The high spectral resolution of the next generation of X-ray telescopes (Chandra, XMM and
Astro-E) will enable us to detect and to separate the three main X-ray lines (resonance,
intercombination and forbidden) of He-like ions.
Concerning Chandra (AXAF), all the main lines (w, x+y, z) for the He-like ions treated
in this paper (C V to Si XIII), can be resolved using either the HRC-S
combined with the LETG (0.08-6.0 keV; 2-160Å), or the ACIS-S with HETG
(0.4-10keV; 1.2-31 Å).
The XMM mission, due to its high sensitivity and high spectral resolution (RGS:
0.35-2.5keV; 5-35Å), will enable us to detect these He-like ions, except for
C V which is outside the detector's energy range. See Fig. 11 for cases
illustrating a pure photoionized plasma and a hybrid plasma for O VII near
The Astro-E XRS will be the first X-ray micro-calorimeter in space.
It will have an energy resolution of 12eV (FWHM) over a broad energy range, 0.4 -
10 keV. Although, this is not sufficient for detailed spectroscopy at low energies, it
will be very useful for the study of He-like ions (see Fig. 8 in Paerels )
with E>2.5keV (i.e. Z>16), i.e. complementary to the Chandra and XMM capabilities.
At some future date, XEUS (X-Ray Evolving Universe Spectroscopy mission), which is a
potential follow-on to ESA's cornerstone XMM (Turner et al. ), will offer to
observers a high energy astrophysics facility with high resolving power
near 1keV with its narrow field imager) combined with a
unprecedented collecting area (initial mirror area of 6m2). This will enable
observers to use these types of plasma diagnostics for Carbon to Iron He-like ions. And, in
addition, for high Z, (Z=26 for iron) He-like lines and their corresponding
dielectronic satellite lines will be resolved and give accurate temperature diagnostics in
the case of hybrid plasmas. Satellite lines to the He-like 1s2-1s2l' parent line
are due to transitions of the type:
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