The Role of the Schwinger Effect in Superradiant Axion Lasers

Abstract: Superradiance can cause the axion cloud around a rotating black hole to reach extremely high densities, and the decay of these axions can produce a powerful laser. The electric field of these lasers is strong enough that the Schwinger effect may become significant, resulting in the production of an electron-positron plasma. We explore the dynamics between axion lasers and this electron-positron plasma. While there are several mechanisms by which the inclusion of a plasma can impact the laser's behavior, the most significant of these mechanisms is that the electron-positron plasma imparts an effective mass on the photon. As the plasma frequency increases, axion decay becomes energetically unfavorable, up to the point where the axion no longer decays into photons, shutting off the laser. We find that the impact of the electron-positron plasma on the dynamics of the system depend heavily on the parameters, specifically the axion mass $m_\phi$ and the superradiant coupling $\alpha$, and that we may divide parameter space into three regimes: the unenhanced, enhanced, and unstable regimes. In the unenhanced and enhanced regime, the system will eventually settle into an equilibrium state, emitting a laser of constant luminosity while the number of axions remains constant. In the unenhanced regime, this equilibrium state can be calculated while neglecting the effects of Schwinger production; in the enhanced regime, the equilibrium luminosity is slightly larger than what it would be without Schwinger production. In the unstable regime, the electron-positron plasma suppresses axion decay to the point where the system is never able to reach equilibrium; instead, the axions continue to grow superradiantly. In all three cases, the production of superradiant axions will eventually cause the black hole to spin down to the point where superradiance ceases.