Enhancing Constraints on Ultralight Axion Dark Matter from Gravitational Capture

Ultralight axions can be gravitationally captured by massive bodies such as the Sun, producing solar-bound gravitational atom states that amplify the local dark matter density through Bose-enhanced capture. For axion masses in the range ${10}^{-14}\text{eV}\lesssim m_a\lesssim {10}^{-13}\text{eV}$, this mechanism becomes exponentially efficient. We show that, for a representative decay constant $f_a\sim 3.5\times {10}^7$ GeV, the local axion dark matter density at Earth can grow to more than ten times the standard Galactic value of ${\rho }_0=0.4{\text{ GeV/cm}}^3$. Incorporating this overdensity, we derive updated limits on the axion-photon coupling from existing satellite and terrestrial measurements and present improved projections for upcoming experiments.