Formation of rapidly rotating dynamic black holes

We investigate the formation of rapidly rotating dynamic black hole through gravitational collapse of rotating relativistic stars by means of 3+1 hydrodynamic simulations in general relativity. We succeed in producing a dynamic black hole of a = 0.98M through the collapse of differentially rotating supermassive stars, and find the following three issues. Firstly, the estimated ratio of the mass between the black hole and the surrounding disk from the equilibrium star is roughly the same as the results from numerical simulation. This suggests that the picture of axisymmetric collapse is adequate, in the absence of nonaxisymmetric instabilities, to illustrate the final state of the collapse. Secondly, quasi-periodic gravitational waves continue to be emitted after the quasinormal mode frequency has decayed. Finally, when the newly formed black hole is almost extreme Kerr, the amplitude of the quasi-periodic oscillation is enhanced during the late stages of the evolution. Geometrical features, shock waves, and instabilities of the fluid are suggested as a cause of this amplification behaviour. This alternative scenario for the collapse of differentially rotating supermassive stars might be observable by LISA.