Speaker
Description
The existence of a superradiant axion cloud around a black hole X-ray binary can heat its accretion disk and be detected in a thermal X-ray spectrum. We consider a derivative coupling between axions and fermions and calculate the emissivity of the inverse bremsstrahlung process, which results in a temperature fluctuation of the disk. The energy injection rate per baryon by axion heating is sizable $\sim0.1(|g_{ae}|/10^{-12})^2\,{\rm eV/s}$ if the accretion disk's densest region overlaps with the cloud's Bohr radius, where $g_{ae}$ is the axion-electron coupling. Based on the Novikov-Thorne thin-disk model and the multicolor disk model, we derive the thermal X-ray spectrum with axion heating. A single bump hunting search of the axion heating signature in the X-ray thermal spectrum can already derive a competitive constraint on axion-electron coupling.