Collapse of differentially rotating supermassive stars: Post black hole formation

TL;DR

This paper uses 3+1 hydrodynamic simulations in general relativity to study the collapse of differentially rotating supermassive stars, revealing the final black hole and disk formation, gravitational wave emission, and potential observability by LISA.

Contribution

It provides a detailed simulation-based analysis of the collapse process, including the black hole-disk mass ratio and gravitational wave signals, highlighting the role of geometrical features and instabilities.

Findings

Black hole to disk mass ratio matches equilibrium estimates.

Quasi-periodic gravitational waves persist after quasinormal mode decay.

Enhanced oscillation amplitude occurs near extremal Kerr black holes.

Abstract

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.