Heating Up the Black Hole X-ray Binary Accretion Disk by Superradiance

Abstract

A superradiant cloud of ultralight axions around a black hole, that is part of an X-ray binary system, can heat up its accretion disk and be detected by the thermal X-ray spectrum emitted by the disk. We consider a derivative coupling of the axions to the plasma fermions and calculate the emissivity of the inverse bremsstrahlung process that results in a temperature fluctuation of the disk. Based on the thin-disk model and the multicolor disk model, we derive the thermal spectrum with axion heating, which shows an enhanced thermal photon flux and a red-/blue- shifted peak spectral frequency. A single bump hunting search of the axion heating signature in the thermal spectrum of a $10M_{\odot }$ black hole X-ray binary with a spectral measurement sensitivity of 10\% (1\%) can derive the constraint on axion-electron coupling $|g_{ae}|\gtrsim 7.5\times 10^{-12}(2.4\times 10^{-12})$ for axion mass $m_a=5.2\times 10^{-12}$eV in a saturated $|211⟩$ state, and $|g_{ae}|\gtrsim 4.5\times 10^{-12}(1.4\times 10^{-12})$ for axion mass $m_a=1.0\times 10^{-11}$eV in a saturated $|322⟩$ state. The projected sensitivities are competitive with those from XENONnT. A detailed continuum fitting can further improve the detectability and provide a complementary bound to the black hole spin-down measurement.