Journal Article

An unexpectedly low oscillator strength as the origin of the Fe xvii emission problem

Graf, A., Beiersdorfer, P., Harman, Z., Hell, N., Porter, F. S., Magee, E. W., Turner, J. J., Eberle, S., Simon, M. C., Bernitt, S., Leutenegger, M., Schlotter, W., Beilmann, C., Kahn, S. M., Rudolph, J. K., Kubiček, K., Keitel, C. H., Mäckel, V., Surzhykov, A., Steinbrügge, R., Epp, S. W., Träbert, E., Schippers, S., Müller, A., Ullrich, J., Clementson, J., López-Urrutia, J. R. Crespo, Brown, G. V.

2012

Overview

Fluorescence of iron ions induced by an X-ray laser allows the relative oscillator strength for Fe xvii emission to be determined; it is found to differ by 3.6 σ from the best quantum mechanical calculations, suggesting that the poor agreement between prediction and observations of the brightest Fe  xvii line is rooted in the quality of the underlying atomic wavefunctions used in the models. New look at highly charged astrophysical iron The interpretation of some of the spectral data from the Chandra and XMM-Newton orbiting X-ray missions has been complicated by discrepancies between theory and observation involving the emission lines from the highly charged Fe 16+ ion, also known as Fe XVII. Specifically, the intensity of the strongest Fe XVII line, one of the brightest X-ray emissions from galaxies and stars, is generally weaker than predicted. Sven Bernitt et al . report the results of laboratory experiments in which a target of iron ions was fluoresced with femtosecond X-ray pulses from a free-electron laser. They find a relative oscillator strength that differs by 3.6 σ from the best quantum mechanical calculations, suggesting that the poor agreement is rooted in the calculations of the underlying atomic dynamics and that the current astrophysical models are not at fault. Highly charged iron (Fe 16+ , here referred to as Fe  xvii ) produces some of the brightest X-ray emission lines from hot astrophysical objects 1 , including galaxy clusters and stellar coronae, and it dominates the emission of the Sun at wavelengths near 15 ångströms. The Fe  xvii spectrum is, however, poorly fitted by even the best astrophysical models. A particular problem has been that the intensity of the strongest Fe  xvii line is generally weaker than predicted 2 , 3 . This has affected the interpretation of observations by the Chandra and XMM-Newton orbiting X-ray missions 1 , fuelling a continuing controversy over whether this discrepancy is caused by incomplete modelling of the plasma environment in these objects or by shortcomings in the treatment of the underlying atomic physics. Here we report the results of an experiment in which a target of iron ions was induced to fluoresce by subjecting it to femtosecond X-ray pulses from a free-electron laser 4 ; our aim was to isolate a key aspect of the quantum mechanical description of the line emission. Surprisingly, we find a relative oscillator strength that is unexpectedly low, differing by 3.6 σ from the best quantum mechanical calculations. Our measurements suggest that the poor agreement is rooted in the quality of the underlying atomic wavefunctions rather than in insufficient modelling of collisional processes.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 639/33/34/864 / 639/766/483/1139 / Agreements / Astronomy / Atomic and molecular data, spectra, and spectral parameters (opacities, rotation constants, line identification, oscillator strengths, gf values, transition probabilities, etc.) / Chemical properties / Collisions (Nuclear physics) / Earth, ocean, space / Emissions / Exact sciences and technology / Fundamental aspects of astrophysics / Fundamental astronomy and astrophysics. Instrumentation, techniques, and astronomical observations / Humanities and Social Sciences / Iron / Lasers / letter / Materials science / multidisciplinary / Observations / Quantum theory / Radiation mechanisms, polarization / Ratios / Science / Wavelengths