Magnetorotational collapse of very massive stars to black holes in full general relativity

We perform axisymmetric simulations of the magnetorotational collapse of very massive stars in full general relativity. Our simulations are applicable to the collapse of supermassive stars with masses M greater than or similar to 10(3)M(circle dot) and to very massive Population III stars. We model our initial configurations by n=3 polytropes, uniformly rotating near the mass-shedding limit and at the onset of radial instability to collapse. The ratio of magnetic to rotational kinetic energy in these configurations is chosen to be small (1% and 10%). We find that such magnetic fields do not affect the initial collapse significantly. The core collapses to a black hole, after which black-hole excision is employed to continue the evolution long enough for the hole to reach a quasistationary state. We find that the black-hole mass is M-h=0.95M and its spin parameter is J(h)/M-h(2)=0.7, with the remaining matter forming a torus around the black hole. The subsequent evolution of the torus depends on the strength of the magnetic field. We freeze the spacetime metric ("Cowling approximation") and continue to follow the evolution of the torus after the black hole has relaxed to quasistationary equilibrium. In the absence of magnetic fields, the torus settles down following ejection of a small amount of matter due to shock heating. When magnetic fields are present, the field lines gradually collimate along the hole's rotation axis. MHD shocks and the magnetorotational instability generate MHD turbulence in the torus and stochastic accretion onto the central black hole. When the magnetic field is strong, a wind is generated in the torus, and the torus undergoes radial oscillations that drive episodic accretion onto the hole. These oscillations produce long-wavelength gravitational waves potentially detectable by the Laser Interferometer Space Antenna. The final state of the magnetorotational collapse always consists of a central black hole surrounded by a collimated magnetic field and a hot, thick accretion torus. This system is a viable candidate for the central engine of a long-soft gamma-ray burst.