The Role of the Schwinger Effect in Superradiant Axion Lasers

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 $m_ϕ$ and the superradiant coupling $α$, 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.

💡 Research Summary

This paper investigates the dynamic interplay between superradiant axion lasers, known as BLASTs (Black hole Lasers powered by Axion SuperradianT instabilities), and electron-positron plasmas generated via the Schwinger effect. Superradiance around rotating black holes can exponentially amplify a dense cloud of axions in a quasi-bound “gravitational atom” state. These axions can then undergo stimulated decay into photons, producing an extremely powerful, coherent electromagnetic emission—a laser. A key prediction is that the electric field strength within this laser can approach the critical Schwinger limit, triggering non-perturbative quantum vacuum decay and the prolific production of electron-positron pairs.

The central finding is that this Schwinger-produced plasma fundamentally alters the laser’s lifecycle by endowing photons with an effective mass equal to the plasma frequency. When this effective mass exceeds half the axion mass, the axion’s decay into two photons becomes kinematically forbidden, effectively shutting off the laser. The paper develops an extended set of Boltzmann equations that couple the evolution of axion and photon numbers to the density of the electron-positron plasma.

Through a stability analysis of these equations, the authors map the system’s fate onto a two-dimensional parameter space defined by the axion mass (m_φ) and the superradiant coupling constant (α). They identify three distinct regimes:

1. The Unenhanced Regime: For certain parameters, Schwinger production is negligible or its back-reaction is minimal. The system behaves as a standard BLAST, reaching a steady-state equilibrium where superradiant growth balances axion decay, resulting in continuous laser emission.
2. The Enhanced Regime: Here, Schwinger production is significant. The plasma imparts a mass to photons, but the system can still reach a new equilibrium. Interestingly, this equilibrium luminosity is found to be slightly higher than in the plasma-free case.
3. The Unstable Regime: For a specific region of parameter space (typically lower axion masses), the feedback is catastrophic. The plasma builds up so rapidly that it completely suppresses axion decay before the system can reach equilibrium. Consequently, the axion cloud continues to grow superradiantly without the balancing act of decay, ultimately spinning down the black hole until the superradiance condition itself is violated.

A significant technical aspect of the work is the method used to calculate the Schwinger production rate within the chaotic, multi-directional photon bath of the BLAST. Instead of using average field values—which would greatly underestimate the rate due to the effect’s extreme nonlinearity—the authors compute a statistical average over the probable distributions of electric and magnetic field strengths within Compton-volume patches of the axion cloud.

The study concludes that Schwinger pair production is not a mere side effect but a decisive factor determining the stability and observational signature of superradiant axion lasers. The three identified regimes provide a clear theoretical framework for predicting whether a BLAST will emit steady radiation, pulsed bursts, or lead to uninterrupted axion cloud growth. The analysis is primarily applied to primordial black holes, which are favored candidates for hosting such extreme conditions. The paper also notes that the axion-assisted Schwinger effect, which would enhance pair production, is not included, suggesting that its findings might represent a conservative lower bound on the plasma’s impact.