1. Astrophysical Priorities for Accurate X-ray Spectroscopic Diagnostics Nancy S. Brickhouse Harvard-Smithsonian Center for Astrophysics In Collaboration with Randall K. Smith Acknowledgments to Guo-Xin Chen, Svetlana Kotochigova and Kate Kirby ITAMP Workshop High Accuracy Atomic Physics in Astronomy Harvard-Smithsonian Center for Astrophysics 8 Aug 2006

2. Introduction Case Studies from X-ray Spectroscopy – Fe XVII 3C/3D – Ne IX density and temperature diagnostics – Fe XVIII and XIX temperature diagnostics Conclusions Outline

3. Overview: X-Ray Spectroscopy High Resolution –λ/Δλ ~ 1000 from gratings, compared with CCD λ/Δλ ~ 10 - 50 –Strong lines of H- and He-like ions and Fe L-shell –Most line profiles unresolved Spectral models –Collisionally ionized plasmas: stellar coronae, SNR, galaxies, clusters of galaxies –Photoionized plasmas: X-ray binaries, AGN, planetary nebulae

4. Benchmarking the ATOMDB ATOMDB (http://cxc.harvard.edu/atomdb)http://cxc.harvard.edu/atomdb - Astrophysical Plasma Emission Database (APED) input atomic data - Output collisional ionization models from the Astrophysical Plasma Emission Code (APEC) http://cxc.harvard.edu/atomdb/WebGUIDE (Smith et al. 2001) Emission Line Project Goal to use the Chandra calibration data to benchmark the collisional models What accuracy do we need and why?

5. Physical Conditions Determined from X-ray Spectroscopy Electron Temperature and Temperature Distribution Electron Density Elemental Abundances - Relative - Absolute (lines/continuum) Opacity Charge State in Time-Dependent (Non- Equilibrium Ionization) Plasma Yohkoh We really want to understand physical processes: e.g. coronal heating, shocks, accretion, winds

6. Fe XVII “3C/3D” In general, neon-like Fe XVII is formed over a very broad temperature range. We observe Fe XVII lines in most stellar coronal spectra. In solar active regions, it is formed near the peak temperature and thus produces very strong emission lines. The solar 3C line has long been thought to be “resonance scattered” (gf =2.7) in the solar corona. 3C 2s 2 2p 6 1 S 0 - 2s 2 2p 5 3d( 2 P) 1 P 1 λ15.014 3D 2s 2 2p 6 1 S 0 - 2s 2 2p 5 3d( 2 P) 3 D 1 λ15.261 TRACE Image in Fe IX

7. Solution[s] to the Long-Standing Fe XVII 3C/3D Problem Fe XVII 3C -- -- Fe XVII 3D -- Fe XVI -- O VIII Ly γ Anomalously low 3C/3D line ratios in solar active regions from resonance scattering? ( Rugge & McKenzie 1985) τ ~ 2.0 ( Schmelz et al. 1997 ) Sample Data from Solar Maximum Mission FCS Brickhouse & Schmelz 2006

8. Recent Results 3D is blended w/ inner shell Fe XVI Brown et al. 2001 Experiment: Laming et al. 2001; Brown et al. 1998 Theory: Chen & Pradhan 2002; Doron & Behar 2002; Loch et al. 2006; Gu 2003 → still ~15% higher than lab Chen 2006 → 5-10% (also Chen et al. 2006, PRA on Ni XIX) For same observed ratio, optical depth depends on predicted value: 3C/3D = 4.20 → τ = 0.42 3C/3D = 3.30 → τ = 0.17 3C/3D = 2.85 → τ = 0.032 The 3C line is optically thin in solar active regions! Brickhouse & Schmelz 2006

9. Therefore, the TRACE Fe XV line is not optically thick either! Fe IX Fe XII Fe XV Active Region Prominence Brickhouse & Schmelz 2006

11. Ne IX “R-ratio” and “G-ratio” Classic He-like diagnostics “R-ratio” = f/i is density-sensitive. “G-ratio” = (f + i)/r is temperature- sensitive. f = forbidden 1s 2 1 S 0 - 1s2s 3 S 1 i = intercombination 1s 2 1 S 0 - 1s2p 3 P 2 1s 2 1 S 0 - 1s2p 3 P 1 r = resonance 1s 2 1 S 0 - 1s2p 1 P 1

12. Capella Ne IX Spectral Region Temperature from Ne IX G-ratio is too low Ness et al. 2003 Mg XI and O VII also give temperatures too low in other stars Testa et al. 2004

13. Blending with Fe XIX in the Ne IX Spectral Region Model Fe XIX wavelengths from HULLAC (1% accuracy) With EBIT λ measurements (Brown et al. 2002, 5-10 mÅ)

14. Fe XIX Model Wavelengths from Dirac-Fock-Sturm Method Kotochigova, in progress With this Fe XIX model we can match the positions of all features in the spectrum.

15. Recent Results Derived T from Capella in better agreement G-ratio agrees with LLNL EBIT measurements of Wargelin (PhD Thesis 1993) New Ne IX G-ratio calculations (Chen et al. 2006, PRA)

16. Fe XVIII and XIX Line Ratios LETG 355 ks HETG/HEG 300 ks HETG/MEG 300 ks LETG 355 ks -- Fe XVIII I EUV Ω EUV [Te] —— = ———— exp (-ΔE/kTe) I X-ray Ω X-ray [Te]

17. Observed/Predicted Line Ratios Desai et al. 2005 All X-ray/EUV line ratios are larger than predicted (by all codes). For the strongest lines, the codes agree: discrepancies are 30% for Fe XVIII and a factor of 2 for Fe XIX. Predictions are based on the EMD with its peak at 6 MK. X-ray Fe XVIII Fe XIX EUV FUV

18. T e -Dependence of Fe XVIII and XIX Line Ratios 6 MK EMD peak Discrepancies not from: –excitation rate uncertainties –calibration uncertainties –absorption –time variability Simple T e diagnostics not consistent with the ionization state of the plasma Motivated consideration of time-dependent NEI effects in impulsively heated loops.

19. Non-Equilibrium Ionization ? EMD models assume collisional ionization equilibrium: Flux ~ ε(T e ) ∫N e 2 dV In an NEI plasma, the charge state lags the instantaneous temperature T e N e Δt determines the charge state For a given N e and T e, ionization is very fast compared with recombination Mass conservation (N e dV = const) implies that a coronal loop, impulsively heated and then cooled by radiation and conduction, will emit primarily during recombination.

20. Additional data from other stars Courtesy P. Desai Fe XVII / Fe XVIII Fe XIX / Fe XVIII

21. Ionization Balance? Recombination rate coefficients accurate to ~30% Ionization rate coefficients? Decreased ionization rate x 2

22. Conclusions Accurate atomic data are a big investment: priorities should be based on astrophysical importance and needs For most important diagnostics, line ratios accurate to 10% or better are possible Interesting astrophysical processes can be explored (e.g. non-equilibrium ionization) with accurate diagnostics Wavelengths need to be accurate to 1 mÅ 3-way collaboration among astrophysics, experiment and theory is needed Experiments can’t substitute for theory