Nonthermal X-rays from low-energy cosmic rays: Application to the 6.4 keV line emission from the Arches cluster region. (arXiv:1210.2108v1 [astro-ph.HE])

Nonthermal X-rays from low-energy cosmic rays: Application to the 6.4 keV line emission from the Arches cluster region. (arXiv:1210.2108v1 [astro-ph.HE]):
The iron line at 6.4 keV provides a valuable spectral diagnostic in several
fields of X-ray astronomy. It often results from the reprocessing of external
X-rays by a neutral or low-ionized medium, but it can also be excited by
impacts of low-energy cosmic rays. This paper aims to provide signatures
allowing identification of radiation from low-energy cosmic rays in X-ray
spectra showing the 6.4 keV line. We study in detail the production of
nonthermal line and continuum X-rays by interaction of accelerated electrons
and ions with a neutral ambient gas. Corresponding models are then applied to
XMM-Newton observations of the X-ray emission emanating from the Arches cluster
region near the Galactic center. Bright 6.4 keV line structures are observed
around the Arches cluster. This emission is very likely produced by cosmic
rays. We find that it can result from the bombardment of molecular gas by
energetic ions, but probably not by accelerated electrons. Using a model of
X-ray production by cosmic-ray ions, we obtain a best-fit metallicity of the
ambient medium of 1.7 plus-minus 0.2 times the solar metallicity. A large flux
of low-energy cosmic ray ions could be produced in the ongoing supersonic
collision between the star cluster and an adjacent molecular cloud. We find
that a particle acceleration efficiency in the resulting shock system of a few
percent would give enough power in the cosmic rays to explain the luminosity of
the nonthermal X-ray emission. Depending on the unknown shape of the kinetic
energy distribution of the fast ions above 1 GeV per nucleon, the Arches
cluster region may be a source of high-energy gamma-rays detectable with the
Fermi Gamma-ray Space Telescope. At present, the X-ray emission prominent in
the 6.4 keV Fe line emanating from the Arches cluster region probably offers
the best available signature for a source of low-energy hadronic cosmic rays in
the Galaxy.