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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

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Astrophysical Priorities for Accurate X-ray Spectroscopic DiagnosticsNancy S. BrickhouseHarvard-Smithsonian Center for AstrophysicsIn Collaboration with Randall K. SmithAcknowledgments 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

  • Introduction
  • Case Studies from X-ray Spectroscopy
    • Fe XVII 3C/3D
    • Ne IX density and temperature diagnostics
    • Fe XVIII and XIX temperature diagnostics
  • Conclusions
overview x ray spectroscopy
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

benchmarking the atomdb
Benchmarking the ATOMDB
  • ATOMDB (

- Astrophysical Plasma Emission Database (APED) input atomic data

- Output collisional ionization models from the Astrophysical Plasma Emission Code (APEC)

(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?
physical conditions determined from x ray spectroscopy
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


We really want to understand physical processes:

e.g. coronal heating, shocks, accretion, winds

fe xvii 3c 3d
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 2s2 2p61S0 - 2s2 2p5 3d(2P) 1P1λ15.014

3D 2s2 2p61S0 - 2s2 2p5 3d(2P) 3D1λ15.261

TRACE Image in Fe IX

solution s to the long standing fe xvii 3c 3d problem
Solution[s] to the Long-Standing Fe XVII 3C/3D Problem
  • Anomalously low 3C/3D line ratios in solar active regions from resonancescattering? (Rugge & McKenzie 1985)
  • τ ~ 2.0(Schmelz et al. 1997)

Fe XVII 3C --

-- O VIII Ly γ

-- Fe XVI

-- Fe XVII 3D

Sample Data from Solar Maximum Mission FCS

Brickhouse & Schmelz 2006

recent results
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

therefore the trace fe xv line is not optically thick either
Therefore, the TRACE Fe XV line is not optically thick either!





Brickhouse & Schmelz 2006

ne ix r ratio and g ratio
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 = forbidden1s21S0 - 1s2s 3S1

i = intercombination1s21S0 - 1s2p 3P2

1s21S0 - 1s2p 3P1

r = resonance1s21S0 - 1s2p 1P1

capella ne ix spectral region
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
blending with fe xix in the ne ix spectral region
Blending with Fe XIX in the Ne IX Spectral Region

Model Fe XIX wavelengths

from HULLAC (1% accuracy)

With EBIT λmeasurements

(Brown et al. 2002, 5-10 mÅ)

fe xix model wavelengths from dirac fock sturm method kotochigova in progress
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.

recent results15
Recent Results

New Ne IX G-ratio calculations (Chen et al. 2006, PRA)

G-ratio agrees with LLNL EBIT

measurements of Wargelin

(PhD Thesis 1993)

Derived T from Capella

in better agreement

fe xviii and xix line ratios
Fe XVIII and XIX Line Ratios


LETG 355 ks

LETG 355 ks



300 ks


300 ks


—— = ———— exp (-ΔE/kTe)


observed predicted line ratios
Observed/Predicted Line Ratios

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.






Desai et al. 2005

t e dependence of fe xviii and xix line ratios
Te-Dependence of Fe XVIII and XIX Line Ratios
  • Discrepancies not from:
    • excitation rate


    • calibration uncertainties
    • absorption
    • time variability
  • Simple Te diagnostics not
  • consistent with the ionization
  • state of the plasma
  • Motivated consideration of
  • time-dependent NEI effects in
  • impulsively heated loops.

6 MK EMD peak

non equilibrium ionization
Non-Equilibrium Ionization ?
  • EMD models assume collisional ionization equilibrium:

Flux ~ ε(Te) ∫Ne2 dV

  • In an NEI plasma, the charge state lags the instantaneous temperature Te
  • NeΔt determines the charge state
  • For a given Ne and Te , ionization is very fast compared with recombination
  • Mass conservation (Ne dV = const) implies that a coronal loop, impulsively heated and then cooled by radiation and conduction, will emit primarily during recombination.


Additional data from

other stars

Courtesy P. Desai

ionization balance
Ionization Balance?

Decreased ionization rate x 2

  • Recombination rate coefficients accurate to ~30%
  • Ionization rate coefficients?
  • 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