A novel scenario for the possible X-ray line feature at 3.5 keV: Charge exchange with bare sulfur ions

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Motivated by recent claims of a compelling 3.5 keV 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 3.5 keV, produced mostly by S XVI transitions from principal quantum numbers n > 8 to the ground. Such high-n states can only be populated by the charge exchange process. In this scenario, the observed 3.5 keV line flux in clusters can be naturally explained by an interaction in an effective volume of 1 kpc³ between a 3 keV temperature plasma and cold dense clouds moving at a few hundred km/s. The S XVI lines at 3.5 keV also provide a unique diagnostic of the charge exchange phenomenon in hot cosmic plasmas.