2015A&A...584L..11G

Motivated by recent claims of a compelling ∼3.5keV emission line from nearby galaxies and galaxy clusters, we investigate a novel plasma model incorporating a charge exchange component obtained from theoretical scattering calculations. Fitting this kind of component with a standard thermal model yields positive residuals around 3.5keV, produced mostly by SXVI transitions from principal quantum numbers n≥9 to the ground. Such high-n states can only be populated by the charge exchange process. In this scenario, the observed 3.5keV line flux in clusters can be naturally explained by an interaction in an effective volume of ∼1kpc³ between a ∼3keV temperature plasma and cold dense clouds moving at a few hundred keV^–1. The SXVI lines at ∼3.5keV also provide a unique diagnostic of the charge exchange phenomenon in hot cosmic plasmas.