Onset of superradiant instabilities in the hydrodynamic vortex model

Shahar Hod
Phys. Rev. D 90, 027501 – Published 11 July 2014

Abstract

The hydrodynamic vortex, an effective spacetime geometry for propagating sound waves, is studied analytically. In contrast with the familiar Kerr black hole spacetime, the hydrodynamic vortex model is described by an effective acoustic geometry which has no horizons. However, this acoustic spacetime possesses an ergoregion, a property which it shares with the rotating Kerr spacetime. It has recently been shown numerically that this physical system is linearly unstable due to the superradiant scattering of sound waves in the ergoregion of the effective spacetime. In the present study we use analytical tools in order to explore the onset of these superradiant instabilities which characterize the effective spacetime geometry. In particular, we derive a simple analytical formula which describes the physical properties of the hydrodynamic vortex system in its critical (marginally stable) state, the state which marks the boundary between stable and unstable fluid configurations. The analytically derived formula is shown to agree with the recently published numerical data for the hydrodynamic vortex system.

  • Received 29 May 2014

DOI:https://doi.org/10.1103/PhysRevD.90.027501

© 2014 American Physical Society

Authors & Affiliations

Shahar Hod

  • The Ruppin Academic Center, Emeq Hefer 40250, Israel and The Hadassah Institute, Jerusalem 91010, Israel

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Issue

Vol. 90, Iss. 2 — 15 July 2014

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