Collapse of differentially rotating relativistic stars: Post black hole formation stage

We investigate the collapse of differentially rotating supermassive stars (SMSs) by means of 3+1 hydrodynamic simulations in general relativity. We particularly focus on the onset of collapse to understand the final outcome of collapsing SMSs. We find that the estimated ratio between the black hole (BH) and the surrounding disk from the equilibrium star is roughly the same as the results from numerical simulation. This statement suggests that the picture of axisymmetric collapse is adequate in the absence of nonaxisymmetric instabilities for illustrating the final state of the collapse. We also find that when the newly formed BH is almost an extreme Kerr, a corotation resonance can be triggered by the oscillation of a BH. In this case, nonaxisymmetric instabilities are triggered by corotation resonance and make a significant difference in the gravitational waveforms. This alternative scenario for the collapse of differentially rotating SMSs might be observable by LISA.