Lawrence Livermore National Laboratory, Livermore CA, USA # also at AIRUB, Bochum, Germany

Chandra spacecraft observes CapellaComparison 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

APiP 21 July 2011

Elmar Träbert #, Peter Beiersdorfer, Joel H. T. Clementson

Stellar and Laboratory XUV/EUV Line Ratios in Fe XVIII and Fe XIX

NASA project funding

see Poster

L62 DESAI ET AL. Vol. 625

Fig. 1. The observed-to-predicted ux ratios of strong lines in the X-ray,EUV, and FUV spectral regions. Shown for comparison are the ratios obtainedusing the APEC, CHIANTI, and SPEX spectral codes and the FAC rates. Thedensity is Ne 1010 cm 3, except for SPEX. Top: Comparison for Fe xviiilines, normalized to

93.92. The X-ray lines plotted here are

14.208,

15.625,

and

16.071. Bottom: Comparison for Fe xix lines, normalized to

108.37.The X-ray lines plotted are 13.518, 14.664, and 15.079.

Fig. 2. The observed-to-predicted ux ratios of X-ray lines using FACand APEC. Lines from Table 1 excluding heavily blended Fe xviii

16.004

are shown. Note the 3d 2p lines are between 14 and 15 for Fe xviii and Ashortward of 14 for Fe xix. Ratios are calculated at Ne 1010 cm 3. Dash- Adotted lines represent agreement within a factor of 2. Top: Comparisonfor Fe xviii, normalized to

14.208. There are no published FAC models for

Fe xviii 4d 2p lines around 11.4 . Bottom: Comparison for Fe xix, nor- Amalized to

13.518.

Observed flux (Capella) / predicted flux as presented by Desai et al., ApJ 625, L59 (2005)Watch out for log scales!

Transmission for Capella

50 100 150 200 250 300 350 400Wavelength

0.2

0.4

0.6

0.8

Interstellar extinction: 8% loss of EUV signal vs. XUV signal

(A)o

Fe XIX O-like Fe XVIII F-like

VUVEUV

XUV

VUV

EUV

XUV

M1M1 M1

E2

2s 2p

2s 2p

2s 2p nl

2

2

5

6

4

2s 2p

2s 2p

2s 2p nl

2

2

4

5

3

The Livermore electron beam ion trap is the archetypical EBIT

E TIAL B

20 Years of

LIVERMOREsince 1986

Spectroscopy

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

0

500

1000

1500

2000

2500

0 5 10 15 20 25

Ionization potential of Fe ions

Charge state q+

Electron beam

Electron energy

Electron beam energy relative to IP determines the highest charge state present.

IP (e

V)

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

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 basesKelly & Palumbo, CHIANTI, NIST ASD, Mewe/Kaastra/Liedahl, ...

tend to be grossly incomplete, not up to date, sometimes faulty - but eventually improving

ModelingHULLAC ! Hebrew University Lawrence Livermore Atomic CodeAPEC ! 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)

86 KOTOCHIGO

13.4 13.5 13.6 13.7 13.8Wavelength (Angstroms)

0

100

200

300

400

500

600

700

Cou

nts

Fe XIX (HULLAC98)Capella

Fe XIX (Lab)

Fe XIX (Kotochigova)

Figure 1. Chandra spectrum of Capella (black line) in the spectral regionbetween 13.4 and 13.8 ¯ (Desai et al. 2005) shown in comparison with threespectral models. The three models for Fe xix (in magenta) use data from theAPEC 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 shadedfor the observed spectrum. Upper panel: model using the Fe xix wavelengthsfromHULLAC (D. Liedahl 1997, private communication).Middle panel: Fe xixwavelengths include the experimentallymeasured values reported inBrown et al.(2002). Lower panel: Fe xix wavelengths are from this work and Kotochigovaet al. (2007) using the MDFS method. Adapted from Brickhouse (2007).

S. Kotochigova et al., The Astrophysical Journal Supplement Series, 186:85—93, 2010

0

1

2

3

4

5

13 14 15 16 17 18

Capella vs. APECT = 6 MK

Wavelength (A)

Flux

sig

nal

o

XUV flux seen is higher than the model prediction (tied to EUV)

Interpretation A: XUV/EUV excess

Interpretation B: XUV underprediction by APEC (and FAC etc.)

H 7 components

Ly

Principal quantum number n

1

2

3

α

s p d

Calculate line ratioall n=2-3 vs n=1-3

O VIII 102 Å vs 16.0 Å

EUV

XUV

ββ

0

200

400

600

800

1000

12 14 16 18 20 22

Coun

ts

O VII

O VIII

0

50

100

150

200

80 90 100 110 120 130 140

Coun

ts

Wavelength (Å)

O VIII

Spectra of CO2 (mostly oxygen lines in the regions shown) dispersed with a 1200 l/mm grating in a R=5.6 m flat-field grating spectrometer and recorded with a CCD camera at an EBIT

0

50

100

150

200

80 90 100 110 120 130 140

SFFS CO2

Coun

ts

Wavelength (Å)

O VIII

0

200

400

600

800

1000

1200

12 14 16 18 20 22 24

Coun

ts

O VII

O VII

O VIIO VIII

O VIII

0

100

200

300

400

500

13 14 15 16 17 18

Coun

ts

Wavelength (A)

XVIII XVII

O

OO

O

XVII + XIX

XIX

XIXXIXXIX

XIX

XVII

XVIII

XVIII

o

a

b

EBIT reference spectrum of oxygen

EBIT spectrum with Fe (CO)5 injection

Spectra dispersed with an R=44.3 m 2400 l/mm flat-field grating and recorded with an MCP-based detector.

0

200

400

600

800

80 90 100 110 120 130 140

SFFS Fe I2

Cou

nts

Wavelength (Å)

Fe XVIII

Fe XIXFe XIX

Fe XIX

Fe XIX

Fe XIX

500

400

300

200

100

01716151413

Cou

nts

Wavelength (A)o

Fe Electron beam energy 2 keV

Plenty of lines in EBIT - mostly Fe, some O - what about stellar spectra?

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

90 95 100 105 110 115 120 125

Capella vs APEC & EBIT

Cap

ella

exc

ess

ratio

Density dependence

APEC

Wavelength (A)o

Experiment with error barsFe XIX Fe XVIII

APEC open circles

0.1

1

10

12 13 14 15 16 17 18 19 20

APEC

& E

BIT

vs. C

apel

laR

atio

Wavelength (Å)

APEC vs. Capella

EBIT Fe XIXEBIT Fe XVIIIEBIT experiment vastly exceeds

Capella flux - something is wrong in this analysis! --> EBIT has an electron beam, Capella has a thermal plasma; need to simulate this difference.

APEC falls short of Capella flux - may be a modeling problem

XUV EBIT and XUV APEC compared to Capella

0

500

1000

1500

2000

2500

0 5 10 15 20 25

Ionization potential of Fe ions

Electron beam

Electron energy

Maxwellian

Number of electrons

XUV

EUVVUV

IP (

eV)

0

0,5

1

1,5

2

2,5

3

3,5

4

13 14 15 16 17 18

Capella XUV excess at 6MK

Cap

ella

exc

ess

ratio

Wavelength (A)

Agreement much improved by using Maxwellian model, but the data slope points to a systematic problem.

0

1

2

3

4

5

13 14 15 16 17 18

Fe XVIIIFe XIX

T = 7 MK

Flux

sig

nal

Wavelength (A)o

Assuming a higher temperature (7 MK instead of 6 MK) improves the agreement with models.

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!