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M. F. Gu, S. M. Kahn, D. W. Savin (Columbia Astrophysics Laboratory, Columbia University), P. Beiersdorfer, G. V. Brown, D. A. Liedahl, K. J. Reed (Department of Physics and Space Technology, Lawrence Livermore National Laboratory), C. P. Bhalla, S. R. Grabbe (Department of Physics, Kansas State University)
Using the Electron Beam Ion Trap facility at Lawrence Livermore National Laboratory, we have measured relative cross sections for Fe XXIV line emission at electron energies between 0.7 and 3.0 keV. The measurements include line formation by direct electron-impact excitation (DE), radiative cascades, resonant excitation (RE), and dielectronic recombination (DR) satellites with captured electron in n\ge5 levels. Good agreement with R-matrix and distorted wave calculations is found. In collisionally ionized plasmas, at temperatures near where the ion abundance peaks (kTe ~ 1.7 keV), the RE contributions are found to be \lesssim 5% of the line emission, while the DR satellites contribute \lesssim 10%. While good agreement with state-of-the-art atomic physics calculations is found, there is less good agreement with existing spectral synthesis codes in common astrophysical use. For the Fe XXIV 3p3/2arrow 2s1/2, 3d3/2arrow 2p3/2, and 3d5/2arrow 2p3/2 transitions, the synthesis code MEKAL underestimates the emissivity in coronal equilibrium by ~ 20% at temperatures near where the ion abundance peaks. In situations where the ionization balance is not solely determined by the electron temperature, RE and DR satellites may contribute a considerable fraction of the line emission.
Work at Lawrence Livermore National Laboratory was performed under the auspices of the US Department of Energy under contract No. W-7405-ENG-48. This program is supported by a NASA High Energy Astrophysics X-Ray Astronomy Research and Analysis grant NAGW-4185 (Columbia University) and work order W-19127 (LLNL). The work at Kansas State University is supported by Division of Chemical Science, Office of Basic Energy Research, U.S. Department of Energy.
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