Collapse of Rotating Supramassive Neutron Stars to Black Holes: Fully General Relativistic Simulations

We study the final state of the gravitational collapse of uniformly rotating supramassive neutron stars by axisymmetric simulations in full general relativity. The rotating stars provided as the initial condition are marginally stable against quasiradial gravitational collapse and its equatorial radius rotates with the Kepler velocity (i.e., the star is at the mass-shedding limit). To model the neutron stars, we adopt the polytropic equations of state for a wide range of the polytropic index as n = 2/3, 4/5, 1, 3/2 and 2. We follow the formation and evolution of the black holes, and show that irrespective of the value of n (2/3 ≤ n ≤ 2), the final state is a Kerr black hole and the disk mass is very small (< 10 of the initial stellar mass). Subject headings: black hole physics – relativity – hydrodynamics – stars: rotation