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This Legacy journal article was published in Volume 6, August 1995, and has not been
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Update of MEKA: MEKAL
R. Mewe, J.S. Kaastra
(SRON Laboratory for Space Researhc, The
Netherlands)
and D.A. Liedahl
(Lawrence Livermore National Laboratory)
1. Introduction
The analysis of the X-ray spectrum of the Centaurus cluster as measured with
the ASCA satellite by Fabian et al. (1994) has demonstrated that the
observed intensity distribution in the dominant Fe L-shell feature around ~
1-1.5 keV is not consistent with the predictions of the commonly used optically
thin plasma codes such as Raymond-Smith and MEKA. In particular the
observational data indicate a higher value than the plasma models predict of
the ratio Fe (3 -> 2)/(4 -> 2) of the two line complexes n=3 -> n=2
and n=4 -> n=2 at about 1.1 and 1.5 keV, respectively. Such a discrepancy
could not be removed by varying the relative metal abundances nor by invoking a
multi-temperature distribution (cf. Fabian 1994, Fabian et al. 1994).
1) Though the ASCA observations with a modest spectral resolving power of about
20 in this region permit the separation of the two blended components, they do
not allow the resolution of the fine structure of each of these Fe L components
which are composed of thousands of lines from ions in the range Fe XVII-XXIV
that are formed in the temperature region 6.5 o(<,~) log
T(K) o(<,~) 7.3. Though the current models, which have
concentrated only on the stronger lines (nearly 250 in total), still give a
good order-of-magnitude estimate of the fluxes, more detailed calculations are
now required to reproduce the observed structure in the Fe L-shell complex for
the broad-band ASCA observations and even more detailed calculations are needed
for the high-resolution XMM and AXAF spectra.
These problems have motivated us to make use of recent calculations of the Fe L
complex with the HULLAC code at Livermore and to implement the results in an
upgrade of our previous plasma code, that was installed end 1992 in the latest
release of XSPEC (version 8.23) under the name MEKA (cf. Kaastra & Mewe
1993, Mewe & Kaastra 1994). Some preliminary calculations by Liedahl et
al. (1994) using about 1000 of the strongest lines from Fe XXII-XXIV as
calculated with HULLAC showed that a revision was warranted. Some more detailed
computations by Liedahl et al. (1995) have confirmed this.
2) Our experience with the data analysis of EUVE spectra of cool stars (Mewe
et al. 1995) has led us to make some revisions to the list of lines of
Landini & Monsignori Fossi (1990) that we used since 1992 for the extension
to the 300-2000 Å wavelength region. We modified several excitation
rates, corrected the wavelengths of about 110 lines between 80-500 Å,
while in a dozen of cases we have split up the average multiplet into its
separate components.
3) Discrepancies in the comparison between observations of the diffuse emission
from the hot ISM with the Diffuse X-ray Spectrometer (DXS) in the 44-84 Å
range and currently used spectral models (Sanders 1995) has led us to
recalculate the lines from Fe VIII to Fe XVI in the wavelength region ~35-2000
Å using data from the literature for collision strengths and radiative
branching ratios.
4) Finally, we added a number of dielectronic recombination (DR) satellites to
the He-like Mg lines at 1.3 keV and use the Arnaud-Raymond (1992) ionization
balance for Fe instead of the Arnaud-Rothenflug (1985) ionization balance which
is still used for the other elements.
To summarize, the forthcoming release of MEKA under the name `MEKAL' contains
the following improvements:
(i) improved calculations of the Fe-L complex between 0.65-1.83 keV based on
the HULLAC results; (ii) addition of about 300 far-UV lines between 300-2000
Å from Landini & Monsignori Fossi (1990) with some lines corrected
for excitation rates and wavelengths and/or split in multiplets based on the
analysis of EUVE spectra; (iii) update of Fe VIII-XVI lines in the 35-1950
Å region; (iv) addition of about 60 DR lines to the He-like Mg lines at
1.3 keV; (v) Arnaud-Raymond (1992) ionization balance for iron.
2. Calculations of the Fe L-shell spectrum
In calculating the Fe L-shell spectrum one of us (DAL) has used the atomic
physics package HULLAC (Hebrew University/Lawrence Livermore Atomic Code)
developed by Klapisch and co-workers (Klapisch 1971, Klapisch et al.
1977). Excitation collision strengths are calculated in the quasi-relativistic
distorted wave approximation (Bar-Shalom et al. 1988). Results for Fe
XXIV are within a few percent in agreement with the results by Zhang et
al. (1990) computed in the relativistic distorted wave approximation.
At the moment more than 40 0000 lines have been calculated from the ions Fe
XXI-XXIV and calculations for the remaining ions Fe XVII-XX are in progress.
The energy-dependent excitation Gaunt factor parameter method as originally
proposed by Mewe (1972) and Mewe & Schrijver (1978) was used for the
calculation of line emission in the MEKA code. Our newer work extends this idea
by introducing, apart from Mewe's formula for the majority of the lines,
different fitting formulae for other lines, in particular forbidden
transitions. These results will, apart from a higher accuracy, also allow the
calculation of the density dependence of the Fe L and Fe M shell lines.
However, for the implementation in MEKAL on a short term we have still used the
old formalism but we have extended and improved our line list by implementing
the 2300 of the strongest lines calculated by the HULLAC code for the
wavelength region 6.8-19.0 Å from the ions Fe XVII-XXIV including the 3-2
and 4-2 lines, and sometimes also 5-2 lines. For Fe XXIII also the effects of
dielectronic recombination (DR) were included. In more extended calculations
for Fe XXIII and Fe XXIV by Liedahl et al. (1995) also radiative
recombination (RR) was taken into account. In the present calculations we have
calculated the contribution of RR to the strength of lines from the ions Fe
XXIII and Fe XXIV in an approximate way by using Eqs. (33)-(35) from Mewe &
Gronenschild (1981). The contribution from DR for the ions Fe XVII-XXII and Fe
XXIV we have taken approximately into account by using Eqs. (43)-(45) from Mewe
& Gronenschild (1981) for the estimate of the total intensity of satellites
close to a resonance line. Note that Eq. (43) contains a printing error: the
constant should be 8.133x10-13 instead of 8.14x10-14;
note also that the electron temperature Te is expressed in units of
106 K and the excitation energy
[[chi]]o(E,z) in eV.
The total number of lines in the Fe L spectrum between 6 and 20 Å
included in MEKAL is 2652 as compared to 244 in MEKA.
2.1. Comparisons and conclusions
Figures 1 and 2 compare simulated X-ray spectra for the ASCA-SIS with the new
MEKAL and the old MEKA for the temperatures 0.8 and 2 keV using AR92 for Fe and
AR85 for the other elements. A few differences are noticed. At T = 0.8 keV the
peak in the spectrum shifts from ~ 1 keV (from Fe XXI) in MEKA to a larger peak
at ~ 0.9 keV (from Fe XIX) while the Fe XX feature at 1.23 keV shifts to 1.31
keV. The features at 1.47 keV and 1.85 keV are dominated by Mg XII and Si XIII
emission, respectively. At T = 2 keV the peak at 1.1 keV is determined in both
codes by Fe XXIV lines but shows for MEKAL a low-energy `shoulder' due to Fe
XXIII emission. In MEKAL the `gap' between 1.3-1.4 keV is filled by Fe XXII
lines. Mg XII emission still gives a significant contribution to the ~ 1.5 keV
blend while the 2 keV emission is entirely due to Si XIV. Figures 3 and 4 show,
apart from different line excitation calculations, the use of different
ionization balances for iron (AR92 in MEKAL vs. AR85 in the original MEKA
code). The last two plots show the full difference between the MEKA and MEKAL
calculations and show evidence that the new calculations significantly enhance
the 3-2/4-2 ratio due to the combined effects of line excitation and ionization
balance (cf. also Liedahl et al. 1995). However the contributions from
e.g. Mg XI and Mg XII emission to the `4-2' blend can not be neglected in the
considered temperature range.
Figures 1-4
3. Calculations of Fe VIII-XVI
On the basis of data taken from the literature for excitation collision
strengths and radiative branching ratios we have revised the intensities from
the lines of the ions Fe VIII-XVI in the wavelength region 35-1960 Å for
the low-density case. The results for Fe XII and Fe XVI we have checked in
personal correspondence with the calculations by Nancy Brickhouse. For the
dependency on electron density of the long-wavelength
(o(>,~) 80 Å) lines of these ions we refer to her
calculations (Brickhouse et al. 1995). The total number of lines from
these ions increased from 245 in MEKA to 477 in MEKAL.
4. SPEX
More extended calculations with the HULLAC code of the Fe L complex and also
long-wavelength lines of these ions are in progress. New model calculations for
the wavelength range ~ 1-2000 Å, will include non-equilibrium ionization,
photo-ionization, X-ray absorption, and density effects. These and
hydrodynamical models for extended sources like supernova remnants and clusters
of galaxies, multi-temperature (DEM) models, including extended diagnostic and
display facilities, are now available in our new spectral code SPEX (cf. Mewe
& Kaastra 1994, Kaastra et al. 1995).
SPEX can be obtained by anonymous ftp from the NASA High Energy Astrophysics
Archive Research Center (HEASARC) (legacy.gsfc.nasa.gov) in the
directory software/plasma_codes/spex. The user should read first the
README file which gives relevant information on the installation, possible
platforms, file conversion, etc. For convenience also a postscript file
containing an extended table of line fluxes (wavelength range 1-2000 Å,
temperature range 104-109 K) is included. The fluxes are
calculated for the collisional ionization equilibrium (CIE) model.
References
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Arnaud, M., Rothenflug, R.: 1985, Astron. Astrophys. Suppl. Ser.
60, 425
Bar-Shalom, A., Klapisch, M., Oreg, J.: 1988, Phys. Rev. A38,
1773
Brickhouse, N.S., Raymond, J.C., Smith, B.W.: 1995, Astrophys. J. Suppl.
97, 551
Fabian, A.C.: 1994, in New Horizons in X-ray Astronomy - First Results from
ASCA, eds. F. Makino, T. Ohashi, Universal Academy Press, Inc., Tokyo, p.
265
Fabian, A.C., Arnaud, K.A., Bautz, M.W., Tawara, Y.: 1994, ApJ
436, L63
Kaastra, J.S., Mewe, R.: 1993, Legacy 3, 16
Kaastra, J.S., Mewe, R., Nieuwenhuijzen, H.: 1995, in Proc. 11th Colloquium on
UV and X-ray Spectroscopy of Astrophysical and Laboratory Plasmas, Nagoya,
Japan, ed. T. Watanabe, in press
Klapisch, M.: 1971, Computer Phys. Comm. 2, 239
Klapisch, M., Schwab, J.L., Fraenkel, J.S., Oreg, J.: 1977, J. Opt. Soc.
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Landini, M., Monsignori Fossi, B.C.: 1990, Astron. Astrophys. Suppl.
Ser. 82, 229
Liedahl, D.A., Osterheld, A.L., Goldstein, W.H.: 1995, ApJ, in press
Liedahl, D.A., Osterheld, A.L., Mewe, R., Kaastra, J.S.: 1994, in New
Horizon in of X-ray Astronomy - First Results from ASCA, eds. F. Makino, T.
Ohashi, Universal Academy Press, Inc., Tokyo, p. 629
Mewe, R.: 1972, Astron. Astrophys. 20, 215
Mewe, R., Gronenschild, E.H.B.M.: 1981, Astron. Astrophys. Suppl. Ser.
45, 11
Mewe, R., Kaastra, J.S.: 1994, European Astron. Soc. Newsletter Issue
8, p. 3
Mewe, R., Kaastra, J.S., Schrijver, C.J., van den Oord, G.H.J., Alkemade,
F.J.M.: 1995, Astron. Astrophys. 296, 477
Mewe, R., Schrijver, J.: 1978, Astron. Astrophys. 65, 99
Sanders, W.T.: 1995, in Astrophysics in the Extreme Ultraviolet, ed. S.
Bowyer, Cambridge Univ. Press, in press
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Tables 44, 31
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