An Experimental Program to Achieve High Accuracy Atomic Rates for the X-ray Astrophysics Recovery Mission and Beyond

We propose an experimental program to generate high accuracy atomic data for the Fe XXV and Fe XXVI emission diagnostics. Theoretical calculations of specific rate coefficients have estimated accuracies ranging from 5 to 30%. The individual uncertainties may translate into uncertainties on the measurement of interest, e.g. electron temperature, of 30%. EBIT measurements will test the existing theoretical calculations, both the line emission and the individual atomic rates for various processes, including excitation, recombination and ionization. We also measure the spectra of H- and He-like Ar for comparison to theory. For Ar the resonance line series will provide another benchmark for the atomic physics data. We will conduct the experiments at electron beam ion traps (EBITs) at NIST and Clemson University. We will explore in detail broad ranges in parameter space for which the accuracy of the atomic data might be low, e.g. near the threshold where resonances contribute to the collision strength. The EBIT measurements will help set priorities for future theoretical calculations. Our program will use the NIST EBIT combined with a pair of crystal spectrometers to (a) measure diagnostic line ratios over a range in parameter space; (b) conduct detailed experiments to assess the accuracy of rates for individual atomic processes; and, (c) evaluate the existing theory and determine its accuracy for input to AtomDB. We will use the EBIT at Clemson to extract and recombining ions to test recombination rates. We expect to make relatively rapid progress by regularly communicating the results from our experiments and assessments to the atomic physics and astrophysics communities. Cosmology using clusters of galaxies relies on a highly accurate measurement of the electron temperature. With current CCD spectra, the shape of the high energy continuum, dominanted by bremsstrahlung, provides the best temperature determination. The microcalorimeter on the X-ray Astrophysics Recovery Mission, as demonstrated by Hitomi, will easily determine the ratio of the fluxes of the strong allowed transitions of Fe XXV and Fe XXVI, as well as for the temperature-sensitive line ratio of the He-like Fe XXV for high signal-to-noise cluster data. Improvements to the accuracy of these diagnostics will significantly reduce the systematics from cluster cosmology. These data will also be of broad use for other sources as well. Our team will make improved atomic data available to the astronomy community in a timely manner through AtomDB. More »