Laboratory measurements compellingly support charge-exchange mechanism for the 'dark matter' $\sim$3.5 keV X-ray line

TL;DR

Laboratory measurements of charge exchange in highly ionized sulfur ions produce an X-ray line at 3.47 keV, supporting the hypothesis that this line observed in space could originate from charge exchange processes related to dark matter.

Contribution

This study provides experimental evidence linking charge exchange in sulfur ions to the 3.5 keV X-ray line, supporting a non-dark matter origin for the astrophysical signal.

Findings

Observed X-ray feature at 3.47 keV from charge exchange

Energy matches astrophysical 3.5 keV line

Supports charge exchange as origin of the line

Abstract

The reported observations of an unidentified X-ray line feature at $\sim$3.5 keV have driven a lively discussion about its possible dark matter origin. Motivated by this, we have measured the \emph{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 produce $\mathrm{S}^{16+}$ and $\mathrm{S}^{15+}$ 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-exchanged-induced X-ray feature at the Lyman series limit (3.47 $\pm$ 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 and Kaastra (A \& A \textbf{584}, {L11} (2015)).