Talk given by E. Träbert, P. Beiersdorfer , J. Clementson at the 17th International Conference on Atomic Processes in Plasmas, Belfast, UK, 19-22 July 2011.

1. Stellar and Laboratory XUV/EUV Line Ratios in
Fe XVIII and Fe XIX
Elmar Träbert #, Peter Beiersdorfer, Joel H. T. Clementson
Lawrence Livermore National Laboratory, Livermore CA, USA
# also at AIRUB, Bochum, Germany
NASA project funding APiP 21 July 2011
Chandra spacecraft observes Capella
Comparison of observed XUV (13-18 Å), EUV (90-120 Å), VUV (1100 Å) line intensities
with predictions by the APEC model
XUV appears brighter than expected
- APEC model incorrect or
- Capella peculiar?
Interstellar absorption involved
Laboratory study under way:
Electron beam ion trap, two flat-field spectrometers
Detection efficiency calibration (experimental data)
Modeling of the excitation process (using the FAC Flexible Atomic Code by M. F. Gu)
! ! ! ! (monoenergetic electron beam vs. Maxwellian electron energy distribution)
Density and temperature effects
Possible insights see Poster

2. L62 DESAI ET AL. Vol. 625
Fig. 1. The observed-to-predicted ux ratios of strong lines in the X-ray, Fig. 2. The observed-to-predicted ux ratios of X-ray lines using FAC
EUV, and FUV spectral regions. Shown for comparison are the ratios obtained and APEC. Lines from Table 1 excluding heavily blended Fe xviii 16.004
using the APEC, CHIANTI, and SPEX spectral codes and the FAC rates. The are shown. Note the 3d 2 lines are between 14 and 15 A for Fe xviii and
p
density is Ne 1010 cm 3, except for SPEX. Top: Comparison for Fe xviii shortward of 14 A for Fe xix. Ratios are calculated at Ne 1010 cm 3. Dash-
lines, normalized to 93.92. The X-ray lines plotted here are 14.208, 15.625, dotted lines represent agreement within a factor of 2. Top: Comparison
and 16.071. Bottom: Comparison for Fe xix lines, normalized to 108.37. for Fe xviii, normalized to 14.208. There are no published FAC models for
The X-ray lines plotted are 13.518, 14.664, and 15.079. Fe xviii 4d 2 lines around 11.4 A. Bottom: Comparison for Fe xix, nor-
p
malized to 13.518.
Observed flux (Capella) / predicted flux as presented by Desai et al., ApJ 625, L59 (2005)
Watch out for log scales!

3. Transmission for Capella
0.8
0.6
0.4
Interstellar extinction:
0.2
8% loss of EUV signal vs. XUV signal
50 100 150 200 250 300 350 400
o
Wavelength (A)

4. Fe XIX O-like Fe XVIII F-like
2s2 2p3 nl
2s2 2p4 nl
XUV XUV
2s 2p 5
EUV 2s 2p 6
E2
EUV
2s2 2p 4 M1 M1 VUV 2s2 2p 5 M1 VUV

5. The Livermore electron beam
ion trap is the archetypical EBIT
Electron collector
Several layers of cooling and cryogenic
shields at the temperatures of liquid He
and liquid N2
Superconducting magnets
(pair of Helmholtz coils, B = 3 T)
Drift tubes at electrostatic potentials trap
ions axially; with openings for optical access
Electron gun
20 Years of
Spectroscopy
E
LA BI T
LIVERMORE
since
1986

6. Electron energy
Ionization potential of Fe ions
2500
2000
Electron beam
1500
IP (eV)
1000
500
0
0 5 10 15 20 25 Electron beam energy relative
to IP determines the highest
Charge state q+
charge state present.

7. G. V. Brown et al., Astrophys. J.
Suppl. Ser. 140, 588 (2002)
(Fe XVIII - Fe XXIV in an EBIT)

8. What to expect in a spectrum?
Atomic structure
Element abundance
Collisional excitation f(T, n)
Radiative de-excitation
Photoionization and -excitation
--> Ionization balance f(T), spectral intensity distribution, "emissivity"
Data bases
Kelly & Palumbo, CHIANTI, NIST ASD, Mewe/Kaastra/Liedahl, ...
tend to be grossly incomplete, not up to date, sometimes faulty - but eventually improving
Modeling
HULLAC ! Hebrew University Lawrence Livermore Atomic Code
APEC ! Astrophysical Plasma Emission Code
FAC ! Flexible Atomic Code (M. F. Gu)
produce thousands of levels and tens of thousands of transitions
... need benchmarking
(testing some testable parameters such as key level energies and some transition rates)

9. 86 KOTOCHIGO
700
Fe XIX (HULLAC98)
Capella
600
500
400
Counts
Fe XIX (Lab)
300
200
Fe XIX (Kotochigova)
100
0
13.4 13.5 13.6 13.7 13.8
Wavelength (Angstroms)
Figure 1. Chandra spectrum of Capella (black line) in the spectral region
between 13.4 and 13.8 ¯ (Desai et al. 2005) shown in comparison with three
spectral models. The three models for Fe xix (in magenta) use data from the
APEC code v1.3 (Smith et al. 2001) with only the wavelengths changed. Ne ix
(dark blue) and other Fe L-shell (light blue) lines in the region are shaded
for the observed spectrum. Upper panel: model using the Fe xix wavelengths
from HULLAC (D. Liedahl 1997, private communication). Middle panel: Fe xix
wavelengths include the experimentally measured values reported in Brown et al.
(2002). Lower panel: Fe xix wavelengths are from this work and Kotochigova
et al. (2007) using the MDFS method. Adapted from Brickhouse (2007).
S. Kotochigova et al., The Astrophysical Journal Supplement Series, 186:85—93, 2010

10. 5
XUV flux seen is higher
than the model Capella vs. APEC
prediction (tied to EUV) 4 T = 6 MK
Interpretation A: 3
XUV/EUV excess
Flux signal
Interpretation B: 2
XUV underprediction
by APEC (and FAC etc.)
1
0
13 14 15 16 17 18
o
Wavelength (A)

11. Principal
quantum
number n
s p d
3
H α 7 components EUV
2
Ly
β
Calculate line ratio XUV
all n=2-3 vs n=1-3
1 O VIII 102 Å vs 16.0 Å

12. O VII
1000
Spectra of CO2 (mostly oxygen
lines in the regions shown) 800
dispersed with a 1200 l/mm
Counts
grating in a R=5.6 m flat-field 600
grating spectrometer and
recorded with a CCD camera at 400 O VIII
an EBIT
200
0
12 14 16 18 20 22
200
150
Counts
100
O VIII
50
0
80 90 100 110 120 130 140
Wavelength (Å)

13. SFFS CO2
200
150
Counts
100
O VIII
50
0
80 90 100 110 120 130 140
Wavelength (Å)

14. O VII
a
1200
1000
800
Counts
O VIII
600
O VII
O VIII
400 O VII
200
EBIT reference spectrum of oxygen 0
12 14 16 18 20 22 24
EBIT spectrum with Fe (CO)5 injection XVII + XIX
b
XVIII XVII
500
XVIII
XIX XIX
400
XIX
XIX
Counts
300 XIX XVIII
O
XVII O
200
O
Spectra dispersed with an R=44.3 m 2400 l/mm O
100
flat-field grating and recorded with an MCP-
based detector. 0
13 14 15 16 17 18
o
Wavelength (A)

15. SFFS Fe I2
Fe XIX
Fe XVIII
800
600 Fe XIX
Counts
Fe XIX Fe XIX
400
Fe XIX
200
0
80 90 100 110 120 130 140
Wavelength (Å)

16. Fe Plenty of lines in EBIT
Electron beam energy 2 keV - mostly Fe, some O -
500 what about stellar spectra?
400
300
Counts
200
100
0
13 14 15 16 17
o
Wavelength (A)

17. Capella vs APEC & EBIT
2
1.8
1.6 Density
Capella excess ratio dependence
1.4
1.2 APEC
1
0.8
0.6
0.4
Experiment with error bars 90 95 100 105 110 115 120 125
Fe XIX o
Fe XVIII Wavelength (A)
APEC open circles

18. Ratio APEC & EBIT vs. Capella
XUV EBIT and XUV APEC 10
compared to Capella
EBIT Fe XIX
EBIT experiment vastly exceeds
Capella flux - something is wrong in EBIT Fe XVIII
this analysis! --> EBIT has an electron
beam, Capella has a thermal plasma;
need to simulate this difference. 1
APEC falls short of Capella flux - may APEC vs.
be a modeling problem Capella
0.1
12 13 14 15 16 17 18 19 20
Wavelength (Å)

19. Ionization potential of Fe ions Electron energy
2500
2000
Electron beam
1500
IP (eV)
1000
XUV
Maxwellian
500
EUV
Number of electrons VUV
0
0 5 10 15 20 25

20. Capella XUV excess at 6MK
4
3,5
3
Capella excess ratio
2,5
Agreement much improved
by using Maxwellian model,
but the data slope points to 2
a systematic problem.
1,5
1
0,5
0
13 14 15 16 17 18
Wavelength (A)

21. 5
Fe XVIII
4 Fe XIX
T = 7 MK
3
Flux signal
2
Assuming a higher 1
temperature (7 MK instead
of 6 MK) improves the
agreement with models.
0
13 14 15 16 17 18
o
Wavelength (A)

22. Conclusions (preliminary):
Relative calibration XUV / EUV achieved relatively simply
Calibration within each range good to ± 10% (maybe)
(Chandra LETGS / HETGS are better known)
Transfer electron beam / Maxwellian via FAC code (M. F. Gu) seems reasonable;
details are still being worked on
Interpretation of Chandra spectra not fully achieved;
the spectrum is possibly richer than previously assumed;
modeling approach of varying the experimental wavelengths seems dubious;
alternative: additional blending lines from whatever elements
XUV / EUV excess seems different for Fe XIX and Fe XVIII
Possible interpretation: underlying temperature 6 MK may be too low
Moreover, the APEC (HULLAC, FAC) model may well be incomplete and
insufficient
! ! ! ! ! ... much more work needs to be done
If you have questions or suggestions see the poster!