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The Astrophysical Journal, 625:L59--L62, 2005
20
The Astronomical
All rights reserved. Printed
in U.S.A.
ASSESSMENT THE AND LINE RATIOS FROM THE CHANDRA
GRATING OBSERVATIONS CAPELLA
Desai,
1 Brickhouse,
J.
J. Drake, Dupree,
1 Edgar,
1
R. Hoogerwerf,
1
V. Kashyap,
Wargelin,
1
R. Smith,
2 Huenemoerder,
3 and Liedahl
4
Received 2005 March accepted published May
3
ABSTRACT
Observations xviii and xix X­ray, extreme­UV, far­UV line emission, formed
at peak
Capella's Aurigae's) emission measure distribution ubiquitous spectra many cool stars galaxies,
provide
a unique opportunity the robustness xviii xix spectral models. The Astrophysical
Plasma Emission Code (APEC) used identify over lines these alone,
to compare
model predictions spectra obtained Chandra Energy Transmission Grating High Energy
Transmission Grating Spectrometers, Far Ultraviolet Spectroscopic Explorer (FUSE), and Extreme
traviolet Explorer. Some flux discrepancies larger than factor found between observations xviii
lines predictions APEC other models common
In particular, X­ray resonance
both ions stronger predicted models relative EUV resonance lines. multi­
wavelength observations demonstrate importance including dielectronic recombination proton­impact
excitation, using accurate wavelengths spectral codes. These ions provide important diagnostic tools
7
K plasmas currently observed Chandra, XMM­Newton, and FUSE.
Subject headings: atomic data atomic processes individual (Capella) ultraviolet:
X­rays:
1. INTRODUCTION
The Capella system (pHD 34029,
a Aurigae), consisting
principally
of giant
# III),
strongest coronal X­ray sources, offers
portunity benchmark models used interpretation
X­ray spectra astrophysical plasmas. Although plasma
codes have gone through major improvements (e.g., Mewe
et
1995; Brickhouse 1995; Smith 2001; Young
et
2003), their accuracy and completeness diagnostic analy­
at spectral resolution has fully assessed.
Letter compares current spectral models
with Chandra Emission Project (ELP) observations
Capella (Brickhouse Drake 2000)
as
a step toward
suring astrophysical interpretations spectra based
sound understanding physical processes involved.
Capella spectrum studied, showing evidence flares
(Brinkman
et 2001; Canizares 2000). The emission
measure distribution (EMD) Capella system shows
a
strong narrow peak MK, near temperature peak
emissivity xviii producing numerous strong
transitions. Modest variability (#20%)
of fluxes from these
ions over timescales months years Dupree
2005, preparation) validates combined analysis mul­
tiple observations.
Although L­shell X­ray offer powerful diagnostic
potential collisionally ionized plasmas, discrepancies
between models observations now arising from Chan­
dra spectra. example, (2002) observed
3s--2p/3d--2p
in elliptical galaxy NGC 4636
1 Harvard­Smithsonian Center Astrophysics, Garden Street,
bridge, 02138; pdesai@cfa.harvard.edu.
2 X­Ray Astrophysics Laboratory, Code NASA Goddard Flight
Center, Greenbelt, 20771; Department Physics Astronomy,
Johns Hopkins University.
3 Center Space Research, Vassar Street, Cambridge, 02139.
4 Lawrence Livermore National Laboratory, L­41, Livermore,
94551.
higher than predicted APEC similar
observed Capella. analogous shows
same pattern discrepancy solar observations
et
1999), suggesting common atomic physics origin. Labo­
ratory programs (Brown 1998; Laming 2000; Beiers­
dorfer 2002) have only recently addressed mod­
eling issues. The ELP observations Capella offer
a unique
opportunity compare models and observations broad
spectral range. Comparisons among far­ultraviolet (FUV),
extreme­ultraviolet (EUV), X­ray lines xviii
particularly useful, since strong
2 EUV
lines essentially entirely produced
by direct collisional
excitation and should easier interpret than FUV
X­ray lines, which include contributions from other
cesses, such proton­impact excitation dielectronic
combination (DR).
2. DATA ANALYSIS
Observations Data Reduction
Multiple spectra
of Capella, acquired between August
and 2002 October, include pointings with the Chandra
Energy Transmission Grating (HETG) ACIS­S detec­
total exposure
of 182.2 and with
Energy Transmission Grating (LETG) HRC­S detector
total exposure 234.2 HETG and LETG data,
obtained from Chandra archive, reprocessed using
CIAO version
5 minor deviations from stan­
dard pipeline procedures. Effective areas were generated
each data Chandra calibration database CALDB
2.8 exposure­time weighted
to create average effec­
tive areas summed spectra.
Extreme Ultraviolet Explorer (EUVE) spectra obtained
in
1999 September, which are nearly simultaneous with Chan­
dra LETG/HRC­S pointing, processed using standard
5 http://cxc.harvard.edu/ciao.

L60 DESAI
EUVE Guest Observer software (IRAF). agreement
tween LETG and EUVE fluxes lines discussed
in
Letter
is good within about 5%, henceforth LETG fluxes
will used. Far Ultraviolet Spectroscopic Explorer (FUSE)
line fluxes taken from spectra Young
et (2001).
Astrophysical Plasma Emission Code version
(APEC; Smith 2001)
to predict Capella spectrum.
6
The APEC models xviii xix contain
994 fine­structure levels, respectively, principal quantum
number They include the effective collision strengths
and atomic transition probabilities calculated using Hebrew
University Lawrence Livermore Atomic Code (HULLAC;
dahl 1995).
Fe xviii, collision strengths
5
2
5
2 transition include resonance excitation from
R­matrix calculations (Berrington 1998). Proton­impact
excitation rates within ground state included
xviii (Foster 1994) xix Reid 1999, private
communication). Laboratory X­ray wavelengths (Brown
2002) been incorporated. APEC currently includes
rates excited levels
of
Fe and He­like ions,
L­shell ions. Similarly, satellite lines
present
in APEC xvii (Safronova 2001),
xviii and xix.
2.2. Spectral Models and Measurements
calculate global continuum spectra produced
bremsstrahlung, radiative recombination continuum, two­
photon emission over observed Chandra spectral range.
then temperature continuum model line­
free regions HETG spectrum, identified both from
APEC visual inspection, which yields
a tem­
perature
6 MK, near peak EMD. Since LETG
spectrum contaminated high­order emission, same
continuum model derived from HETG data
is applied
LETG fitting. adopted abundances Brickhouse
(2000), found evidence deviation from
solar abundances
of Anders Grevesse (1989). Individual
fluxes from Chandra spectra measured using Sherpa
(Freeman 2001) functions approximating
strumental line profiles. Plus minus orders were fitted
arately, with requirement that line fluxes same.
narrow range the FWHM allowed HETG, 0.01--
0.0135
, LETG, 0.045--0.06
), standard binning
maintained, Cash statistic was applied (Cash 1979).
Table gives the observed fluxes xviii
lines with
j errors.
2.3. Model Assumptions
continuous EMD (Brickhouse 2000), composed
eight temperature components dex grid, used
estimate the contribution line blends from ions over entire
temperature range. This EMD
is normalized flux
l93.92 resonance and used predict
fluxes given Table note there only few­percent
difference between single­temperature
6 MKmodel and
EMD
of interest. Since some line emis­
sivities show modest density sensitivity between low­
density limit and densities expected under coronal conditions,
APEC
to compute models
a wide
6 APEC models, calculated
at low­density limit
(
e
),
atomic rate produce them available http://
cxc.harvard.edu/atomdb. Higher density models
are available request.
range densities. most affected line ratio that
l101.55
to l108.37.
e
10 the predicted
0.347 photon units), compared with 0.261
at standard
APEC low­density limit, better agreement the observed
flux ratio 0.328.
Lack significant variability further supports assump­
tion plasma conditions stable, individual lines
xviii, and xix show modest flux changes
(!10% deviation from average value) between Chandra
pointings, and light curves show low levels variability
(#8%) during
a single pointing. There evidence
challenge standard assumptions negligible optical depth
(Canizares 2000; Brown
et 1998).
3. RESULTS DISCUSSION
Figure
1 compares observed xviii
fluxes with those predicted spectral codes, APEC,
CHIANTI version (Dere 1997; Young 2003),
and SPEX version (Kaastra
et
al. 1996), which incorporates
MEKAL model (Mewe
et
al 1995). Emissivities provided
M.­F. (2004, private communication) Flexible
Atomic Code (FAC; 2003) also compared. The fluxes
scaled fluxes respective strong EUV reso­
nance lines, which direct excitation dominates. models
figure calculated
a single temperature,
T
e
6 MK,
and same density,
e
, except SPEX, which
available
at low­density limit.
Most striking discrepancy between EUV and X­ray
lines: observed X­ray fluxes stronger predicted
fluxes models. Even X­ray resonance lines,
l14.208 l13.518, underpredicted
ative their EUV counterparts more than 30% factor
respectively. Since these factors larger than expected
from calibration errors blending, possible that
accuracy
of direct excitation coefficients might explain
predicted weakness l14.208 Brown 2005);
however, difficult reconcile larger
crepancy l13.518.
The xviii and FUV forbidden­line fluxes
good agreement fluxes l93.92
l108.37 APEC models, somewhat better than
not calculate proton­impact excitation
rates, which included both APEC CHIANTI.
APEC models, proton­impact excitation increases forbidden­
line emissivities
by 8% l974.86 l1118.07,
respectively. predicted FUV fluxes begin increase
with density above
e
. APEC models
agreement
e #10
12
#3 also consistent within
observational errors lower coronal density range.
Figure
1 shows some discrepancies among
strongest X­ray lines, reflecting 3s--2p/3d--2p pattern.
these transitions, largest difference among predictions
results from number processes calculated with each
model. example, even though APEC CHIANTI have
similar collision strengths l15.625 line,
ditional line
in APEC produced direct excitation
levels, followed radiative cascades, while
CHIANTI currently includes levels
to
n
3.
other hand, xviii l16.07 APEC CHIANTI both
show differences
of more
a factor
of
2 from FAC because
neither includes effects the upper level population,
which included
in FAC.
Comparisons
of APEC predictionsNo. 2005 xviii AND xix LINE RATIOS FROM CHANDRA
TABLE Line Measurements
Instrument
ref
)
š
A
l
(
) Transition
U
--
J
L
Model Flux
a
(photons
#2
ks
) Observed
(photons
#2
ks
)
FUSE
b 974.86 974.85
5
2
1/2
2
--
1
3
2
2 5.063 5.50
LETG 103.93 103.98
6
2
S
5
2
1/2
--
1
1
2
2 1.625 1.69
LETG 93.923
6
2
S
5
2
3/2
--
1
3
2
2 4.441 4.44
MEG 17.623 17.620
4
2
3/2 --2s2p
6
2
S
--
3
1
2
2 0.300 0.30
MEG 16.159 16.163
3s
2
P --2s2p
2
S
--
3
1
2
2 0.164 0.13
MEG
c 16.071 16.073
4
(
4
5/2
2
P
--
5
3
2
2 0.418 0.82
HEG
c 16.071 16.076
4
(
4
5/2
2
P
--
5
3
2
2 0.418 1.00
HEG
d 16.004 16.008
4
(
2
3/2
2
P
--
3
3
2
2 0.768 0.81
HEG 15.870 15.873
4
(
2
3/2
2
--
3
1
2
2 0.095 0.34
HEG 15.824 15.831
4
(
4
3/2
2
P
--
3
3
2
2 0.179 0.29
HEG 15.625 15.628
4
(
1 D)3s
2
5/2
2
3/2
--
5
3
2
2 0.290 0.43
HEG 14.571 14.559
4
(
4
3/2
2
P
--
3
3
2
2 0.110 0.21
HEG 14.534 14.539
4
(
2
5/2
2
--
5
3
2
2 0.210 0.39
HEG 14.373 14.376
4
(
3 P)3d
2
5/2
2
3/2
--
5
3
2
2 0.278 0.55
HEG
e 14.256 14.261
4
(
2
1/2
2
--
1
3
2
2 0.087 0.42
...
...
2
5/2
5
2
P
p
--
5
3
2
2 0.141
HEG 14.208 14.208
2
5/2
5
2
P
p
--
3
3
2
2 0.381 1.40
...
...
4
(
1 D)3d
2
5/2
2
3/2
--
5
3
2
2 0.695
HEG 11.527 11.528
2
5/2
5
2
P
2
p
p
--
5
3
2
2 0.032 0.17
...
...
4
(
3 P)4d
2
5/2
2
3/2
--
5
3
2
2 0.061
HEG 11.423 11.424
4
(
2
5/2
2
--
5
3
2
2 0.080 0.13
11.427
3p
2
1/2
--
3
1
2
2 0.007
HEG 11.326 11.327
4
(
2
1/2
2
--
1
3
2
2 0.019 0.13
...
...
4
(
2
3/2
2
--
3
3
2
2 0.031
...
...
4
(
1 D)4d
2
5/2
2
3/2
--
5
3
2
2 0.038
FUSE
b,f 1118.07
4
3
P
1
3 1.833 1.74
LETG 120.00 120.04
3
4
3
1 0.836 0.97
LETG 111.70 111.74
3
4
3
1 0.326 0.46
LETG 109.97 109.99
3
4
3
0 0.413 0.46
LETG 108.37 108.39
3
4
3
2 3.091 3.13
LETG 101.55 101.59
3
4
3
2 0.838 1.02
LETG 91.02 91.054
1
P
4
1
2 0.241 0.45
HEG 16.110 16.111
1/2
3p --2s2p
5
3
2
2
p 0.120 0.14
HEG 15.198 15.204
2 3s--2s2p
3
2
p
p 0.080 0.39
HEG 15.079 15.083
2p
(
4
S
2
3
P 0.094 0.33
HEG 14.664 14.671
3
2
D
3
3 0.079 0.21
HEG 13.795 13.795
2
5/2
3
P
p 0.105 0.24
...
...
3
2
3
P
2
3
P 0.012
HEG 13.518 13.523
3
2
D
3 --2p
4
3
2 0.262 0.52
HEG 13.497 13.507
2
3/2
3
P
p 0.118 0.32
13.507 13.507
2
3
D
1
2
p 0.025
HEG 13.462 13.470
3
2
3
S
1
3 0.072 0.25
HEG 13.447 13.446
2
0 --1s2p
1
1 0.397 0.40
blends
are separately
if contribute more
of interest model). Fluxes (including
blends) normalized l93.92 predicted
by APEC density
1 listed. observed fluxes
been corrected
for interstellar absorption using
N (Piskunov
et
al. 1997), neutral helium, H/He abundance
(Kimble 1993). largest correction
at l120.0 amounts
to only
Young 2001.
measurements
to show cross­calibration. preferred analysis because
of
its better
spectral resolution.
Contribution
of
O
to
is than
LETG measured
to cross­check calibration
of LETG
vs. HETG. LETG
is somewhat blended,
but
is within 30% HETG
FUSE measurement uncertain,
is blended. Solar­network spectra used
to estimate contribution
i
to
observed fluxes the X­ray lines listed Table
1 are shown
Figure confirm
a general 3s--2p/3d--2p discrepancy
pattern APEC models largely removed with FAC
calculations. The 3s--2p/3d--2p ratios summed fluxes
from APEC smaller observed ratios #20%,
whereas agreement within 10%. inclusion
in
the FAC models produces additional 3s--2p emissivity.
Another significant disagreement between the models
observations occurs radiative transitions terminate
excited levels, namely, xviii l15.870, l16.159, l17.623
and l15.198 l16.110. Although the APEC line
which reasonably complete
in spectral region, does
include DR satellite lines from either xviii xix, blend­
ing with satellite lines lines from other ions cannot explain
the extent underprediction. possible that large
theoretical wavelength inaccuracies these lines,L62 DESAI
observed­to­predicted ratios
of X­ray,
EUV, FUV spectral regions. Shown comparison obtained
using APEC, CHIANTI, SPEX spectral codes
the rates.
density
is
e
10
#3
, SPEX. Comparison
for
Fe
lines, normalized l93.92. plotted here l14.208, l15.625,
l16.071. Bottom: Comparison lines, normalized
to l108.37.
plotted l13.518, l14.664, l15.079.
observed­to­predicted ratios X­ray
APEC. Lines Table excluding heavily blended l16.004
shown. between xviii
š
shortward Ratios calculated
at
e
10
cm
. Dash­
dotted represent agreement within
a
of
2. Comparison
xviii, normalized
to l14.208. There published FAC models
xviii 4d--2p lines around
. Bottom: Comparison xix,
š
A
malized
to l13.518.
percent, have misidentifications
in laboratory mea­
surements. Blending nearby lines from same could
produce
a pattern under­ overprediction.
xviii l15.870, latter explanation consistent new
wavelength calculations (Kotochigova
et
al. 2005; 2005).
4. CONCLUSIONS
surprising result benchmark spectral modeling
study
is the large discrepancy between modern theory and
Capella observations X­ray EUV resonance lines
(30%) and (factor New FAC calcu­
lations including dielectronic recombination bring most X­ray
lines into good agreement observations; however, puzzling
discrepancies
as
a factor remain some
relatively strong lines. Additional laboratory theoretical
work
is needed eliminate largest remaining problems.
Meanwhile, errors largely
be minimized judicious
choice line diagnostics consideration
of appropriate
atomic processes.
This work supported
in Chandra X­Ray Center
(NAS 8­39073). thank CXC staff, particularly Harvey
Tananbaum, supporting efforts obtain these data, and
developers other public spectral modeling codes SPEX
and CHIANTI, well M.­F. atomic structure
code FAC.
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