Probing Axions via Spectroscopic Measurements of S-stars at the Galactic Center

Abstract

Axions, encompassing both QCD axions and axion-like particles, can generate loop-induced quadratic couplings to electromagnetic field strength tensors, resulting in oscillatory shifts of the fine-structure constant. Near a Kerr black hole, an axion field with a Compton wavelength comparable to the event horizon can exponentially grow through the superradiance mechanism, potentially reaching a maximum amplitude near the decay constant, provided this scale is below approximately $10^{16}$ GeV. The saturated axion cloud formed around the black hole induces characteristic oscillations in the fine-structure constant, with a period of $10$-$40$ minutes determined by the axion mass, and a spatial profile governed by the axion wavefunction and its coupling strength. At lower axion masses, axion dark matter can form a soliton-like core characterized by a nearly constant amplitude, extending measurable variations of the fine-structure constant to greater distances. Precise spectroscopic measurements of S-stars orbiting the supermassive black hole Sgr A$^*$ provide a powerful probe of these predictions, potentially excluding substantial regions of parameter space for quadratic scalar couplings to photons, owing to the high boson density near the Galactic Center.