Boldt, E. A.

Holt, S. S.

Serlemitsos, P. J.

English

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Iron Line Emission From X-Ray Sources
by
S. S. Molt, E. A. Boldt and P. J. Serlem:itsos
NASA, Goddard Space Flight Centex, Greenbelt, Maryland
ABSTRACT
X-ray sources for which thermal models have been proposed should
exhibit line structure for energies below 10 keV. The (1s) 2 - 1s 2p
transition in Fe XXV at 6.6: keV is that emission line which is
characteristically the most easily observable, and its presence (or
absence) in experimentally observed spectra is shown to severely
'-.	 constrain proposed models.
1
r	 ;
INTRODUCTION
Tucker and Gould (1966) have developed a framework in which the
radiation from an optically than plasma at temperatures in excsss of
106 °K may be studied. Using this approach, Tucker (1967) has constructed
temperature- and elemental-abundance dependent models for thermal X-ray
sources, and has pointed out that the differential structure of the
X-ray emission spectrum below 10 keV can yield specific limitations
on the physical nature of the source. Sartori and Morrison (1967)
have postulated that presumed non-thermal radio sources (such as the
Crab Nebula may have X-ray emission spectra which are, in fact, thermal
in origin. It is the purpose of this letter to.point out that such
thermal models may not be chosen to empirically fit the observed continuum
with thermal continua without considering the effect of line emission
t	 .1 y 1
CEDING PAGE BLANK NOT MU T'_'-
-4-
In the construction of Figure Z we have used the line strengths
for the three composition models as a function of temperature given
1
T.	 by Tucker (1967). The bremsstrahlung component of the continuum
was computed using the standard free-free emission prescription (cf.
Allen, 1955), where we took a Gaunt factor of	
.^.
n
for hydrogen, and unity for the other elements in the abundances given
by Tucker for his three models. The recombination radiation contri-
bution to the continuum was evaluated using the functional form given
by Tucker and Gotld (1966) normalized to the models of Tucker.
An X-ray experiment with energy resolution of — 25% at 6.9 keV,
and a statistical uncertainty of N 1C% in the flux in a 1 keV energy bin
ac that energy, would be capable of detecting the iron line emission at
the 1Q level for the universal abundance mode, (model T) over the
most interesting part of the temperature range, and could easily detect
4
the iron line emission from models which are iron-rich.
Conclusions
Our laboratory has used a detector which possesses an energy
resolutions of , 25% (FWHK) at 6.9 keV in a rocket borne x-ray observation
of the Crab Nebula and the Taurus region of the sky (Bolds, Desai, and
BJit 1968). The data from this exposure to the Crab Nebula were lumped
into 1 keV bins as suggested above for three separate proportional
4 ,	
counters, and no positive indication of iron in any of the detector
records was obtained, with a statistical error of w 15% in each energy
-5-
bin. If we combine the three independent records, the statistical error
in the composite 1 keV channel at 6.9 keV is 9%.
The low temperature region chosen for the Crab by Sartori and
Morrison is 2 keV (2.82 x 10 7 °K). According to Figure 1, universal
abundances in the Crab at this temperature would give us an increase
at the lcr level. Model II (Mo supernovarnova shell) would give almost
2a, and model III (— 30 M. supernova shell) would give us about 8Q.
The statistics in our results for models I and II are hardly compelling,
but are certainly not indicative of a thermal source.
The two temperatures chosen by Sartori and Morrison are not unique.
The presently available data, in fact, would necessitate more than
two temperature regions in order to achieve consistency with the entire
spectrum from 1 keV to 200 keV. We wish to point out, however, that
any temperature region chosen between 20 and 100 million degrees will
give observable iron line emission for most of the possible elemental
abundance models. The continuum from lower and higher temperature
regions will be insignificant in the neighborhood of 7 keV compared
to the contribution from the 20-100 million degree region, if the
20-100 million degree region is a necessary constituent of the model.
The x-ray source which has been most consistently reconciled
with a thermal model is Sco X-1, at a temperature of 50-60 million
degrees. We would expect the iron emission from this source to be
observable. The most recent Sco X-1 data which we have seen (one
axposure by Hill et al (1968), and two exposures by Gorenstein et al
(1968) all seem to exhibit an excess in photon intensity in precisely
iv
tl
q
-6-
the energy region where we would predict the iron emission. We suggest
that all experimenters reexamine their data with this criterion in mind.
We thank Dr. A. Kl.imas and Dr. G. Sandri for valuable discussions.
I
.
M
Ir
-7-
References
Allen, G. W., Astrophysical Quantities, 1,955, University of London.
Boldt, E., Aesai, U. and Holt, S., 1968 to be published.	 i
Gorenstein, P., Gursky, H., and Garmire, G., 1968, Ap. J., iii press.
Hill, R. W., Grader, R. J. and Seward, F. W., 1968, Ap. J., in press.
Sartori, L. and Morrison, P., 1967 0 Ap. J., 150, 385.
Tucker, W., 1967, Ap. J., 148, 745.
Tucker, , W.,and Gould, R. J., 1966 0 Ap. J., 144, 244.
I-8-w
Fi ure Ca Lion
1. Ratio of power emitted in re line emission to power emitted in
continuum (bremsstrahlung plus recombination radiation) emission
in a 1 keV window centered at 6.9 keV, as a function of temperature.
The three traces 1, 11 and Txx refer to the three models of Tucker
(1967) .
.
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0


Iron line emission from X-ray sources

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