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 WorkshopHigh Accuracy Atomic Physics in Astronomy

Harvard-Smithsonian Center for Astrophysics8 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

Outline

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• 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?

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

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 2p6 1S0 - 2s2 2p5 3d(2P) 1P1 λ15.014

3D 2s2 2p6 1S0 - 2s2 2p5 3d(2P) 3D1 λ15.261

TRACE Image in Fe IX

Solution[s] to the Long-Standing Fe XVII 3C/3D Problem

Fe XVII 3C --

-- Fe XVII 3D--

Fe

XV

I

-- O

VII

I L

y 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

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!

Fe IX Fe XII Fe XV

Active Region

Prominence

Brickhouse & Schmelz 2006

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 1s2 1S0 - 1s2s 3S1

i = intercombination 1s2 1S0 - 1s2p 3P2

1s2 1S0 - 1s2p 3P1

r = resonance 1s2 1S0 - 1s2p 1P1

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

Model Fe XIX wavelengths from HULLAC (1% accuracy)

With EBIT λ measurements

(Brown et al. 2002, 5-10 mÅ)

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 Results

Derived T from Capella in better agreement

G-ratio agrees with LLNL EBITmeasurements of Wargelin

(PhD Thesis 1993)

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

Fe XVIII and XIX Line Ratios

LETG 355 ks

HETG/HEG 300 ks

HETG/MEG 300 ks

LETG 355 ks

-- Fe XVIII

-- Fe XVIII

IEUV ΩEUV [Te]—— = ———— exp (-ΔE/kTe)IX-ray ΩX-ray[Te]

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

Te-Dependence of Fe XVIII and XIX Line Ratios

6 MK EMD peak

•Discrepancies not from: –excitation rate uncertainties –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.

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

Fe XVII / Fe XVIII

Fe XIX / Fe XVIII

Ionization Balance?

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

Decreased ionization rate x 2

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