Jan K. Rudolph

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

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

X-Ray Resonant Photoexcitation: Linewidths and Energies of Kα Transitions in Highly Charged Fe Ions

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.

Studies of highly charged iron ions using electron beam ion traps for interpreting astrophysical spectra

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 fur Kernphysiks FLASH-EBIT operated at third and fourth generation advanced light sources, including a discussion of some of the results are presented.

X-ray laser spectroscopy with an electron beam ion trap at the free electron laser LCLS

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.

MaxQuant goes Linux

To the Editor: We report a Linux version of MaxQuant1 (http://www.biochem. mpg.de/5111795/maxquant), our popular software platform for the analysis of shotgun proteomics data. One of our main intentions in developing MaxQuant was to ‘take the pain out of ’ quantifying large collections of protein profiles2. However, unlike, for instance, the Trans-Proteomic Pipeline3, the original version of MaxQuant could be run only on Microsoft Windows, and thus its use was restricted in high-performance computing environments, which very rarely use Windows as an operating system. When we began developing MaxQuant, Windows was the only operating system supported by vendor-provided raw data access libraries. Therefore, we wrote MaxQuant in the C# programming language on top of the Windows-only .NET framework. Windows support for cloud platforms is more expensive, and the operating system is harder to use and less scalable compared with Linux. We recently carried out a major restructuring of the MaxQuant codebase, and we made it compatible with Mono (https://www.mono-project.com/), an alternative cross-platform implementation of the .NET framework. Furthermore, we now provide an entry point to MaxQuant from the command line without the need to start its graphical user interface, which allows execution from scripts or other processing tools. Meanwhile, Thermo Fisher Scientific has released its platform-independent and Monocompatible implementation of its raw data access library (http://planetorbitrap.com/ rawfilereader), and hopefully more vendors will follow soon. Together, this leads to a situation in which large-scale computing of proteomics data with MaxQuant becomes feasible on all common platforms. When we parallelized the MaxQuant workflow over only a few central processing unit (CPU) cores, we hardly noticed a difference in performance between Linux and Windows (Fig. 1). However, in benchmarking of a highly parallelized MaxQuant run on 120 logical cores, we observed that the Linux version showed highly superior parallelization performance, with speed 64% faster than that observed under a Windows server operating system using identical hardware. MaxQuant uses operating system processes, rather than the intrinsic multi-threading mechanism of C#, to realize parallel execution, and it manages the load-balancing of an arbitrarily large set of raw data files over a specified number of processors by itself. We hypothesize that this allows Linux to optimize parallel execution to the high extent that we observed. A larger benchmark study is under way, in which we will investigate the dependence of the increased speed on hardware such as, for instance, the type of CPU and storage systems. MaxQuant has already been adapted in several forms for cloud and highperformance computing applications, as described, for instance, by Judson et al.4 and on the Chorus platform (https://chorusproject.org). We expect that the number of applications will increase with our Linux-compatible MaxQuant version. We envision that proteomics core facilities, for instance, will benefit from the combination of command-line access and Linux compatibility, which enables standardized high-throughput data analysis. The MaxQuant code base is identical for Windows and for Linux; thus there is only a single distributable running on both operating systems, which can be downloaded from http://www.maxquant. org (version 1.6.1.0). MaxQuant is freeware, and contributions to new functionality are collaboration-based. The code of open source parts is available at https://github. com/JurgenCox/compbio-base. ❐

Absolute radiative and Auger transition rates of K-shell excited few-electron iron ions

The iron Kα lines are among the most prominent features in the x-ray spectra of many celestial sources. However, transition rates used to model these spectra were only known by theoretical calculations. Here we present an absolute measurement of iron K-shell transition rates for Li-like to C-like iron ions. These ions were created with an electron beam ion trap and excited with x rays from the PETRA III synchrotron source. Measuring x-ray fluorescence and Auger decay simultaneously allows to determine absolute decay rates independent of most experimental parameters.

Measurement of the angular distribution of Dielectronic Recombination into highly charged Krypton ions

Angular distribution of x-rays emitted in the process of Dielectronic Recombination (DR) was studied at the Electron Beam Ion Trap. For this the photon emission spectra were observed along and perpendicular the electron beam propagation direction. X-ray line intensities differ drastically between the two acquired spectra. This indicates a strong alignment of the total angular momentum vector of the excited states populated by DR with respect to the electron beam propagation direction.

Electron-impact ionization of 4d-shell xenon and tin ions

Electron-impact single ionization of xenon and tin ions for charge states where the 4d-subshell is the outermost has been investigated. Measured cross sections have been analyzed in detail by comparing with configuration-averaged distorted wave (CAWD) calculations. Contributions of different direct- and indirect ionization processes have thus been quantitatively revealed.

Photoionizing Trapped Highly Charged Ions with Synchrotron Radiation

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.

Electron-impact single and double ionization of tin ions

Motivated by the requirements for modeling EUV light sources for semiconductor lithography, a systematic study of electron-impact ionization cross sections of Snq+ ions (q = 1, ..., 13) in the energy range up to 1000 eV was performed. Detailed analysis of all in all 25 measured cross-section functions revealed strong contributions of indirect ionization mechanisms which need to be taken into account in EUV-source plasma modeling.