Observations made with the solid state spectrometer aboard the Einstein Observatory confirm Kepler's SNR as an X-ray source with an intensity between 1-3 KeV of 7.2 x 10 to the-11th power ergs/sq cm-s. The X-ray spectrum is similar to those of Cas A and Tycho, with strong line emission from the helium-like species of Si, S, and Ar. Direct comparisons to Tycho's SNR suggest a distance of Kepler's SNR of greater than or equal to 5 kpc

Becker, R. H.

Boldt, E. A.

Holt, S. S.

Serlemitsos, P. J.

White, N. E.

English

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NASA
Technical Memo; andum 80637
X-Ray Spectrum of Kepler's
SNR
(NA"' A -TM- HC637)	 X-RAY SPECTRUM OF KEPLFR'S	 N30-17974
SNP (NASA)
	
11 p HC 402/MF A01	 CSCL 019
Uncles
63/93 12887
R. H. Becker, E. A. Boldt,
S. S. Holt, P. J. Serlemitsos,
and N. E. White
i
1
JANUARY 1980
National Aeronautics and
Space Administration
Goddard Space Flight Center
Greenbelt, Maryland 20771
0
coo	 It 
g^
X-kAV SPECTRUM OF KEPLER'S SNR
R H.. Becker , E.A Boldt, S.S. k'alt, P.J Serlemitsos, and N E Whitel
Laboratory for High Energy Astrophysics
NASA/Gcddard Space Flight Center
Greenbelt, Maryland 20771
ABSTRACT
Observations made with the Solid State Spectrometer (SSS) aboard the
Einstein Observatory confirm Kepler's SNR as an X-ray source with an intensity
between 1-3 keV of 7.2 x 10-11 ergs/cm 2 -s. The X-ray spectrum is similar
to those of Cas P and Tycho, with strong line emission from the helium-like
species of Si, S and Ar. Direct comparEsons to Tycho's SNR suggest a distance
to Kepler's SNR of ' 5 kpc.
I Also Dept. Physics & Astronomy, Univ. of Maryland
- 2 -
I. INTRODUCTION
Historical SNR are generally known to be X-ray sources. Radio and optical
measurements indicate that Kepler's SNR shares many properties in common
with other such remnants and therefore is a likely candidate for X-ray
emission. Recently two independent groups (Bonner 1979; Tuohy at al. 1979)
have reported X-ray emission from the vicinity of Kepler's SNR, but the relatively
large field of view of their respective instruments precluded positive identi-
fication. In this paper, we report on the results of an observation of Kepler's
SNR with the Solid State Spectrometer (SSS) at the focus of the HEAO-2 (Einstein)
X-ray telescope and discuss the results in the context of the emission observed
from Cas A and Tycho's SNR with the same instrument.
IIo EXPERIMENT AND ANALYSIS
For a detailed description of the SSS detector and electronics we
refer the reader to Joyce et al. (1978). To summarize, the experiment consists
of a cooled Si(Li) detector, which can be placed at the focus of the Einstein
X-ray telescope. So located, the SSS has an effective area of ti 100 cm2
between 1-3 keV which drops monotonically to ti 10 cm2
 at 4.5 keV. The field
of view of the SSS is a ti 6 arc min diameter circle. The SSS has an energy
resolution of ,,  160 eV (FWHM).
The spectral analysis procedures have been discussed previously by
Holt et al. (1979) and Recker et al. (1979a). In general, theoretical X-ray
spectra are folded through the experiment response and compared to the obser-
vational data. Model parameters are varied to minimize the chi-square between
the data and the model. In this case, we utilized a model which predict3
the emission expected from a hot plasma in collisional equilibrium (Raymond
and Smith 1977, 1979).
The extent of the radio emission from Kepler's SNR is - 3 arc min
(Hermann and Dickel 1973), somewhat smaller than the SSS field of view,
so that these measurements should reflect the total X-ray emission from
Kepler's SNR.
II
V'r
-3-
III. RESULTS AND DISCUSSION
The SSS pulse height spectrum of Kepler's SNR is shown in Figure 1.
As in the case of other SNR, the spectrum cannot be well represented by
a single temperature, but requires at least two thermal components. The
best estimate for Nhe kT of the softer component is controlled by the line
emission and is 0.54 keV. The data are not capable of restricting the kT
of the high temperature component and therefore kT of this component has
been arbitrarily fixed at 4 keV similar to values previously reported for
Cas A and Tycho (Davison et al. 1976). If we require identical elemental
abundances for both components, the best fit values so determined for Mg,
Si, S, Ar, and Fe are given in Table 1. The best fit column density was 6 ± 1 x
1021 atoms/cm2 (2o errors). In these fits, the abundances of He, C, 0, N, and Ne
are all assumed to be solar (see Becker et al. 1979x). The best fit model
has been superimposed upon the data in Figure 1. based on this model, we
infer an X-ray intensity of 7.2 x 10-11 ergs/cm2-s between 1-3 keV.
It is hardly surprising that Kepler's SNR is an X-ray source. None-
theless these measurements are important because they allow a new channel
for comparisons among the young remnants of Cas A, Tycho, and Kepler. It
is widely believed that Cas A evolved from the explosion of a very massive
star (Chevalier 1977) while Tycho's and Kepler's SNR resulted from Type
J SN (Minkowski 1966). The SSS has now observed all three of these remnants,
k	 with the results for Cas A and Tycho already reported (Becker et al. 1979a,
1979b).
Direct comparisons among Kepler and the other remnants are complicated
by the uncertainty in the distance and column density to Kepler's SNR.
Tuohy et al. (1979) suggest a distance between 3.5 and 6 kpc based on the
brightness of the historical SN. However, Bunner (1979) estimated a hydrogen
column dens i ty of X1071 atoms/cm2 from observations of the X-ray spectrum,
suggesting Kepler's SNR is closer than previously accepted values. Our
0
w _
k-4-
data do not indicate such a low column density. At the other extreme, van
den Bergh and Kamper (1977) suggest Kepler's SNR is within the galactic
nuclear bulge, ti 9 kpc distant.
If Kepler's SN is indeed similar to Tycho's SN, we would conclude
the correct distance to Kepler's SNR is greater than . 5 kpc (assuming Tychn's
SNR is ^ 1.5 kpc distant (Hughes, Thompson, and Calvin 1911)). The angular
extent of Kepler's SNR is only %, 35% that of Tycho's SNR, while its X-ray
intensity is only ti 8% of Tycho's SN.t (based on HEAD-1 observations of Tycho
by Pravdo et al. 1979), thus suggesting Kepler's S14R to be ti 3 times more
distant than Tycho's SNR. At a distance of 5 kpc, the X-ray luminosity
of Kepler's SNR between 1-:3 keV is 3.3 x 1035 ergs after correcting
for the effects of interstellar absorption.
Qualitatively, the spectra of Cas A, Tycho, and Kepler are very similar.
All show strong line emission from helium-like species of Si, S, and Ar,
but relatively little emission below 4 keV from ions of Mg and Fe. In all
three sources, the inferred abundance (relative to solar values) of S is
2-3 times that of Si, while Ar is 4-8 times that of Si. Furthermore, results
from observations of MSH11-54 (Clark Tuohy, and Becker 1980) show the same
relative excess of S. When taken together, the spectra of SNR's suggest
that the galactic abundance ratio of S to Si is 2-3 times that usually estimated
for the solar system, with an even greater relative excess existing for
Ar. However, these conclusions will remain questionable until a complete
non-equilibrium model for X-ray emission is developed. The relative under-
abundance of Mg in all three remnants may be indicative of the uncertainties
in our models. Since even in these young SNR, a substantial amount of inter-
stellar material (presumably containing Mg) has been swept-up, a complete
lack of Mg in the SN ejecta still could not explain such low Mg abundances
	
.
in the X-ray emitting plasma. The model also fails to account for an apparent
line feature at 2.9 keV. This energy corresponds to the ls 2 -ls3p transition
of helium-like S. In the Tycho SNR spectrum, an excess is present at 2.2
0
F, .
- 5 -
keV, the corresponding transition for helium-like Si. Enhancement of these
transitions might be indicative of a non-equilibrium situation in which
the electron temperature is higher than that indicated by the line spectrum
as is probably the case in young SNR (Itoh 1977).
In summary, we can not account in detail for the various line strengths
observed in the spectra of young SNR. However, it is clear that qualitatively
similar conditions exist in Cas A, Tycho, and Kepler in spite of the vastly
different origins between Cas A and the two Type 1 remnants. Future work
will extend these comparisons to older, more evolved, remnants.
We wish to acknowledge B. Smith and J. Raymond for helpful discussions.
4
REFERENCES
BeckeM , R.N., Holt, S.S., Smith, B.W., White, N.E., Boldt, E.A., Mushotzky,
R.F., and Serlemitsos, P.J. 1979a, Ap. J. (Letters), 234, L73.
Becker, R.H.. Holt, S.S., Smith, B.W., White, N.E., Boldt, E.A., Mushotzky,
R.F., and Serlemitsos, P.J. 1979b, submitted to Ap. J. (Letters).
Bunner, A.N. 1979, B.A.A.S. 110 461.
Chevalier, R.A. 1971 0
 Ann. N.Y. Acad. Sci. 302, 106.
Clark, D.H., Tuohy, I.R., and Becker, R.H. 1980, in preparation.
Davison, P.J.N., Culhane, J.L., and Mitchell, R.J. 1976, Ap. J. (Letters)
206, L37.
Hermann, B.R. and Dickel, J.R. 1973, Ap. J. 78 9
 879.
Holt, S.S., White, N.E., Becker, R.N., Boldt, E.A., Mushotzky, R.F.,
Serlemitsos, P.J., and Smith, B.W. 1979 1, Ap. J. (Letters), 234, L65.
Itoh, H. 1977, Publ. Astron. So:. Japan 29, 813.
Joyce, R.M., Becker, R.H., Birsa, F.B., Holt, S.S., and Noordzy, M.P. 19780
IEEE Trans. Nuc. Sci. 25, 453.
Meyer, J.P. 1978 9 22nd Liege International Astrophysical Symp.
Minkowski, R. 1966 9
 A.J. 71 9 371.
Pravdo, S.H., Smith, B.W., Charles, P.A., and Tuohy, I.R. 1979, Ap. J.
(Letters), in press.
Raymond, J.C. and Smith, B.W. 1977,'Ap. J. Suppl. 35, 41%).
Raymond, J.C. and Smith, B.W. 1979, in preparation.
Tuohy, I.R., Nugent, J.J., Garmire, G.P., and Clark, D.H. 1979, Nature 279, 139.
van den Bergh, S., and Kamper, K.W. 1977, Ap. J. 218, 617.
II
P+t
FIGURE CAPTIONS
Figure 1 - X-ray spectrum of Kepler's SNR as observed by the SSS on
HEAO 2. Superimposed upon the data 1s the expected response
of the SSS to a two-component thermal model. The lower second
trace is the estimated contribution to the spectrum from
emission by H, fie, C, 0 9 N, and Ne. The three peaks in the
spectrum at 1,85, 2.45, and 3.1 keV are attributable to line
emission from Si XIII, S XV, and Ar XVII respectively.
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BIBLIOGRAPHIC DATA SHEET
1. Report No. 7 2. Government Accession No. 3.	 Catalog No.TM 80637
4. Title and Subtitle 5. Repoort Cate
X-Ray Spectrum of Kepler's SNR January 198Q
e. Performing Organization Code
661
7. Authw(s) R.H.	 BECKER o E.A.	 BOLDT t S.S.	 HOLT i, 8. Performing Organization Report No.
P.J. SERLEMITSOS, N.E. WHITE
9CORrefor10i0	 meOrganizatior Na
	
and Address
	 10. Work Uia;t No.
Laboratory for High Energy Astrophysics
NASA/Goddard Space F l ight Center
	
11. Contract or Grant No.
Greenbelt, MD
	
20771
13. Type of Report and Period Covered
12. Sponsoring Agency Name and Address
14. Sponsoring Agency Code
15. Supplementary Notes
Accepted for publication in Ap. u.
	 (Letters)
16. Abstract
Observations made with the Solid State Spectrometer (SSS) aboard the
Einstein Observatory confirm Kepler's SST^IIR as an X-ray source with an
intensity between 1-3 keV of 7.2 x 10" TT
 ergs/cm2 -s.	 The X-ray spectrum is
similar to those of Cas A and Tycho, with strong line emission from the
helium-like species of Si, S and Ar.
	 Direct comparisons to Tycho's SNR
suggest a distance of Kepler's SNR of > 5 kpc.
17. Key Words (Selected by Author(s)) 18. Distribution Statement
19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of lages T22. Price'
U U 9
nor saie oy [ne rvationai i ecnnicai inrormation bervice, Springrler 1, Virginia Z1101.	 G5FC 25 .44 (30/77)


X-ray spectrum of Kepler's SNR

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