Stellar Superradiance and Low-Energy Absorption in Dense Nuclear Media

TL;DR

This work examines whether ultralight bosons coupled to neutrons, tightly constrained by stellar cooling, can also drive neutron-star superradiance. By formulating a microphysical absorption framework and translating it to the superradiant, bound-state regime, the authors identify two suppression mechanisms: gravitational-atom (bound-state) effects and long-wavelength collective scattering in dense nuclear matter. The key result is that, despite naive rates suggesting rapid superradiant growth, the medium-induced suppression F_sup ≈ ω_b^2/(ω_b^2+Γ_col^2) with Γ_col from degenerate nuclear matter dramatically reduces the SR rates, rendering stellar superradiance via these channels astrophysically negligible. Consequently, NS spin-down constraints do not currently compete with stellar cooling bounds for axions and dark photons in the neutron-star context, though the framework sets the stage for exploring other long-wavelength channels and broader dense-matter environments in the future.

Abstract

Ultralight bosons such as axions and dark photons are well-motivated hypothetical particles, whose couplings to ordinary matter can be effectively constrained by stellar cooling. Limits on these interactions can be obtained by demanding that their emission from the stellar interior does not lead to excessive energy loss. An intriguing question is whether the same microphysical couplings can also be probed through neutron star superradiance, in which gravitationally bound bosonic modes grow exponentially by extracting rotational energy from the star. Although both processes originate from boson-matter interactions, they probe very different kinematic regimes. Stellar cooling probes boson emission at thermal wavelengths, while superradiance is governed by modes whose wavelength is comparable to or greater than the size of the star. Previous work has attempted to relate the microphysical neutron-nucleon scattering and inverse-bremsstrahlung absorption rates directly to the macroscopic growth rate of superradiant bound states. In this work, we re-examine this connection and show that a naive extrapolation of the microphysical absorption rate to the superradiant regime would imply superradiant rates comparable to astrophysical timescales characterised by pulsar spindown. These naive rates are especially high for vector fields. However, we demonstrate that this conclusion changes once collective multiple-scattering effects in dense nuclear matter are taken into account. Repeated nucleon collisions modify the effective low-energy absorption experienced by the bosonic bound state, strongly suppressing the rate relevant for superradiance.

Figures (1)

• Figure 1: One of the eight Feynman diagrams for nucleon–nucleon bremsstrahlung, involving four external neutrons $(\psi)$ with momenta $p_i$ and one boson with momentum $p_b$, mediated by neutral pion $(\pi^0)$ exchange. Boson emission in the final state dominates stellar cooling, whereas boson absorption in the initial state dominates superradiance. The remaining diagrams are obtained by attaching the boson to other external neutron legs and by exchanging $p_3$ and $p_4$.