New solar x-ray constraints on keV axionlike particles

The decay of axionlike 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(alpha gamma gamma), m) by considering ALPs production via photon coalescence. For g(alpha e) not equal 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(alpha e). Out of the transitional region, the solar x-ray constraints on ALPs are exclusively governed by g(alpha gamma gamma).