Vol. 584
In section 1. Letters

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

by L. Gu, J. Kaastra, A. J. J. Raassen, et al. A&A 584, L11


Recently, a narrow X-ray band around 3.5 keV has been detected toward bright galaxy clusters and the Andromeda galaxy. Since no compatible thermal emission lines from a hot plasma are expected, some interpretations have been oriented toward the decay of - 7 keV sterile neutrino dark-matter particles. Assuming these 3.5 keV features are real, the authors propose a novel scenario in terms of charge exchange processes between hot cosmic plasma and cold neutral hydrogen gas. They calculate the line emission due to a set of S xvi transitions from level n ≥ 9 to the ground. Such high-n transitions can be produced exclusively by charge transfer between bare sulfur ions and neutral particles, both being present in galaxy clusters and central regions of spiral galaxies. This scenario might be confirmed by observing other charge exchange lines with the future Astro-H mission.