Ergoregion instability of exotic compact objects: Electromagnetic and gravitational perturbations and the role of absorption
Abstract
Spinning horizonless compact objects may be unstable against an "ergoregion instability." We investigate this mechanism for electromagnetic perturbations of ultracompact Kerrlike 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 timescale 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 blackhole 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 χ ≲0.7 (χ ≲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 ≈60 % 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. Finally, correcting previous results, we give the transformations that bring the Teukolsky equation in a form described by a real potential also in the gravitational case.
 Publication:

Physical Review D
 Pub Date:
 March 2019
 DOI:
 10.1103/PhysRevD.99.064007
 arXiv:
 arXiv:1807.08840
 Bibcode:
 2019PhRvD..99f4007M
 Keywords:

 General Relativity and Quantum Cosmology;
 Astrophysics  High Energy Astrophysical Phenomena;
 High Energy Physics  Phenomenology;
 High Energy Physics  Theory
 EPrint:
 v2: 11+3 pages, 5 figures. Refereed version: various improvements, including numerical computation of the gravitational instability, conclusions unchanged. v3: 11+4 pages, 5 figures. Refereed version: analytical and numerical derivation of gravitational quasinormal modes changed, qualitative results and conclusions unchanged