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

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Highly charged iron (Fe16+, here referred to as Fe xvii) produces some of the brightest X-ray emission lines from hot astrophysical objects1, 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 predicted2,3. This has affected the interpretation of observations by the Chandra and XMM-Newton orbiting X-ray missions1, 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 laser4; 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 3.6σ 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.

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Figure 1: X-ray spectrum of Capella, and measured as well as calculated values of the 3C/3D intensity ratio.
Figure 2: Time coincidence between the free-electron laser (FEL) pulse and detected X-ray fluorescence.
Figure 3: Measured X-ray fluorescence spectra.
Figure 4: Predicted and measured intensity ratios of lines 3C and 3D.


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We thank the staff at MPIK, LLNL and SLAC, especially D. Layne (LLNL) who provided technical support. Portions of this research were carried out on the SXR instrument at the LCLS, a division of SLAC National Accelerator Laboratory and an Office of Science user facility operated by Stanford University for the US Department of Energy. The SXR instrument is funded by a consortium whose membership includes the LCLS, Stanford University through the Stanford Institute for Materials Energy Sciences, Lawrence Berkeley National Laboratory, University of Hamburg through the BMBF priority programme, and the Center for Free Electron Laser Science. The present work was performed in part at LLNL under the auspices of the US Department of Energy and supported in part by the Helmholtz Alliance. P.B. performed part of the work reported here while at the Department of Chemistry and the Chemical Physics Program, University of Puerto Rico. A.S. was supported by the Helmholtz association and Z.H. was supported by EMMI. N.H. acknowledges support from BMBF, and E.T., A.M., J.K.R. and S.S. acknowledge support from the Deutsche Forschungsgemeinschaft.

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J.R.C.L.-U., P.B. and J.U. conceived the project; G.V.B., A.M. and S.M.K. contributed to the original proposal. S.B., J.K.R., R.S., E.W.M, C.B. and J.R.C.L.-U. prepared the FLASH-EBIT for operation at LCLS. C.B. planned the integration of the FLASH-EBIT at the SXR beamline. S.B., G.V.B., J.K.R., R.S., C.B., A.G., N.H., M.L., S.E., S.W.E., K.K., V.M., M.C.S., S.S., E.T. and J.R.C.L.-U. operated the FLASH-EBIT and detectors. F.S.P. provided technical assistance for the operation of one of the detectors. W.S. and J.J.T. prepared and operated the SXR instruments. A.S., J.C., P.B, Z.H. and C.H.K. performed supporting calculations. R.S. converted raw data for further processing and analysis. S.B. performed the data analysis. S.B., G.V.B., A.G., M.L., S.W.E., S.S., E.T., J.U., P.B. and J.R.C.L.-U. interpreted the results. P.B. wrote the manuscript with input from S.B. and J.R.C.L.-U.; Z.H., C.H.K., S.B., A.S. and P.B. wrote the Supplementary Information. All authors were involved in the discussion of results and commented on the manuscript.

Correspondence to S. Bernitt.

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Supplementary Information

This file contains Supplementary Text and Data 1-3, additional references, Supplementary Tables 1-2 and Supplementary Figures 1-4. (PDF 1060 kb)

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Bernitt, S., Brown, G., Rudolph, J. et al. An unexpectedly low oscillator strength as the origin of the Fe xvii emission problem. Nature 492, 225–228 (2012) doi:10.1038/nature11627

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