New solar X-ray constraints on keV Axion-Like Particles

The decay of Axion-Like Particles (ALPs) trapped in the solar gravitational field would contribute to the observed solar X-ray flux, hence constraining ALP models. We improve by one order of magnitude the existing limits in the parameter space \((g_{a\gamma\gamma}, m)\) by considering ALPs production via photon coalescence. For \(g_{ae} \neq 0\), we demonstrate that trapped ALPs can be Compton-absorbed while crossing the Sun, resulting in two regimes in the exclusion limits, with a transition triggered by \(g_{ae}\). Out of the transitional region, the solar X-ray constraints on ALPs are exclusively governed by \(g_{a\gamma\gamma}\).