Gregory Brown (16-ERD-020)
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'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, including Hitomi.
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 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 LLNL models, as well as those used by the astrophysics community.
The production of anchor measurements that can be used by the x-ray astrophysics community contributes to the LLNL cyber security, space, and intelligence strategic focus area. 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 strategic focus area in stockpile stewardship science as well as the spectroscopy effort at Livermore’s National Ignition Facility.
FY16 Accomplishments and Results
In FY16 we (1) began measurements of electron-impact excitation cross sections of the iron electron K-shell (closest shell to the nucleus) transitions using LLNL's EBIT-I electron-beam ion trap, including measurements from both K-shell ions and the more challenging L-shell ions; (2) began training as EBIT operators to participate independently in laboratory astrophysics experiments; (3) analyzed data measured using the NuSTAR satellite; and (4) participated in the check-out phase of the Hitomi x-ray observatory, launched February 17, 2016, and attended a Hitomi flight software workshop and science working group meeting. Unfortunately, after a successful launch, the Hitomi satellite tumbled out of control five weeks later. Therefore, we are shifting our focus to the European Space Agency's Athena (Advanced Telescope for High Energy Astrophysics) x-ray observatory.
Publications and Presentations
- Hell, N., et al., "Laboratory measurements of the K-shell transition energies in L- shell ions of Si and S." Astrophys. J. 830(1), 26 (2016). LLNL-JRNL-688670. http://dx.doi.org/10.3847/0004-637X/830/1/26
- Hitomi Collaboration, "The quiescent intracluster medium in the core of the Perseus cluster." Nature 535, 117 (2016). LLNL-JRNL-696939. http://dx.doi.org/10.1038/nature18627