2016ApJ...833...52S

The reported observations of an unidentified X-ray line feature at ∼3.5 keV have driven a lively discussion about its possible dark matter origin. Motivated by this, we have measured the K-shell X-ray spectra of highly ionized bare sulfur ions following charge exchange with gaseous molecules in an electron beam ion trap, as a source of or a contributor to this X-ray line. We produced S¹⁶⁺ and S¹⁵⁺ ions and let them capture electrons in collision with those molecules with the electron beam turned off while recording X-ray spectra. We observed a charge-exchange-induced X-ray feature at the Lyman series limit (3.47 ±0.06 keV). The inferred X-ray energy is in full agreement with the reported astrophysical observations and supports the novel scenario proposed by Gu et al.