Stellar and laboratory XUV/EUV line ratios in Fe XVIII and Fe XIX
Stellar and Laboratory XUV/EUV Line Ratios inFe XVIII and Fe XIXElmar Träbert #, Peter Beiersdorfer, Joel H. T. ClementsonLawrence Livermore National Laboratory, Livermore CA, USA# also at AIRUB, Bochum, GermanyNASA project funding APiP 21 July 2011Chandra spacecraft observes CapellaComparison of observed XUV (13-18 Å), EUV (90-120 Å), VUV (1100 Å) line intensities with predictions by the APEC modelXUV appears brighter than expected- APEC model incorrect or- Capella peculiar?Interstellar absorption involvedLaboratory study under way:Electron beam ion trap, two flat-field spectrometersDetection 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 effectsPossible insights see Poster
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 FACEUV, and FUV spectral regions. Shown for comparison are the ratios obtained and APEC. Lines from Table 1 excluding heavily blended Fe xviii 16.004using 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 pdensity 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: Comparisonand 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 forThe 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!
Transmission for Capella 0.8 0.6 0.4Interstellar extinction: 0.28% loss of EUV signal vs. XUV signal 50 100 150 200 250 300 350 400 o Wavelength (A)
Fe XIX O-like Fe XVIII F-like2s2 2p3 nl 2s2 2p4 nl XUV XUV2s 2p 5 EUV 2s 2p 6 E2 EUV2s2 2p 4 M1 M1 VUV 2s2 2p 5 M1 VUV
The Livermore electron beamion trap is the archetypical EBITElectron collectorSeveral layers of cooling and cryogenicshields at the temperatures of liquid Heand liquid N2Superconducting magnets(pair of Helmholtz coils, B = 3 T)Drift tubes at electrostatic potentials trapions axially; with openings for optical accessElectron gun20 Years ofSpectroscopyELA BI TLIVERMORE since 1986
Electron energy Ionization potential of Fe ions 2500 2000 Electron beam 1500IP (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.
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 structureElement abundanceCollisional excitation f(T, n)Radiative de-excitationPhotoionization 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 improvingModelingHULLAC ! Hebrew University Lawrence Livermore Atomic CodeAPEC ! Astrophysical Plasma Emission CodeFAC ! 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 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
5XUV flux seen is higherthan the model Capella vs. APECprediction (tied to EUV) 4 T = 6 MKInterpretation A: 3XUV/EUV excess Flux signalInterpretation B: 2XUV underpredictionby APEC (and FAC etc.) 1 0 13 14 15 16 17 18 o Wavelength (A)
Principalquantumnumber 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
O VII 1000Spectra of CO2 (mostly oxygenlines in the regions shown) 800dispersed with a 1200 l/mm Countsgrating in a R=5.6 m flat-field 600grating spectrometer andrecorded with a CCD camera at 400 O VIIIan 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)
SFFS CO2 200 150Counts 100 O VIII 50 0 80 90 100 110 120 130 140 Wavelength (Å)
O VII a 1200 1000 800 Counts O VIII 600 O VII O VIII 400 O VII 200EBIT reference spectrum of oxygen 0 12 14 16 18 20 22 24EBIT 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 OSpectra dispersed with an R=44.3 m 2400 l/mm O 100flat-field grating and recorded with an MCP-based detector. 0 13 14 15 16 17 18 o Wavelength (A)
SFFS Fe I2 Fe XIX Fe XVIII 800 600 Fe XIXCounts Fe XIX Fe XIX 400 Fe XIX 200 0 80 90 100 110 120 130 140 Wavelength (Å)
Fe Plenty of lines in EBIT Electron beam energy 2 keV - mostly Fe, some O - 500 what about stellar spectra? 400 300Counts 200 100 0 13 14 15 16 17 o Wavelength (A)
Capella vs APEC & EBIT 2 1.8 1.6 Density Capella excess ratio dependence 1.4 1.2 APEC 1 0.8 0.6 0.4Experiment with error bars 90 95 100 105 110 115 120 125Fe XIX oFe XVIII Wavelength (A)APEC open circles
Ratio APEC & EBIT vs. CapellaXUV EBIT and XUV APEC 10compared to Capella EBIT Fe XIXEBIT experiment vastly exceedsCapella flux - something is wrong in EBIT Fe XVIIIthis analysis! --> EBIT has an electronbeam, Capella has a thermal plasma;need to simulate this difference. 1APEC 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)
Ionization potential of Fe ions Electron energy 2500 2000 Electron beam 1500IP (eV) 1000 XUV Maxwellian 500 EUV Number of electrons VUV 0 0 5 10 15 20 25
Capella XUV excess at 6MK 4 3,5 3 Capella excess ratio 2,5Agreement much improvedby using Maxwellian model,but the data slope points to 2a systematic problem. 1,5 1 0,5 0 13 14 15 16 17 18 Wavelength (A)
5 Fe XVIII 4 Fe XIX T = 7 MK 3 Flux signal 2Assuming a higher 1temperature (7 MK insteadof 6 MK) improves theagreement with models. 0 13 14 15 16 17 18 o Wavelength (A)
Conclusions (preliminary):Relative calibration XUV / EUV achieved relatively simplyCalibration 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 onInterpretation 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 elementsXUV / EUV excess seems different for Fe XIX and Fe XVIIIPossible interpretation: underlying temperature 6 MK may be too lowMoreover, the APEC (HULLAC, FAC) model may well be incomplete andinsufficient ! ! ! ! ! ... much more work needs to be doneIf you have questions or suggestions see the poster!