Fifteen strong X-ray sources were observed by the X-ray polarimeters on board the OSO-8 satellite from 1975 to 1978. The final results of this search for X-ray polarization in cosmic sources are presented in the form of upper limits for the ten sources which are discussed elsewhere. These limits in all cases are consistent with a thermal origin for the X-ray emission

Hughes, J. P.

Long, K. S.

Novick, R.

English

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46
A SEARCH FOR X-RAY POLARIZATION
IN COSMIC X-RAY SOURCES
John P. Hughes, Knox S. Long, and Robert Novick
Columbia Astrophysics Laboratory, Columbia Vniversity
Received:
ABSTRACT
Fifteen strong X-ray sources were observed by the v-ray polarimeters on
board the OSO-8 satellite from 1975 to 1978. The final results of this search
for X-ray polarization in cosmic sources are presented in the form of upper
limits for the ten sources which have not been discussed elsewhere.	 These
limits in all cases are consistent with a thermal origin for the X-ray emis-
sion.
Subject headings: nebulae: supernova remnants -- polarization --
X-rays: binaries -- X-rays: sources
<<,
11
Im
2i^
1
I. INTRODUCTION
X-ray polarization can serve as a useful probe into the mechanisms and
geometries of high energy emission from celestial objects. When the emission
is nonthermal, e.g., synchrotron radiation or linear bremsstrahlung, studying
the magnitude, direction, and energy dependence of the polarization can yield
information about the nonisotropic electron velocity distribution and associ-
ated magnetic fields and their configurations. Even when the intrinsic emis-
sion mechanism is unpolarized, scattering can lead to polarization of the
I
emergent radiation.	 In binary X-ray sources, for example, the amount and
angle of polarization may constrain the geometry of the source (Angel 1969)
and, for accretion disks, the inclination angle (Chandrasekar 1960).	 The
measurement of polarization does not uniquely determine a source model but
t
polarimetric results may allow one to distinguish between different models
suggested by photometric, spectral, and temporal data.
Novick et al. (1972) were the first to detect X-ray polarization in a
cosmic X-ray source, the Crab Nebula; their sounding rocket observation proved
that the X-ray emission in the Nebula was due to synchrotron radiation.
Subsequent polarimetry experiments were carried out on two satellites, Ariel V
I
(Griffiths et al. 1976, Gowen et al. 1977) and OSO-8.
	 In this article, we	 j
summarize the results which were obtained with the Bragg crystal polarimeters
on OSO-8.
	
A total of 15 celestial X-ray sources were observed using this
experiment; the results of the data from ten of these objects have not been
discussed previously.	 Polarization was not detected in any of these new
sources and therefore all of the new results are in the form of upper limits.
3II. OBSERVATIONS AND DATA ANALYSIS
	
The polarimetry experiment on OSO-8 consisted of two identical polari-	 :j
meters; each polariveter was made up of graphite crystals (2d = 6.704) mounted
upon a parabolic surface which focused Bragg reflected X-rays into a gas
filled proportional counter.	 The axes of the diffracting parcels were co-
aligned with the spin axis of the satellite, which rotated about the line of
sight to a given X-ray source once every 10 sec. Because of the polarization
dependence of Bragg reflection, the diffracted flux from a polarized source
loceted along the spin axis was modulated at twice the rotation period of the
satellite. The phase of the modulation gave the polarization angle and the
amplitude gave the fractional polarization. The mean angle of incidence for
X-rays was 45°, hence first and second order Bragg reflections at the crystal
s
panels occurred at 2.6 keV and S .2 keV. The modulation factor, M (the mea-
sured response to a 100% linearly polarized beam), was 93%. The polarimeters
f
have been described in more detail by Novick (1975).
Fifteen distinct X-ray sources were observed with the OSO-8 polarimeters
during the lifetime of the mission. Observations lasted from 2 to 11 days.
Table 1 lists for each source: (1) the 11huru designation, (2) the common name
of the source, (3) observation dates, (4) the amount of useful data, and in
the last column, (8) the nature of the X-ray source. 	 Columns (5), (6), and
(7) contain spectral parameters and count rates to be described below. Typi-
cally the amount of useful data taking time was 15% of the length of the
observation.	 Data were rejected d , +ring those times when the source was oc-
culted by the earth as well as during those portions of the orbit in which the
charged particle rates were large. The remaining data were sorted into vari-
ous types depending upon, f-)r example, whether the satellite was viewing a
AL
I
I
1
source on axis in the sunlit sky or in the nighttime. There were also several
types of background measurements: those obtained when the polarimeter field
of view was occulted by the earth, also when the spin axis was greater than 50
from the direction to the source, and in some cases when the X-ray source was
in binary eclipse. Pulse height information allowed us to consider the energy
bandwidths separately as well. The data of each type at both energy band-
widths and for each polarimeter were analyzed separately and the results for
the two polarimeters and the day and night types were combined by time averag-
ing.
The data of each type were binned into 90 azimuth bins (4 0 width) re-
ferenced to celestial north and binniig for valid events was determined by the
orientation of the satellite at the photon time of arrival. A count ra .:e was
computed for each bin by dividing the counts per bin by the corresponding
exposure time and then this was fit by the least squares method to the
function :(0):
f(0) = a0 +a l cos0 + b 1 sin0 + a 2 costa + b2 sing©
with 0 as the azimuth angle. The meal source counting rate plus background is
a0 ; the coefficients a 2 and b2 are related to the Stokes paramecers Q and U
(linear polarization in NS-FT; and NESW-NWSE directions) by
Q = -a 2 /M, and U = -b2/fit.
The fractional polarization
a (Q2 + U2) 1/2P	
I
a. ;s
5where I is the mean source counting rate, i.e., aU minus the instrunerital
	
background counting rate, determined during the earth occulted portion of the 	 11
orbit. The polarization angle
-2arctan(Q)
measured positive east of north.
In the absence of systematic errors, the Stokes parameters Q and U are
normally distributed quantities. Polarization, however, is a positive defi-
nite number and therefore random fluctuations in Q and U yield non-zero values
for polarization even for an unmedulated source. The relevant statistic is
the probability n of obtaining the measured polarization or greater by chance
from an unaolarized source,
n = exp[
-(Q2 + U2)/4021'
where a is the error in the counting rate.
Most galactic X-ray sources are highly variable. In order to estimate
the approximate Uhuru counting rate of these sources during our observations
we have adopter: the following procedure. As described earlier, this experi-
ment determines, among other quantities, a count rate at two energies with
known bandwidths. These two spectral points can be used to calculate a photon
index for a power law fit. We used the Crab Nebula as a calibration standard,
viz., by obtaining factors to convert from the polarioeter count rate to
source photon flux at both 2.6 keV and 5.2 keV. These factors were then used
throughout this analysis for all the sources, ignoring differencas in inter-
stellar absorption. The Crab spectrum used was
6dN
dE	
10.0 E-2-10	 (Toor and Seward 1974),
where dN/dE is the differential photon flux. The results are also presented
in Table 1; columns (5) and (6) contain the photon index and equivalent number
of llhuru counts for a power law spectram and column (7) lists the minimum and
maximum range of Uhuru counts obtained from the 4 Uhuru catalog (Forman et al.
1978).
III. RESULTS AND DISCUSSION
Table 2 presents upper limits on the polarization of ten sources sepa-
rated according to ener,, y and year. The source designation appears in column
(1) and the year of observation in column (2). Column (3) contains the source
i
intensity at 2.6 keV after appropriate background subtraction; the 5.2 keV
intensity is in column (6). The probalility, n, that the observed polariza-
tion (or greater) was obtained by chance from an unpolarized source appears in
column (4) for the first order results and in column (7) for second order.
The upper limits to polarization are in columns (5) (2.6 keV) and (8) (5.2
keV) and represent the extreme values of polarization on the 99% confidence
level contours.
a) Binary X-ray Sources
Many of the sources observed by this instrument have Jong been suspected
to involve thermal X-ray emission processes. It has been proposed that the
bulge sources, binary sources, and X-ray bursters radiate X-rays from some
r1%1
7
type of accretion flow onto a compact object. The X-ray polarization proper-
ties expected for different accretion scenarios, e.g., accreting neutron
stars, accretion disks around black holes, or X-rays scattered off companion
stars, have been studied by Rees (1975) and Lightman and Shapiro (1975). The
application of any of these models to a specific object, however, is quite
complicated and certainly involves details of geometry and sometimes time
dependent effects, as in the scattering of X-rays off a companion in a binary
system. The upper limits pr..oented here are not sensitive enough to be able
to constrain detailed geometric effects (except possibly for inclination
an,gle, see the discussion of 4[11820-30 below) or to examine binary phase
dependent polarization.	 Nevertheless, the OSO-8 observations show that the
time-averaged polarization of strong compact X-ray sources is small, certainly
less than 10%.	 At 2.6 keV, the time-averaged polarization of Sco X-1 and
rygnus X-1 is 0.39 + 0.20% (Long et al. 1979) and 2.4 f 1.1% (Long et al.
1980).	 The restrictive upper limits on sources such as Cen X-3 (10.1y),
4LT1636-53 (14.0%), GX 339-4 (9.4%), CX 349+2 (9.17,), and 4U1820-30 (4.0%)
which we present here, are consistent with a thermal origin for the emission
in an electron scattering dominated disk, as was expected considering the lack
o.e. strong narrow line emission in most of these sources. The detailed inter-
pretation of the results depends upon the exact emission geometry. For exam-
pie, in a geometrically thin disk with a large optical depth to electron
i
scattering, the expected polarization varies from zero for viewing a disk
	 i
i
face-on (i s 0°) to a maximum of 11.7% for an edge-on view (i - 90°)
(Chandrasekhar 1960). For one source, 4U1920-30, which is the brightest X-ray
source associated with any globular cluster, the upper limit to th^ polariza-
tion at 2.6 keV implies that the inclination, assuming such an accretion disk
geometry, is less than 73°.
1
L
n W
R,
8
b) Supernova Remnants
The Crab Nebula is the brightest supernova remnant (SNR) in the sky at X-
ray energies above about 2 keV. The observations with the OSO-8 polarimete:s
have yielded a time- and energy-averaged value for the X-ray polarization of
19.2 f 0.97 at a position angle of 155°8 t 1°4 (Weisskopf et al. 1978). We
have also obtained the polarization of the Crab Nebula at 7.8 keV (third-order
Brag; reflection) from the 1978 observations. At this energy, the polariza-
tion of the Nebula (including pulsar contamination) is 54.5% f 28.4% at an
angle of 137° f 16% consistent with the above value for the lower energy 1976
and 1977 averaged results. These values are consistent with the polarization
at optical wavelengths when the optical polarization map (Oort and Walraven
1956) is integrated over the region of the nebula where X-ray emission
arises. The observation of linear polarization in the nebula throughout the
electromagnetic spectrum proves that synchrotron radiation is dominating the
emission.
Cas A, the next brightest SNR at X-ray wavelengths, is twenty times
fainter than the Crab Nebula.	 This SNR has a radio polarization of —4.57
aligned tangentially around the edge of the remnant (Mayer and Hollinger
1968); unlike the Crab Nebula, DAs A is not currently dominated by energy
supplied from a rotating neutron star. The OSO-8 polarimeters were not suffi-
ciently sensitive to measure (as A's X-ray polarization, even if the frac-
tional polarization were as great as that in the Crab Nebula. Furthermore,
high resolution observations of the X ray spectrum of Cas A show that its
emission is dominated by .line radiation from a shock heated plasma. There-
fore, it is not surprising that polarization was not detected; future, much
9more sensitive polarimetric observations are necessary to determine what
portion of the emission is from the synchrotron process.
IV. CONCLUSION
We have used the polarimeteis on board the OSO-8 satellite in an attempt
to determine the linear X-ray polarization of various bright galRctic
sources. We present here the results of the observations in tb p form of upper
limits to the magnitude of polarization which are consistent with emission
from thermal sources.	 To further constrain the X-ray emission from such 	
t
sources more sensitive polarimetric observations, at about the 1% level are
required. Such sensitivities are envisioned for AXAF.
We wish to thank Martin C. Weisskopf and Eric If. Silver who were instru-
mental in analyzing the early OSO-8 data. Some suggestions by Robert Becker
and Gary Chanan nave been incorporated into this article and we appreciate
their input. This work was supported by NASA research grant NAO.4 18. This
article is Columbia Astrophysics Laboratory Contribution No. 224.
10	 URIGINAL PAGE iJ
OF POOR QUALITY
w
I	 ^
TAW, 1
M>43 RILPRI V"MY OASFRVATIONS
(t1CM-DT1 Observation Fzgvsure Power Iaw fit Range of Source
4l1 Source Nam dates (days) Index Oxaits Utxau cts type
( 1 ) (2) (3) (4) (5) (6) (7) (8)
4 UD316+/41 Perseus 1976: 047-049 0.37 Not detected 46.8 - 48.0 Cluster
cluster 1977: 045-051 1.94 -1.22 90 of galaxies
4UD531+21 Orab 1976: 071-077 1. PA -2.104 910 926 - 968 S14R
Nel-ila 1977: 074-079 1.11 -2.10a 920 pulsar
1979: 071-076 1.19 Gain changed
4U1118-60 @n X-3 1975: 199-207 1.59 -0.66 220 < 20 - 200 Binary
1978:, 197-208 1.49 -1.20 50 pulsar
4U1617-15 Sco X-1 1977: 239-242 1.24 -1.75 11900 6800 - 17000 Binary
1978: 236-2..44 2.17 -1.59 12200
401636-53 1976: 249-254 0.52 -1.8C 120 125 - 250 Raster
4U1656+35 Tier X-1 1975 : 241-245 1.11 -0.08 310 10 - 100 Binary pulsar
4U1658 .48 GX 339-4 1978: 255-258 0.66 -4.06 '_00 120 - 350 Binge source
4U1^02-36 G.X 349+2 1975: 254-255 0.12 -1.32 750 375 - 750 Bulge source
4U1759-25 CSC 5-1 1975: 263-2n/+ 0.22 -1.25 1020 575 - 1150 1lulge source
4U1820-30 1976: 20&-2"3 2.10 -1.48 250 110 - 320 Burster in
1978: 269-274 1.57 -1.48 310 globnular cluster
a Crab nebula used as calibration standard; this %alie is by definition
4111837404	 Ser X-1
41n956+35	 C'm X-1
4112030+40	 Cyg X-3
4112141+38	 Cyg X-2
4if2321+59
	
as A
1975: 276-278	 0.14
1975: 311-314 0.59
1976: 30g-315 1.67
1977: 305-314 3.34
1975: 323-329 1.32
1976: 322-327 2.73
1975:. 343-345 0.98
1976: 339-345 1.35
1977: 338-346 1.02
1976: 021-023	 0.24
IXaC 6624
140 - 280	 Burster
235 - 1175	 Binary
190 - 335	 Binary
275 - 550	 Inc+ mass
binary
52.4 - 54.4	 S1%
I^ A
I	 ^
r	 I
i
1
-1.68
	
250
-2.26
	
790
-1.45
	
340
-1.40
	
330
-0.49
	
90
-0.25
	
70
-1.85
	
I40
-1 50
	
370
-1.72
	
580
-3.28
	
70
i^r^^..i.w^ e w
11
ORIGINAL PAGE 09
TAK[F. 2	
OF POOR QUALITY
Pf1 MI7ATIUV t1PPM LIKM
Energy
...........
2.6 keV 5.2 keV
Source Yc.3r n Leper
(Cts/10009ec) limit (Cto/1000Rec) limit
(1) (2) (3)	 (4) (5) (6) (7) (8)
410316+41 1976 24.62 f 5.13 0.58 25.5% Abt detected
1977 21.34 t 0.63 0.52 11.6% 7.14 t 0.50 0.40 29.4%
Onb 21.39 f 0.63 0.88 9.4%
4U111R-60 1975" 44.69 f 0.93 0.55 13.1: 21.25 t 0.74 0.37 18.9%
1970 11.75 f 1.06 0.01 31.57 3.85 t 0.89 0.56 54.6%
ODMb 30.36 f 0.70 0.06 10.17. 14.14 f 0.57 0.74 12.4%
01636-53 1976 36.25 + 1.00 M3 14.0% 7.82 1 0.72 0.33 54.0%
4ti1656+35 1975" 10.76 t 1.26 0.96 58.7% 3.27 t 1.13 0.20 100.07.
4U1658-48 1978 48.19 f 1.09 0.95 9.4% 2.17 t 0.70 0.17 100.0%
4U1702-36 1975 194.25 13.45 0.92 9.1% 58.67 t 2.17 0.55 21.9%
4TV58-25 1975 254.97 f 2.96 0.01 10.0%. 80.99 f 1.92 0.40 13.9%
401620-30 1976 66.x'7 t 0.96 0.(N 4.9% 18.11 t 0.77 0.48 12.1%
1978 85.18 f 1.15 0.40 7.57; 23.05 ± 0.76 0.72 14.6%
CoIS 74.39 t 0.74 0.50 4.07. 20.61 t 0.54 0.42 9.5%
4U1837+04 1975 73.35 t 2.19 0.62 17.6% 17.7.1 t 1.44 0.04 64.2%
4LI2321+58 1976 30.88 - 3.24 0.66 25.3% 2.40 t 2.42 0.26 100.07.
a %nary eclipse backgraaid data subtracted from I, Q, and U.
b Binary eclipse backgrouml rate subtracted from I.
11
-0
12
ORIGINAL PAGE M
OF POOR QUALITY
REFERENCES
Angel, J. R. P. 1969, Ap. J., 158, 219.
w•^
Chandrasekhar, S. 1960, Radiative Tr p.nsfer (New York: Dover).
Forman, W., Jones, C., Cominsky, L., Julten, 1-., Murray, S., Peters, G.,
Tananbaum, H., and Giacconi, R. 1978, ap • J. Suppl., 38, 357.
.M..
Gowen, R. A., Cooke, B. A., Griffiths, R. E., and Ricketts, M. J. 1977,
1!.N.R.A.S., 179, 303.
,..r
Griffiths, R. E., Ricketts, M. J., and Cooke, B. A. 1976, M.N.R.A.S., 177,
.wm
429.
Lightman, A. P., and Shapiro, S. L. 1975, Ap. J. (Letters), 198, L73.
Long, K. S., Chanar., G. A., Fay , W. 11.-M., and Novick, R. 1979, Ap. J.
(Letters), 232, L107.
Long, K. S., Chanan, G. A., and Novick, R. 1980, A;+• J., 238, 710.
w•w^•
Mayer, C. H., and Hollinger, J. P. 1468, Ap. .T., 151, 53.
Novick, R. 1975, Space Sci. Rev., 18, 389.
Novick, R., Weisskopf, M. C., Berthelsdorf, R., Linke, R., and Wolff, R. S.
1972, Ap. J. (Letters), 174, 1.1.As"
Oort, J. H., and Walraven, T. 1956, Bull. Astr. Iust. Netherlands, 12, 285.	 it
Rees, M. J. 1975, M.N.R.A.S., 171, 457.
Silver, E. H. 1978, Ph.D. thesis, Columbia Thiversity.
Silver, F. H., Weisskopf, M. C., KeRtenbsum, H. L.. Long, K. S., Novick, R.,
and Wolff, R. S. 1979, Ap. J., 232, 248.
wm.
Toor, A., and Seward, F. D. 1974, A. J., 79, 995.
Weisskopf, M. C., Silver, F. H., Kestenhaum, H. L., Long, K. S., and Novick,
•
R.	 1978, Ap. J. (Letters), 220, x.117.
wow
13
JOHN F. HUGHES and ROBERT NOVICK: Columbia Astrophysics Laboratory, Depart—
dents of Astronomy and Physics, Columbia University, 538 'west 120th Street, 	 1
New York, New York 10027.
KNOB S. LONG: Department of Physics, Johns Hopkins University, Baltimore,
Maryland 21219.
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A search for X-ray polarization in cosmic X-ray sources

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