Ergoregion instability of exotic compact objects: electromagnetic and gravitational perturbations and the role of absorption. (arXiv:1807.08840v3 [gr-qc] UPDATED)
<a href="http://arxiv.org/find/gr-qc/1/au:+Maggio_E/0/1/0/all/0/1">Elisa Maggio</a>, <a href="http://arxiv.org/find/gr-qc/1/au:+Cardoso_V/0/1/0/all/0/1">Vitor Cardoso</a>, <a href="http://arxiv.org/find/gr-qc/1/au:+Dolan_S/0/1/0/all/0/1">Sam R. Dolan</a>, <a href="http://arxiv.org/find/gr-qc/1/au:+Pani_P/0/1/0/all/0/1">Paolo Pani</a>

Spinning horizonless compact objects may be unstable against an ‘ergoregion
instability’. We investigate this mechanism for electromagnetic perturbations
of ultracompact Kerr-like objects with a reflecting surface, extending previous
(numerical and analytical) work limited to the scalar case. We derive an
analytical result for the frequency and the instability time scale of unstable
modes which is valid at small frequencies. We argue that our analysis can be
directly extended to gravitational perturbations of exotic compact objects in
the black-hole limit. The instability for electromagnetic and gravitational
perturbations is generically stronger than in the scalar case and it requires
larger absorption to be quenched. We argue that exotic compact objects with
spin $chilesssim 0.7$ ($chilesssim 0.9$) should have an absorption
coefficient of at least $0.3%$ ($6%$) to remain linearly stable, and that an
absorption coefficient of at least $approx60%$ would quench the instability
for any spin. We also show that – in the static limit – the scalar,
electromagnetic, and gravitatonal perturbations of the Kerr metric are related
to one another through Darboux transformations.

Spinning horizonless compact objects may be unstable against an ‘ergoregion
instability’. We investigate this mechanism for electromagnetic perturbations
of ultracompact Kerr-like objects with a reflecting surface, extending previous
(numerical and analytical) work limited to the scalar case. We derive an
analytical result for the frequency and the instability time scale of unstable
modes which is valid at small frequencies. We argue that our analysis can be
directly extended to gravitational perturbations of exotic compact objects in
the black-hole limit. The instability for electromagnetic and gravitational
perturbations is generically stronger than in the scalar case and it requires
larger absorption to be quenched. We argue that exotic compact objects with
spin $chilesssim 0.7$ ($chilesssim 0.9$) should have an absorption
coefficient of at least $0.3%$ ($6%$) to remain linearly stable, and that an
absorption coefficient of at least $approx60%$ would quench the instability
for any spin. We also show that – in the static limit – the scalar,
electromagnetic, and gravitatonal perturbations of the Kerr metric are related
to one another through Darboux transformations.

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