Charge Exchange Spectra of Hydrogenic and He-like Iron Page: 5 of 25
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to a few hundred keV amu-1, depending on the element and the shape of the low-energy
cosmic ray flux distribution. The typical energy for Fe emission will be roughly 200 keV
amu-1, corresponding to a line width of -170 eV. (All line widths in this paper are given in
terms of the FWHM.) In comparison, the Doppler broadening for Fe Lya in a plasma with
kT = 10 keV is only ~7 eV. Intriguingly, -170 eV is the line width measured by Koyama et
al. (1996) and Tanaka et al. (2000) in an ASCA spectrum of the GC, and is also consistent
with results from GR spectra (Tanaka 2002). More recently, however, Muno et al. (2004)
observed several GC fields with Chandra and deduced that line broadening was probably no
more than - 100 eV and could be consistent with zero.
Unfortunately then, the energy resolution of the ASCA and Chandra spectra (-200 eV
at 7 keV) is insufficient to permit firm conclusions regarding line broadening, and in both
cases energy calibration uncertainties are large enough to prohibit adequately precise line
centroid determinations that would distinguish between CX and thermal emission (see 2).
The XRS microcalorimeter detector on ASTRO-E2 (Mitsuda et al. 2004), which has 6-eV
resolution, should be able to provide definitive measurements although its small field of view
will necessitate very long exposures.
We next briefly review the CX mechanism and discuss key diagnostics of CX emission
that can be used in the analysis of ASTRO-E2 spectra. In 3 we describe our experiment,
then follow with an explanation of data analysis procedures in 4, discussion of results in 5,
and conclusion in 6.
2. CHARGE EXCHANGE THEORY
CX is the radiationless collisional transfer of one or more electrons from a neutral atom
or molecule to an ion. If the recipient ion is highly charged it is left in an excited state
which then decays via radiative cascades, or else, if the neutral species donates more than
one electron, by autoionization.
Since no photons are emitted during the electron transfer, the sum of the internal
energies of the ion and atom/molecule are conserved, and the donated electron(s) can be
transferred only to specific levels in the ion. The resonant character of the electron transfer
is softened somewhat by distortion of the energy levels of ion and atom during the collision,
so that a range of atomic states is accessible. For low collision energies (up to -100 keV
amu-1), the n level with the largest capture probability for single-electron transfer is given
approximately by Janev & Winter (1985) (rewriting to explicitly include the neutral species
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Wargelin, B J; Beiersdorfer, P; Neill, P A; Olson, R E & Scofield, J H. Charge Exchange Spectra of Hydrogenic and He-like Iron, article, April 27, 2005; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc879558/m1/5/: accessed May 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.