OSO-8 observations of the X-ray flux in the range 2-60 keV from the Virgo, Perseus, and Coma Clusters provide strong evidence for the thermal origin of the radiation, including iron line emission. The data are adequately described by emission from an isothermal plasma with an iron abundance in near agreement with cosmic levels. A power law description is generally less acceptable and is ruled out in the case of Perseus. Implications on the origin of the cluster gas are discussed

Boldt, E. A.

Holt, S. S.

Serlemitsos, P. J.

Smith, B. W.

Swank, J. H.

English

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https://ntrs.nasa.gov/search.jsp?R=19760025007 2020-03-22T13:00:40+00:00Z
• 
1-661-76-205 
PREPRfNT 
7//f9 
X-RADIATION FROM CLUSTERS OF 
GALAXIES: SPECTRAL EVIDENCE 
FOR A HOT EVOLVED GAS 
P. J. SERLEMITSOS 
B. W. SMITH 
E. A. BOLDT 
S. S. HOLT 
J. H. SWAN.K 
SEPTEMBER 1976 
GODDARD SPACE FLIGHT CENTER 
GREENBELT, MARYLAND 
\ 
(NASA - TM-X -711 89) X- RADI ATION FROM CLUSTERS N7 6- 32095 
OF GAL AXIES : SPECTR AL EVID ENCE FOR A HO T 
EVOLV ED GAS (NA.· ) 12 p HC $3. 50 CSCL 03 8 
Unc l as 
G3 / 93 033 74 
X-radiation From Clusters of Galaxies:
Spectral Evidence For A Hot Evolved Gas
P. J. Serlemitsos, B. W. Smith	 E. A. Boldt,
S. S. Holt, and J. H. Swank
NASA/Goddard Space Flight Center
Greenbelt, Marylar:d 20771
ABSTRACT
OSO-8 observations of the X-ray flux in the range
2-60 keV from the Virgo, Perseus, and Coma clusters
provide strong e-idence for the thermal origin of
the radiation, including iron line emission. The
data are adequately described by emission from an
isothermal plasma with an iron abundance in near agreement
with cosmic levels. A power law description is generally
` less acceptable and is ruled out in the case of Perseus.
Implications on the origin of the cluster gas are
discussed.
I. INTRODUCTION
Spectral measurements of X-ray emission from several clusters of
galaxies (Kellogg, Baldwin, and Koch 1975; Davidsen et al. 1975; Scheepmaker
et al., 1976) have already resulted in substantial evidence that favors
the thermal origin of the radiation over the inverse Compton hypothesis.
NAS/NRC Resident Research Associate
r
- 2 -
The Ariel 5 discovery by Mitchell et al. (1976) of an emission feature
near 7 keV in the spectrum of the Perseus cluster has, in addition, in-
troduced implications concerning the history of the emitting gas. If
I
this feature is indeed due to thermally excited iron lines, is it an isolated
case, perhaps due to the presence of the active galaxy NGC1275, or is
it an inherent feature common to other clusters as well? In addition
to Perseus, the Virgo and Coma clusters emit radiation resulting in strong
enough fluxes at the earth to allow a search for iron lines with data
presently at hand. The three clusters form a diverse and, therefore,
interesting sample with differences in richness, morphology, and predominance
of active galaxies.
The Goddard X-ray Spectroscopy Experiment aboard OSO-8 has observed
these three clusters as part of an observing program that will eventually
result in spectra in the range 2-60 keV for most known X-ray sources_
We report in this paper results from an analysis of "Quick Look" data
seeking to confirm the iron line in Perseus and to search for lines in
Virgo and Coma.
II. THE EXPERIMENT
The Perseus, Virgo, and Coma clusters were observed in that order
in February, June, and July, 1976, each observation lasting some 3-6 days.
Only 10-15 percent of the data are presently available corresponding to
exposures of .91, .93 anu .7 0
 x 105 cm2-s and net counts of ti 12 x 104,
6 x 104 and 4 x 104 respectively. The detector involved in all three
measurements is a multi-wire xenon-methane proportional counter fitted
	 I
with a .002 inch beryllium window, having a net area of 263 cm  and 5.1
0- 3 -
degree circular collimation. The center of the field of view is in con-
tinuous motion via a 5 degree precession about the spacecraft spin axis.
This mode of observation both optimizes the exposure to a source and allows
us to obtain source and background spectra each spacecraft revolution
(ti 10 sec). On board calibration confirms that near optimum energy resol-
ution ( ti 1.1 keV at 6.7 keV) has been maintained since launch (21 June
1975), For additional information on this experiment see Serlemitsos
et al. (1976).
III. RESULTS
Our method of analyzing spectra is adequately described by Pravdo
et al. (1976). Net counts from each source were fitted to two types of
spectra: a single temperature thermal continuum and a power law, adding
in each case a narrow line at 6.7 keV and treating its equivalent width
as a free parameter. Absorption by cold matter was also included with
relative abundances as given by Brown and Gould (1970). We utilized the
Gaunt factor computer approximation of Kellogg, Baldwin and Koch (1975)
for hydrogen and helium in our thermal fits, since these elements dominate
the contribution to the continuum emission. All parameters and their
errors were derived from a X 2 comparison between the actual data and the
predictions of the assumed source spectrum folded through the detector
response. In estimating parameter errors, we followed the approach of
Lampton et al. (1976).
In Table 1 we summarize our results. We note that the thermal
model provides acceptable fits although the confidence level for Virgo
is only •L 4 percent, partially due to a possible additional component
- 4 -
indicated at the high energy end. The power law spectrum gives a poorer
fit in each case, being ruled out in the case of Perseus where the sta-
tistical significance of the data is highest. In addition, the power
	 I"
law requires substantially larger absorption. In every case, t::e addition
of the iron line reduces the X 2 substantially. The deduced equivalent
widths are roughly the same for both types of spectra. In the weakest
case, (i.e. Coma with poorer statistics and lower equivalent width), con-
fidence in the presence of the line is at the 3a level. The line stands
out in the inferred incident spectrum for Coma shown in Figure 1 along
with the best fit thermal continuum including the deduced absorption.
In Figure 2 we show the residuals obtained after subtracting from the
three sets of data the best fit continua. In all three cases the data
are consistent with narrow lines, i.e, the observed broadening can be
accounted for by the detector response.
The same detector observed the Crab nebula in March 1976. We fitted
a sample of the Crab data in precisely the same manner using a power law
spectrum with an iron line at 6.7 keV. The 3a upper limit to the equivalent
f
width of line emission from the Crab is 35 eV.
I
IV. DISCUSSION
Results presented in this paper greatly enhance the already mounting
evidence for the thermal origin of the X-ray emission from clusters of
galaxies. They support a model for the emitting plasma more in agreement
with the simple isothermal case of Lea et al. (1973) rather than the more
complex adiabatic model discussed by Mitchell et al. (1976). They confirm
the iron line in Perseus, and add Virgo and Coma to the list of cluster
(All '111 1.,
^; IS POOR
- 5 -
sources with similar lines. Support is given to source models that would
predict iron and presumably other heavy element abundances to be at or
near cosmic levels in the emitting gas.
The gas temperatures derived from this work are generally in agree-
ment with the thermal fits of Kellogg, Baldwin and Koch (1975) including
the required amounts of absorbing matter in the line of sight. We assume
that the reason these authors did not report line emission must arise
from insufficient spectral resolution and/or stability of the UHURU counters
ever the time of the measurements. The statistical significance of their
data certainly appears to be comparable with that of these observations.
In computing the equivalent widths we have used the collisional
equilibrium calculations for spectra of optically thin plasmas by Raymond,
Smith, and Cox (1976). Assuming a cosmic abundance of iron relative to
hydrogen of 4 x 10-5 (Allen 1973), we infer iron abundances for cluster
sources as given in Table 1. Our 0SO-8 observations of Cas A (Pravdo
et al. 1976) have, using the same optically thin model, given a relative
iron abundance of .64 for that supernova remnant, which is within
ti 2a of the abundances deduced for Virgo, Perseus, and Coma. As in the
case of Cas A, had we used the model of Tucker and Koren (1971), we would
have deduced higher iron abundances by about a factor of 3. Mitchell
et al. (1976) would obtain an equivalent width for Perseus in better agree-
ment with our result if they were to use 6.9 keV instead of 6 keV as the
temperature for thi • continuum.
Although the values inferred for plasma temperature and iron line
equivalent width vary significantly among the three clusters considered
FIGURE CAPTIONS
Figure 1	 The inferred incident spectrum for the Coma cluster of
galaxies. The solid line represents the best fit thermal
continuum with parameters as shown in Table 1.
Figure 2	 Residual counts for Virgo, Perseus, and Coma obtained by
subtracting from the data the best fit thermal continua
from Table 1.
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REFERENCES
Allen, C. W. 1973, Astrophysical quantities, 3rd ed. (University c1
London)
Brown, R. L., and Gould, R. J. 1970, Phys. Rev., D1, 2252.
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Lca, S. M. 1976, IAU 16th General Assembly, to be publishe ,1 in Highlights
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Scheepmaker, A., Ricker, G. R., Brecher, 	 Ryckman, S. G., Ballintine,
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X-radiation from clusters of galaxies: Spectral evidence for a hot evolved gas

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