Volume 579, July 2015
|Number of page(s)||6|
|Published online||03 July 2015|
Line absorption of He-like triplet lines by Li-like ions
Caveats of using line ratios of triplets for plasma diagnostics
SRON Netherlands Institute for Space Research,
Sorbonnelaan 2, 3584 CA
2 Department of Physics and Astronomy, Universiteit Utrecht, PO Box 80000, 3508 TA Utrecht, The Netherlands
3 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
4 Astronomical Institute “Anton Pannekoek”, Science Park 904, University of Amsterdam, 1098 XH Amsterdam, The Netherlands
Received: 15 April 2015
Accepted: 22 May 2015
He-like ions produce distinctive series of triplet lines under various astrophysical conditions. However, this emission can be affected by line absorption from Li-like ions in the same medium. We investigate this absorption of He-like triplets and present the implications for diagnostics of plasmas in photoionisation equilibrium using the line ratios of the triplets. Our computations were carried out for the O vi and Fe xxiv absorption of the O vii and Fe xxv triplet emission lines, respectively. The fluorescent emission by the Li-like ions and continuum absorption of the He-like ion triplet lines are also investigated. We determine the absorption of the triplet lines as a function of Li-like ion column density and velocity dispersion of the emitting and absorbing medium. We find O vi line absorption can significantly alter the O vii triplet line ratios in optically-thin plasmas, by primarily absorbing the intercombination lines, and to a lesser extent, the forbidden line. Because of intrinsic line absorption by O vi inside a photoionised plasma, the predicted ratio of forbidden to intercombination line intensity for the O vii triplet increases from 4 up to an upper limit of 16. This process can explain the triplet line ratios that are higher than expected and that are seen in some X-ray observations of photoionised plasmas. For the Fe xxv triplet, line absorption by Fe xxiv becomes less apparent owing to significant fluorescent emission by Fe xxiv. Without taking the associated Li-like ion line absorption into account, the density diagnosis of photoionised plasmas using the observed line ratios of the He-like ion triplet emission lines can be unreliable, especially for low-Z ions.
Key words: techniques: spectroscopic / atomic processes / atomic data / X-rays: general
© ESO, 2015
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