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The iron line at 6.4 keV provides a valuable spectral diagnostic in severalfields of X-ray astronomy. It often results from the reprocessing of externalX-rays by a neutral or low-ionized medium, but it can also be excited byimpacts of low-energy cosmic rays. This paper aims to provide signaturesallowing identification of radiation from low-energy cosmic rays in X-rayspectra showing the 6.4 keV line. We study in detail the production ofnonthermal line and continuum X-rays by interaction of accelerated electronsand ions with a neutral ambient gas. Corresponding models are then applied toXMM-Newton observations of the X-ray emission emanating from the Arches clusterregion near the Galactic center. Bright 6.4 keV line structures are observedaround the Arches cluster. This emission is very likely produced by cosmicrays. We find that it can result from the bombardment of molecular gas byenergetic ions, but probably not by accelerated electrons. Using a model ofX-ray production by cosmic-ray ions, we obtain a best-fit metallicity of theambient medium of 1.7 plus-minus 0.2 times the solar metallicity. A large fluxof low-energy cosmic ray ions could be produced in the ongoing supersoniccollision between the star cluster and an adjacent molecular cloud. We findthat a particle acceleration efficiency in the resulting shock system of a fewpercent would give enough power in the cosmic rays to explain the luminosity ofthe nonthermal X-ray emission. Depending on the unknown shape of the kineticenergy distribution of the fast ions above 1 GeV per nucleon, the Archescluster region may be a source of high-energy gamma-rays detectable with theFermi Gamma-ray Space Telescope. At present, the X-ray emission prominent inthe 6.4 keV Fe line emanating from the Arches cluster region probably offersthe best available signature for a source of low-energy hadronic cosmic rays inthe Galaxy.

Nonthermal X-rays from low-energy cosmic rays: Application to the 6.4 keV line emission from the Arches cluster region