High Resolution Photoexcitation Measurements Exacerbate the Long-Standing Fe XVII Oscillator Strength Problem
Steffen Kühn, Chintan Shah, J. R. Crespo López-Urrutia, Keisuke Fujii, René Steinbrügge, Jakob Stierhof, Moto Togawa, Zoltán Harman, Natalia S. Oreshkina, Charles Cheung, M. G. Kozlov, S. G. Porsev, M. S. Safronova, J. C. Berengut, Michael Rosner, M. Bissinger, Ralf Ballhausen, Natalie Hell, S. Park, М. Chung, Moritz Hoesch, Jörn Seltmann, A. Surzhykov, V. A. Yerokhin, J. Wilms, F. S. Porter, Th. Stöhlker, Christoph H. Keitel, Thomas Pfeifer, G. V. Brown, Maurice A. Leutenegger, Sven Bernitt
Abstract
For more than 40 years, most astrophysical observations and laboratory studies of two key soft x-ray diagnostic 2p-3d transitions, 3C and 3D, in Fe XVII ions found oscillator strength ratios f(3C)/f(3D) disagreeing with theory, but uncertainties had precluded definitive statements on this much studied conundrum. Here, we resonantly excite these lines using synchrotron radiation at PETRA III, and reach, at a millionfold lower photon intensities, a 10 times higher spectral resolution, and 3 times smaller uncertainty than earlier work. Our final result of f(3C)/f(3D)=3.09(8)(6) supports many of the earlier clean astrophysical and laboratory observations, while departing by five sigmas from our own newest large-scale ab initio calculations, and excluding all proposed explanations, including those invoking nonlinear effects and population transfers.