The interpretation of soft x-ray emission lines in the spectra of accretion-powered sources: The effects of thermal stability

X‐ray line emission in the 0.5–1.5 keV band has been detected in the spectra of a growing number of accretion‐powered x‐ray sources. The emission appears to be dominated by strong lines from the Fe L ions, although there are several peculiarities to the emission structure. In particular, the Fe L lines appear not to be accompanied by K‐shell lines of He‐ and H‐like oxygen, which should be several times stronger for most relevant emission mechanisms. If the lines are produced in a photoionized medium surrounding the central source, then the particular species observed may be strongly affected by the thermal stability of the irradiated gas. The coupling between the thermal balance and the ionization balance introduces a strongly non‐linear relation between abundance and strength of the emission lines for the important elements. The absence of strong oxygen emission may simply be due to the fact that the H‐ and He‐like oxygen line emission regions are located entirely in a thermally unstable phase of the medium, as would be predicted to occur for relatively small departures from standard abundances. We explore the fundamental thermal stability properties of an x‐ray photoionized plasma, and their dependence on elemental abundance, and we discuss the obvious importance of these properties for any prospects of determining the chemical composition of accreting gas in accretion‐powered sources from the x‐ray emission spectrum.