  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 of the mass between the black
hole (BH) 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. We also find that
quasi-periodic gravitational waves continue to be emitted after the quasinormal
mode frequency has decayed. We furthermore have found that when the newly
formed BH 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 SMSs might be observable by LISA.
