Charge Exchange Spectra of Hydrogenic and He-like Iron Page: 4 of 25
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Within the past decade, astrophysical X-ray emission via charge exchange (CX) has been
recognized to occcur in comets, the atmospheres of planets including the Earth, throughout
the heliosphere, and around other stars (see review by Cravens (2002) and references therein).
Recently, observations with moderate spectral resolution by Chandra (Wargelin et al. 2004;
Smith et al. 2005) and XMM-Newton (Snowden, Collier, & Kuntz 2004) have detected clear
signatures of geocoronal and heliospheric CX, most prominently in time-variable oxygen line
emission, which may contribute a significant fraction of the soft X-ray background. All the
aforementioned CX emission is from moderately ionized species such as He-like and H-like
C, N, O, and Ne which originate in solar or stellar coronae. Those ions emit X rays when
they CX with: neutral molecules such as H20 in comets; neutral H in the Earth's outer
atmosphere; and neutral interstellar H and He within the heliosphere or astrospheres around
CX has also been proposed (Tanaka, Miyaji, & Hasinger 1999) to explain some and
perhaps most of the line emission from more highly ionized species such as He-like and H-
like Si, S, Ar, Ca, and Fe seen in diffuse emission from the Galactic Ridge (GR) and Galactic
Center (GC) (Koyama et al. 1996; Kaneda et al. 1997; Ebisawa et al. 2001; Muno et al.
2004). According to this hypothesis, the highly charged ions are low-energy cosmic rays
that CX with neutral gas in the plane of the Galaxy. This CX mechanism would naturally
explain the remarkable similarity in the spectral shapes of GC and GR diffuse emission from
widely separated regions of the Galaxy, since the emission arises from essentially the same
population of ions with the intensity level primarily determined by the supply of neutral gas.
A major problem with this idea, however, is that it assumes that cosmic rays remain
nearly fully ionized even at low energies (of order 100 keV amu-1). Because of solar mod-
ulation, there are no reliable measurements of cosmic ray flux or ionization state below -1
GeV amu-1 (Fulks 1975), but theoretical work on cosmic-ray CX (Watson 1976; Bussard,
Ramaty, & Omidvar 1978) predicts that the fraction of fully ionized Fe is negligible below
several MeV amu-1. Given existing uncertainties in the cosmic-ray energy budget and flux
at low energies, however, a CX explanation for at least some of the GR line emission cannot
yet be discounted.
Provided that a 'significant' fraction (> 10-4) of cosmic rays remains nearly fully ionized
at low energies, then even conservative extrapolations of the cosmic ray flux to low energies
can account for the observed GR line flux (Raymond & Wargelin, in preparation), and
the characteristic collision energy for CX emission is straightforward to predict. Because
of the sharp fall-off in CX cross sections at energies above -25q0-5 keV amu-1(Ryufuku &
Watanabe 1979), the emission-weighted average collision energy ranges from several tens
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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/m1/4/: accessed April 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.