Recent calculations of atomic data for Fe xv have been used to generate theoretical line ratios involving n = 3–4 transitions in the soft X-ray spectral region ( \sim 52–83 Å ) , for a wide range of electron temperatures and densities applicable to solar and stellar coronal plasmas .
A comparison of these with solar flare observations from a rocket-borne spectrograph ( XSST ) reveals generally good agreement between theory and experiment .
In particular , the 82.76 Å emission line in the XSST spectrum is identified , for the first time to our knowledge in an astrophysical source , as the 3 s 3 d ^ { 3 } D _ { 3 } –3 s 4 p ^ { 3 } P _ { 2 } transition of Fe xv .
Previous suggested identifications of the 53.11 , 63.97 and 69.65 Å features as the 3 s ^ { 2 } ^ { 1 } S–3 s 4 p ^ { 3 } P _ { 1 } , 3 p ^ { 2 } ^ { 1 } D–3 s 4 f ^ { 1 } F and 3 s 3 p ^ { 1 } P–3 s 4 s ^ { 1 } S lines of Fe xv , respectively , are confirmed .
However the former is blended ( at the \sim 50 % level ) with the S ix 2 s ^ { 2 } 2 p ^ { 4 } ^ { 3 } P _ { 1 } –2 p ^ { 3 } 3 s ^ { 3 } P _ { 2 } transition .
Most of the Fe xv transitions which are blended have had the species responsible clearly identified , although there remain a few instances where this has not been possible .
The line ratio calculations are also compared with a co-added spectrum of Capella obtained with the Chandra satellite , which is probably the highest signal-to-noise observation achieved for a stellar source in the \sim 25–175 Å soft X-ray region .
Good agreement is found between theory and experiment , indicating that the Fe xv lines are reliably detected in Chandra spectra , and hence may be employed as diagnostics to determine the temperature and/or density of the emitting plasma .
However the line blending in the Chandra data is such that individual emission lines are difficult to measure accurately , and fluxes may only be reliably determined via detailed profile fitting of the observations .
The co-added Capella spectrum is made available to hopefully encourage further exploration of the soft X-ray region in astronomical sources .