Lawrence Livermore National Laboratory

Gregory Brown (16-ERD-020)

Executive Summary

This project will provide high-accuracy atomic-parameter measurements of highly charged ions. The data will be used to benchmark models being used to study extrasolar and terrestrial sources and to test models of fundamental atomic physics parameters, such as transition energies and excitation cross sections of multi-electron ions of iron-group elements, used by the astrophysics and nuclear security communities.

Project Description

One of the most rapidly advancing fields of astrophysics is x-ray astronomy. The last decade has seen profound growth in discoveries resulting from the successful operation of orbiting x-ray observatories including the Chandra X-Ray Observatory, XMM-Newton (X-Ray Multi-Mirror Mission), and NuSTAR (Nuclear Spectroscopic Telescope Array). The Japanese Aerospace Exploration Agency's ASTRO-H, also known as Hitomi, orbiting observatory will help decipher the x-ray emission from sources observed by NuSTAR and will extend high-resolution spectroscopy beyond the limits of Chandra and XMM-Newton. At the center of Hitomi's instrument suite is the soft x-ray spectrometer, which will provide the first-ever high-resolution, high signal-to-noise spectra of some of the most exotic, most energetic sources in the universe, including black holes, neutron stars, active galactic nuclei, high-mass x-ray binaries, and supernova remnants. Decoding the high-resolution spectra requires accurate, complete sets of atomic data, most of which are generated using highly sophisticated atomic calculations. Unfortunately, most of the calculated data has not been tested experimentally in a systematic way. The lack of complete, systematic experimental benchmark measurements in the case of multi-electron ions limits and often precludes accurate interpretation of high-resolution spectra. We intend to use Livermore Laboratory's EBIT-I electron-beam ion trap to make anchor measurements of the x-ray emission from multi-electron ions of iron-group elements (elements from chromium to nickel in the periodic table). These data will be used to decode the x-ray emissions from exotic extra-solar sources measured with instruments on orbiting x-ray observatories.

Accurate, complete sets of atomic data are required to fully utilize the high-quality, high-resolution spectra of the soft x-ray spectrometer. The new physics accessible by the soft x-ray spectrometer will make it possible to reveal the large-scale structure of the universe by observing clusters of galaxies. Additionally, the soft x-ray spectrometer will be able to detect the ejecta patterns and presence of rare elements such as chromium, manganese, and nickel in supernova remnants, providing constraints on its explosion mechanism. We expect to aid this effort by providing high-accuracy atomic-parameter measurements of highly charged ions, which are necessary to better understand high-resolution x-ray spectra measured from celestial sources, at Livermore’s EBIT-I facility. Its capability as an astrophysics facility provides a unique resource for assessing atomic data and addressing spectroscopic issues. The data gained from our work will be used to benchmark models being used to study extrasolar as well as terrestrial sources. The results will be used to test fundamental atomic physics parameters, such as transition energies and excitation cross sections, of multi-electron ions of iron-group elements found in Lawrence Livermore models, as well as those used by the astrophysics community.

Mission Relevance

The production of anchor measurements that can be used by the x-ray astrophysics community contributes to the Laboratory's space security mission research challenge. The benchmark data generated by this project will also be valuable in testing models being used to study the interiors of hohlraum target capsules and multi-electron high-atomic-number ions, enhancing the Laboratory's core competency in nuclear weapons science and supporting DOE's goal in nuclear security to strengthen key science, technology, and engineering capabilities and modernize the national security infrastructure.

FY17 Accomplishments and Results

In FY17 we (1) completed the steady-state measurements of cross sections of K-shell (closest shell to the nucleus) transitions in highly charged iron ions; (2) began cross section measurements of K-shell transitions in highly charged manganese and cobalt ions; (3) analyzed Hitomi x-ray spectrum of the Perseus Cluster; (4) analyzed data from NuStar and from Livermore's EBIT facility and used it to help interpret the Perseus spectrum; (5) prepared requests for beam time at the PETRA III advanced synchrotron light source in Hamburg, Germany, so that fluorescence measurements can be made; and (6) participated in the European Space Agency's Athena (Advanced Telescope for High-Energy Astrophysics) spatially resolved x-ray spectroscopy science working group.

Figure 1.
We used observations of the Perseus Cluster performed with the Soft X-ray Spectrometer (SXS) on board the Hitomi X-ray Observatory to detect the resonance emission from chromium, manganese, and nickel. The measurements, combined with the latest atomic models, revealed that these elements have near-solar abundance with respect to iron, in contrast to previous x-ray measurements of cluster abundances. Comparison between our results and modern nucleosynthesis calculations for type Ia supernovae disfavors supernovae Ia progenitors being exclusively white dwarfs with mass well below the Chandrasekhar limit (MCh). The observed abundance pattern of the iron-peak elements can naturally be explained when the combination of near-MCh and sub-MCh supernovae Ia is allowed, adding to the mounting evidence that both progenitor types make a significant contribution to cosmic chemical enrichment. Laboratory measurements at Lawrence Livermore are providing stringent tests of the models and fundamental atomic physics parameters related to the strengths of the chromium and manganese line emission. Emission from manganese is also used to determine the temperature of the hohlraum plasma at the National Ignition Facility. (a) The spectrum (black) in the 1.89.0 keV band modeled with an optically thin thermal plasma based on the atomic code AtomDB (red). The emission from NGC1275 (active galactic nuclei) is indicated by the gray curve. (b) The zoom-in spectrum in the 5.3–6.4 keV band, where the emission from helium-like chromium and manganese are detected. The red-shifted iron-I fluorescence from the active galactic nuclei is resolved as well. (c) The same in the 7.48.0 keV band, highlighting the nickel-XXVII resonance (w) line clearly separated from the stronger iron-XXV He-beta line and other emissions. This enables the first accurate measurement of the nickel abundance in a galaxy cluster. For comparison, an XMM-Newton spectrum extracted from the same spatial region is shown as the blue data points.

Publications and Presentations

Hell, N. 2017. “Benchmarking Transition Energies and Emission Strengths for X-ray Astrophysics with Measurements at the Livermore EBITs.” PhD thesis, Friedrich-Alexander-Universität Erlangen-Nürnberg. LLNL-TH-727717.

Hitomi Collaboration. 2017.Hitomi Constraints on the 3.5 keV Line in the Perseus Cluster," Astrophys. J. Lett. 837 (1):L15. doi:org/10.3847/2041-8213/aa61fa. LLNL-JRNL-731764.

——— 2017. “Measurements of Resonant Scattering in the Perseus Cluster Core with Hitomi SXS," Publ. Astron. Soc. Jpn. (forthcoming). LLNL-JRNL-740200.

——— 2017. “Solar Abundance Ratios of the Iron-Peak Elements in the Perseus Cluster," Nature (forthcoming). LLNL-JRNL-740598.