A laboratory study of Fe XVII x-ray excitation cross sections using the LLNL EBIT facility and NASA/GSFC AstroE microcalorimeters

The mixing of atomic and macroscopic processes taking place in non-terrestrial objects creates complex, dynamic, and intriguing environments. High-resolution x-ray spectra from these sources measured by satellites such as the Chandra X-ray Observatory, XMM-Newton, and the Solar Maximum Mission provide a means for understanding the physics governing these sources. Laboratory measurements of the atomic processes have proved crucial to the interpretation of these spectra. For example, using the LLNL electron beam ion traps EBIT-I & EBHIT-II a detailed study of the x-ray spectrum of Fe xvii has been conducted addressing the large ratio predicted by theory compared to observations for the relative intensity of the 2p-3d ¹P₁ resonance to ³D₁ intercombination line. The difference was often attributed to opacity effects. However, laboratory measurements in the optically thin limit agree with observations demonstrating that the prediction is too large. The laboratory results thus provide a benchmark in the optically thin limit for accurate estimates of opacity effects. To uncover the source of the discrepancy between theory and observation, we have performed a series of experiments that successively uncovered more details about the Fe XVII lines produced in coronal plasmas. Most recently, we used NASA's 32 channel array microcalorimeter from the Astro-E x-ray satellite program to measure the excitation cross section of various Fe XVII lines in the laboratory. Our results show that the theoretically predicted cross section for the resonance line is too large while the value for the intercombination line is correct. These measurements resolve long-standing issues thought to be associated with non-equilibrium processes.

Work at LLNL was completed under the auspices of the U.S. D.o.E by the University of California Lawrence Livermore National Laboratory under contract W-7405-Eng-48 and supported by NASA's Astronomy and Physics Research and Analysis Program under work order S-06553-G