X ray spectra of discrete sources in Cygnus data from rocket-borne proportional counte

Bleach, R. D.

Boldt, E. A.

Holt, S. S.

Schwartz, D. A.

Serlemitsos, P. J.

English

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X-RAY SPECTRA OF DISCRETE
SOURCES IN C1fGNUS
D.
P.
R. D. BLEACH
E. A. BOLDT
S. S. H O L T
A. SCHWARTZ
	
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1
X-RAY SPECTRA OF DISCRETE SOURCES IN CYGNUS
R.D. Bleacht, E.A. Boldt, S.S. Holt, D.A. Schwartz*, &
P.J. Serlemitsos, Goddard Space Flight Center, Greenbelt, Md. 20771
Abstract
X-ray spectral data from a rocket-borne proportional counter
exposure to discrete sources in Cygnus are presented. The data
from Cyg X-1 9 Cyg X-2, and Cyg X-3 have sufficient statistical
significance to clearly indicate mutually exclusive spectral forms
for the three. Upper limits are presented for X-ray intensities
above 2 keV for Cyg X-4 and Cyg X-5 (Cygnus Troop).
t Permanent Address: Dept. of Physics, University of Maryland
* NAS-NRC Resident Research Associate t
t
-1-
I. INTRODUCTION
All five of the previously reported sources {.n Cygnus were searched
for in a rocket-borne exposure obtained on September 21, 1970. For all
except Cyg X-5 (Cygnus Loop), the exposure consisted of a single scan
of a 2° (FWHM) wide collimator across the source at a roll rate of
ti 1/2°/sec, with a total collecting area of — 1300 cm 2 . In the cap,
of the Cygnus Loop, a dwell time of - 20 sec was spent with the center
of the 2° x 8° collimator response near the center of the supernova
remnant.
The data were obtained in two multi-anode, multi-layer proportional
counters, Bach with 2° x 8° FWHM collimation and with net areas of
650 cm 2. In the top layers of each counter there was three-sided anti-
coincidence for each 1/2" cell, and four-sided anticoincidence in the
lower layers. The bottom layer and all anodes adjacent to the side
walls were used only for anticoincidence. One of the counters was
filled with P10 (90% argon, 10% methane), and the other with xenon-
methane in like proportions. The operating range of the argon counter
was 1.6 - 24 keV, and that of the xenon counter was 2 - 32 keV. Both
counters had windows of 1 mil aluminized mylar supported by the colli-
mators.
r6
-2-
The high statistical significance of the data allows us to
severely constrain the model parameters for each of the sources.
Having two independent counters with different gases, and layers
within each counter with differing response owing to the opacity of
the higher layers, we can report spectra which are relatively systematics-
free as a result of the non-de&enerate response functions of the various
detecting layers.
II. SPECTRAL OBSERVATIONS
(a) Cyg X-1: Essentially all of the Cyg X-1 spectra reported
from both rocket- and balloon-borne experiments are at least as hard
as the Crab Nebula, i.e., spectral number index "^ 2 (c.f. Oda et al.,
1971). The spectruw which we measure for Cyg X- 1 is considerably softer,
as shown in Figure 1:
dN_
	
+2.4	 -(2.6 +.3) 2	 -1	 -1
dE 7.4-1.3 E	 —	 cm sec keV	 (1)
The limits in the above expression are at the 90% confidence level of
the X2 distribution, as are all other limits in this communication unless
otherwide stated. We are reasonably confident that this soft new spectrum
does not arise from a systematic effect in our experiment, as both the
xenon- and argon-filled counters (as well as the argon layers individually)
are in agreement with this spectrum.
i
ti
i-3-
It ; s interesting to note that we observed this spectrum during
the same few seconds in which we were able to detect marked varia-
bility in the source intensity on time scales of tenths of a second
and greater (Holt et al., 1971). The UHURU satellite, in observing
similar temporal variations, sees spectral indices for Cyg X-1 whist°i
are also > 2 (Giacconi, personal communication, 1971, correcting Chita
et al., 1971). Furthermore, balloon measurements in Sept. 1970 at
higher energies (Matteson and Peterson, personal communication, 1970)
are consistent with the present softer Cyg X-1 spectrum up to 100 keV.
The power law fit is more than adequate. The other "standard"
approximation spectra, thermal continua and black-body, cannot be
reconciled with our data, at all. This would imply a non-thermal
origin for the X-ray output, possibly synchrotron emission as in the
case of the nebular X-ray output of the Crab Nebula. As pointed out
by Oda, et al. (1971), however, there is good reason to suppose that
f
the Cyg X-1 X-ray emitting source is considerably smaller. There is
no visible or radio nebula at the Cyg X-1 position, and the shortest
time scale variations observed by us (, 300 msec), imply a linear
dimension for the source region which is smaller than a solar radius.
Because of the relatively high energy threshold of our instrument,
we are unable to make a very sensitive measurement of interstellar
	 X
absorption by cold material in the line of sight. Our data are con-
sistent with no absorption, with a 90% confidence upper limit of 5 r 1021
atoms/cm2 using the formalism of Brown and Gould (1970). This is to
4.
i
-4-
be compared with l.6 x 10 21 atoms/cm2 as deduced by Gursky et al. (1971)
from an analysis of X-ray and 21 cm data.
(b) Cyl 2: Unlike Cyg X-1, with which a non-thermal spectrum
has always been associated, Cyg X-2 has always been reported as being
well fi ? by a thermal continuum. Our measurement confirms this trend,
to the extent that a power law cannot be fit to the data in Figure 2.
The temperature and intensity are in good agreement with previous
measurements:
dN
dE
	
	
1.4 ± .3 exp r 4.5E+ 7 cm 2sec -1keV 1	 (2)
E_ •
Again, the relatively thick transmission window limits our ability
to study interstellar absorption, but the observed Cyg X-2 spectrum is
barely reconcilable with no absorption at the 90% confidence level,
with an upper limit of 2.1 x 1022 atoms /cm2. Although the acceptability
of equation (2) at the 90% confidence level does not allow us to meaning-
fully add another parameter to the analysis, we remark that the inclusion
	
of absorption would decrease the best fit temperatur:.; by 	 1/2 k,--V.
The optical identification of Cyg X-2 (Giacconi et a'., 19n1) and
its interpretation in terms of a binary system (Burbidge et,., 1967)
seem to be fairly well established. If an accretion model for Cyg X-2
is correct, we should expect a compact thermal source with X-ray opacity
arising from the cold infalling matter, as well as free-free and photo-
If
1-5-
electric absorption in the source itself which should exceed inter-
stellar photoelectric opacity. Our upper limit on the amount of
material in the line of sight does not rule out such self-absorption, as
it is more than an order of magnitude above that expected from inter-
stellar opacity alone.
(c) Cyg X-3:  The spectrum from Cyg X-3 cannot be adequately fit
by any of the three simple forms: power law, thermal continuum or
black-body spec rum. Furthermore, the data have high enough statistical
significance to also exclude a power law or thermal continuum masked by
cold interstellar matter using the approximate forms of Bell and Kingston
(1967) or Brown and Gould (1970). The position of the maximum in the
observed counting rate at 5 keV impl:
induced) more than 1023 atoms/cm2 in
absorption by cold material plays an
portion of our observed spectrum, an
might bring out the absorption edges
Les (if the turnover is absorption
the line of sight to Cyg X-3. If
important role in the low energy
order of magnitude more exposure
(especially that due to argon)
which are hinted at in the present observation. Our data above 15 keV
(not displayed in Figure. 3), indicate a hardening of the spectrum at
higher energies with respect to the best fit displayed.
(d) gl& X-4: This source, reported by Giacconi et al. (1967),
at a strength of — .19 keV cm-2 sec-1 in the energy range 2 - 5 keV,
was not detected. The 3 Q upper limit we can place on this object is
i-6-
N .04 keV can 2se ►
-1
 in the same energy range, or about 1% of the
intensity of Cyg X-1.
(e) ga X-5: We did not observe the Cygr.us Loop with a 3 J uppeL
limit of 10-2cm-2sec -1 above 2 keV, assuming a source of uniform
surface brightness coincident with the supernova remnant. This would
imply agreement with a thermal model for the source (c.f. Tucker, 1971)
as the power law fit to the data of Grader et al. (1970) should have
been clearly observable (about an order of magnitude above our upper
limit), while the thermal fit to that same data at a temperature of
a few hundred eV would be below our threshold of detectability.
III. SUMMARY
Cyg X-1 appears to be a compact non-thermal emitter. Temporal
variations in she source on time scales from — 100 ms to - 1 year have
been reported by several groups. The spectra observed by those investi-
gators who have observed rapid temporal fluctuations in Cyg X-1 have a
considerably softer non-thermal spectrum than that observed previously
from the source (by about a full unit in spectral index).
Cyg X-2 appears to be a compact thermal emitter, consistent with
its interpretation as an optical binary system. No rapid temporal
variations of the type observed from Cyg X-1 were seen in the Cyg X-2
exposure (which was identical to that from Cyg X-1 in all respects
save approximately a factor of two in maximum counting rate). Our
-7-
best fit without absorption is barely acceptable at the 90% confidence
level, and the inclusion of absorption would improve it, in qualitative
agreement with the apparent opacity we would expect to see in the direc-
tion of a source which is accreting matter.
Cygnus Loop, by its absence above 2 keV, would appear to be an
extended thermal source associated with the evolved supernova remnant.
Cygnus X-3 may be a compact source of the X-1 or X-2 variety, with
several qualifications. First, it cannot be exactly like X-2 because
it has been observed at energies > 30 keV (Peterson, 1970), and is
observed to be considerably harder than thermal above — 10 keV in our
zperiment, as well. Second, there a:e no temporal fluctuations as in
the case of X-1. Third, the spectrum is very different than that of
the other two, an effect which may arise from absorption or be intrinsic
to the source. In either case, the effect is distinct: ti 1023 atoms/cm2
in the line of sight or a peaking of the X-ray output at 5 keV are both
very different situations than obtained for the other Cygnus sources.
t
-8-
REFERENCES
Bell, K. L., and Kingston, A.E. 1967, M.N.R.A.S., ', 241.
Brown, R., and Gould, R. 1970, Phys. Rev. D, 1 9 2252.
Burbidge, E.M., Lynda, C.R., and Stockton, A.N. 1967, Ap. J. (Letters),
150, L95.
Giacconi, R., Gorenstein, P., Gursky, H., Usher, P.D., Waters, J.R.,
Sandage, A., Osmer, P., and Peach, J.V. 1967, Ap. J. (Letters),
148, L129.
Giacconi, R., Gorenstein, P., Gursky, H., and Waters, J.R. 1967,
Ap. J. (Letters), 148, L119.
Grader, R.J., Hill, R.W., and Stoering, J.P. 1970, Ap. J. (Letters),
161, L45.
Gursky, H., Gorenstein, P., Kerr, F.J., and Grayzeck, E.J. 19710
Ap. J. (Letters), 167, L15.
Holt, S.S., Boldt, E.A., Schwartz, D.A., Ser.emitsos, P.J., and
Bleach, R.D. 1971, Ap. J. (Letters), 166, W5.
Oda, M., Gorenstein, P., Gursky, H., Kellogg, E., Schreier, E.,
Tananbaum, H., and Giacconi, R. 1971, Ap. J. (Lettersl 166, L1.
Peterson, L.E. 1970, in Non-Solar X- and Gamma-Ray Astronomy, ed. by
L. Gratton, (D. Reidel Publishing Co., Dordrecht-Holland), p.59.
Tucker, W. 1971, Science, 172, 372.
x-9-
FIGURE CAPTI'IONS
1. Apparent flux observed from Cyg X-1. The data points represent
the observed count: rate divided by t1w notii nal exposure and
energy range, for they first layer of the xenon-filled counter.
No efficiency corrections are made to the data displayed. The
solid line represents the best-fit spectrum 7.4 L - ' b folded
forward through our cr^mplete detector response, and the dashed
line represents a spectral shape proportional to E -1.7 similarly
folded.
2. Apparent flux observed from Cyg X-
rr
2. The solid line represents
the best-fit spectrum 1-4 exp j- 4 5
E	
folded forward through
our complete detector response. tt
3. Apparent flux observed from Cyg X-3. The solid line represents
the best-fit spectrum 87 f(E, EO)E -3.9 folded through the detector
response, where f(E,EO) denotes the Brown and Gould interstellar
opacity at a characteristic energy EO of 4.0 keV. Note that this
best-fit was not acceptable at the 90% confidence level.
z
10
V
W
U)
N
V
X
M
-i
LL 
.01
H
Z
W
Q
a.
CL
Q
.j
1
CYG X-I
	 1
I
1001
1	 10
ENERGY ( keV)
FIGURE 1
1taiY
IV
W
N
U
V
X
J
LL.	
.01
!-
Z
W
cc
Q
a.
a.Q
.001
r
A
1
10
ENERGY ( keV)
FIGURE 2
t+l	 }^}f
t
CYG X-3
1	
.1
Y
1
U
W
N
U
x
J
La.
	
.01
Z
W
G.
a.
Q
I
1001
10
ENERGY ( keV )
FIGURE 3


X-ray spectra of discrete sources in Cygnus

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