Theory and astrophysical consequences of a magnetized torus around a rapidly rotating black hole.
VAN PUTTEN M.H.P.M. and LEVINSON A.
Abstract (from CDS):
We analyze the topology, lifetime, and emissions of a torus around a black hole formed in hypernovae and black hole-neutron star coalescence. The torus is ab initio uniformly magnetized, represented by two counteroriented current rings, and develops a state of suspended accretion against a ``magnetic wall'' around the black hole. Magnetic stability of the torus gives rise to a new fundamental limit E_B_/Ek<0.1 for the ratio of poloidal magnetic field energy to kinetic energy, corresponding to a maximum magnetic field strength Bc≃1016G7M☉/MH6MH/R2MT/0.03MH1/2. The lifetime of rapid spin of the black hole, effectively defined by the timescale of dissipation of spin energy Erotin the horizon, hereby satisfies T≃40s(MH/7M☉)(R/6MH)4(0.03MH/MT) for a black hole of mass MHsurrounded by a torus of mass MTand radius R. The torus converts a major fraction Egw/Erot∼10% into gravitational radiation through a finite number of multipole mass moments and a smaller fraction into MeV neutrinos and baryon-rich winds. At a source distance of 100 Mpc, these emissions over N=2x104 periods give rise to a characteristic strain amplitude N1/2hchar≃6x10–21. We argue that torus winds create an open magnetic flux tube on the black hole, which carries a minor fraction Ej/Erot≃10–3 in baryon-poor outflows to infinity. We conjecture that these are not high-σ outflows, owing, in part, to magnetic reconnection in surrounding current sheets. The fraction Ej/Erot~(1)/(4)(MH/R)4 is standard for a universal horizon half-opening angle θH≃MH/R of the open flux tube. We identify this baryon-poor output of tens of seconds with gamma-ray bursts with contemporaneous and strongly correlated emissions in gravitational radiation, conceivably at multiple frequencies. Ultimately, this leaves a black hole binary surrounded by a supernova remnant.