inhomogeneous disk before settling into a quasistationary state. At the end of the simulation, the NS matter settles into a high density, low temperature torus of matter surrounding the BH.

Fig. 2-1 Initial Frame

Fig. 2-2 Final Frame

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last updated 15 December 2014 by aakhan3

Center for Theoretical Astrophysics---University of Illinois at Urbana-Champaign

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