We present absolute measurements of radiative and Auger decay rates of K-shell vacancies in highly charged iron ions. The ions were resonantly excited with monochromatic x rays from the PETRA III synchrotron source. By measuring x-ray fluorescence and Auger decay simultaneously, absolute transition rates could be determined independently of most experimental parameters. The results confirm theoretical calculations, which are essential to model the photoexcited plasmas in x-ray binary stars and active galactic nuclei.

For over a decade, the x-ray astrophysics community has enjoyed a fruitful epoch of discovery largely as a result of the successful launch and operation of the high resolution, high sensitivity spectrometers on board the Chandra, XMM-Newton and Suzaku x-ray observatories. With the launch of the x-ray calorimeter spectrometer on the Astro-H x-ray observatory in 2014, the diagnostic power of high resolution spectroscopy will be extended to some of the hottest, largest and most exotic objects in our Universe. The diagnostic utility of these spectrometers is directly coupled to, and often limited by, our understanding of the x-ray production mechanisms associated with the highly charged ions present in the astrophysical source. To provide reliable benchmarks of theoretical calculations and to address specific problems facing the x-ray astrophysics community, electron beam ion traps have been used in laboratory astrophysics experiments to study the x-ray signatures of highly charged ions. A brief overview of the EBIT-I electron beam ion trap operated at Lawrence Livermore National Laboratory and the Max-Planck-Institut für Kernphysik's FLASH-EBIT operated at third and fourth generation advanced light sources, including a discussion of some of the results are presented.

Photoabsorption by and fluorescence of the Kα transitions in highly charged iron ions are essential mechanisms for x-ray radiation transfer in astrophysical environments. We study photoabsorption due to the main Kα transitions in highly charged iron ions from heliumlike to fluorinelike (Fe24+ to Fe17+) using monochromatic x rays around 6.6 keV at the PETRA III synchrotron photon source. Natural linewidths were determined with hitherto unattained accuracy. The observed transitions are of particular interest for the understanding of photoexcited plasmas found in x-ray binary stars and active galactic nuclei.

We review our recent high resolution experiments on photoabsorption by Fe14+ [M. C. Simon, et al., Phys. Rev. Lett. 105, 183001 (2010)], Fe15+, and Ar12+ [M. C. Simon, et al., J. Phys. B-At. Mol. Opt. Phys. 43, 065003 (2010)] at photon energies up to 1 keV. These ions play an essential role in photoionized astrophysical plasmas. Diagnostics of X-ray binary systems rely heavily on precise identification and knowledge of absorption lines. Novel experiments using an electron beam ion trap, FLASH EBIT, in combination with monochromatic synchrotron radiation allow us to investigate ions in charge states hitherto out of reach. Trapped ions can be prepared in any charge state at target densities sufficient to measure absorption cross sections below 0.1 Mb. This results in benchmark state-of-the-art predictions of the transitions wavelengths, widths, and absolute cross sections.

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 3.6σ 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. 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 Fe16+ 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 3.6σ 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 (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.

We present a first laser spectroscopy experiment in the keV energy regime, performed at the Free-Electron Laser LCLS at Stanford. An electron beam ion trap was used to provide a target of highly charged O, F and Fe ions. The resonant fluorescence spectra obtained for various transitions were calibrated to simultaneously measured Lyman lines of hydrogenic ions.