A comparison of theoretical and solar-flare intensity ratios for the Fe XIX X-ray lines

Atomic data consisting of energy levels, gf-values and wavelengths are presented for the Fe XIX$$2{s}^{2}2{p}^{4}-2{s}^{2}2{p}^{3}3s,\,2{s}^{2}2{p}^{3}3d$$ arrays that give rise to lines in solar-flare and active-region X-ray spectra. Collision strengths and theoretical intensity ratios are given for the $$2{s}^{2}\,2{p}^{4}-2{s}^{2}\,2{p}^{3}\,3d$$ lines, which occur in the 13.2–14.3 Å range. Solar spectra in this range include a large number of other intense lines, notably those due to He-like Ne (Ne IX). Although the Ne IX lines are potentially the most useful indicators of electron density in solar X-ray spectra, blending with the Fe XIX lines has been a major problem for previous analyses. Comparison of observed spectra with those calculated from the Fe XIX atomic data presented here and Ne IX lines from other work indicates that there is generally good agreement, especially for spectra in which the Fe XIX lines are less strong than the Ne IX lines. For spectra in which the Fe XIX lines are very strong, such as those at the maxima of hot flares, two observed line features due to Fe XIX have larger intensities than calculated. One of these includes a strongly density-dependent line – the only one in this Fe XIX array –but this is not the reason for the discrepancy. We use the calculated Fe XIX and Ne IX line spectra and several observed spectra during a flare previously analysed to estimate electron density from Ne IX line ratios, thus for the first time properly taking into account blends with Fe XIX lines. The very high values of $${N}_\text{e},\,\sim\,{10}^{12} \text{cm}^{-3},$$ at flare maximum obtained in the earlier work are confirmed.