Atomic data needs for X-ray spectroscopy of photo-ionised plasmas

High-resolution X-ray spectroscopy of Active Galactic Nuclei has become possible thanks to the launch of XMM-Newton and Chandra with their grating spectrometers, and will be explored further after the expected launch of ASTRO-E2 with its XRS detector. In several AGN the X-ray spectra show the signatures of on outflowing, photo-ionised wind. Also several X-ray binaries show a similar imprint of a photo-ionised gas. The clearest signatures are formed by the broad range of absorption lines, mostly from the ground states of a wide range of ionisation states of the abundant elements. In addition to absorption lines due to the valence electrons, the spectra show many inner-shell absorption lines. Examples are the K-shell transitions of the most abundant metal, oxygen, in the 19-23 A band, and 2p-3d transitions of lowly ionised iron in the 15-17 A region. These transitions have an extremely important diagnostic value, as other transitions of the same ions frequently occur in the unobservable extreme ultraviolet. Several of these inner-shell transitions, however, lack accurate experimental or theoretical wavelengths, which makes the spectral analysis complicated and ambiguous. This is even more the case for transitions from metastable levels, which can be used as density diagnostics. Finally, attention is paid to the role of atomic data in the photo-ionisation equilibrium calculations. Uncertainties in for example dielectronic recombination rates cause large uncertainties in the predicted absorption line strengths