5 Current and future X-ray satellites opportunities

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 0.57keV. 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 [1999]) 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. [1997]), will offer to observers a high energy astrophysics facility with high resolving power ( $E/\Delta E\sim1$ 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:

$${\rm 1s}^{2}nl-{\rm 1s}2l{'}nl~~~~~~~n\geq2.$$ (16)

The main dielectronic satellite lines of Ca XIX and Fe XXV He-like ions correspond to n=2 and 3 and are most important for temperature diagnostic purposes. For more details see the review by Dubau & Volonte ([1980]).