Balloon-borne X ray telescope data limiting angular size of source Cyg XR-

Floyd, F. W.

English

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CENTER FOR SPACE RESEARCH 
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A Limit on the Angular Size

of Cyg XR-1 
by 
Franklin W. Floyd 
CSR-P-69-10 
February 1969 
This work was supported in part by NASA grant NGL 22-009-015
A Limit on the Angular Size of Cyg XR_lt 
by 
Franklin W. Floyd 
Center for Space Research 
Massachusetts Institute of Technology 
Both nebular and stellar objects have been identified 
as discrete sources of cosmic x-rays. Representative of 
the first is the Crab Nebula, the size of which has been 
measured to be 1 to 2 arc minutes in the energy range 1-6 key 
according to observations during a lunar occultation
(1)
 and 
with a modulation collimator 	 . This size is comparable 
to the optical size of the nebula. Representative of stellar 
sources is Sco X-1,the x-ray size of which has been shown 
to be less than 20 arc seconds by a measurement with a mOdu- 
lation collimator, also in the energy rang 1-6 keV 2 . The 
optical counter part of Sco X-1 is an irregularly variable 
blue star (3). No other discrete x-ray source has been observed 
with equipment capable of measuring sizes less than 10 ar 
minutes. However the tentative identifications of Cas XR-1 
with the supernova remnant, Cas A 4 , and of several other 
x-ray sources with blue stars (Cyg X-2 5 , Cen XR-2 6 ) ob-
viously suggest that these particular sources can be assigned 
to the nebular and the stellar classes, respectively. 
The bright x-ray source Cyg XR-1 presents a specially 
interesting problem for size measurement because it has a 
tThis work was supported in part by NASA grant NGL 22-009-015
-2-
hard "power-law" x-ray spectrum extending to 100 keV and 
beyond (7) like the Crab Nebula, and is apparently highly 
variable 8' like Sco X-l. Moreover, no optical counterpart 
of Cyg XR-1 has been identified. Thus a direct measurement 
of its x-ray size may be the only way to determine whether 
it is a variable x-ray nebula or a stellar object emitting 
x-rays with a power law spectrum extending beyond 100 key. 
The purpose of this note is to report an observation 
that places an upper limit of 1.4 minutes of arc on the 
angular diameter of the x-ray source Cyg XR-1 in the energy 
range from 25 to 100 key . This result is based on data 
obtained in a balloon experiment carried out on 16 July 1968 
with an instrument comprising a pair of x-ray telescopes with 
gyro-stabilized modulation collimators. 
The collimators are similar to the one used by Oda 
et al. (2)	 a rocket measurement of the angular diameter 
of the Crab Nebula in the 1-6 keV range. Each collimator 
consists of two parallel grids of 10 mil gold wires mounted 
35 inches apart in a cylindrical tube. When an x-ray source 
passes across the field of view along an arc perpendicular 
to the wires, the transmitted intensity is modulated with 
a period of 2.0 arc minutes and an amplitude that depends 
on the angular diameter of the source. For a point source 
the modulation is nearly one hundred per cent, and for a 
source much larger than 2 arc minutes it is nearly zero. 
A slow and steady source scan is achieved by an intertial 
stabilization system. Two rate integrating gyroscopes are
ISM 
used to stabilize the collimators against random motions 
of the balloon gondola while rotating them at a uniform 
rate of several arc minutes per minute. 
The x-rays transmitted by the collimators are detected 
by two Na I () crystal scintillator detectors which are 
mounted behind the collimators. The total effective area 
of the instrument in the direction of a transmission maxi-
2  
mum is 280 cm . Its overall field of view is 10 1 by 10 
full width at half maximum. The instrument is oriented in 
azimuth and elevation and is controlled by radio command 
from the ground. The x-ray counting rate in each detector 
is telemetered to the ground once per second and recorded. 
The instrument was launched from the National Center 
for Atmospheric Research (NCAR) balloon base at Palestine, 
Texas. Command and telemetry systems were provided by NCAR. 
The data gathering portion of the flight lasted for 5 hours, 
30 minutes at an atmospheric depth of 3.74 cm/cm 2 . A source 
was observed when the telescope was pointed in the direction 
given by Giacconi et al.	 for Cyg XR-1. The intensity of 
Cyg XR-1 at the altitude of observation was found to be 
.056±.007 photons cm 2sec	 in the 25 to 100 key energy 
range. Because the command equipment malfunctioned during 
most of the flight, only one good scan, of 8 minutes duration, 
was made of Cyg XR-1 by the collimators. A Fourier trans-
formation of the data recorded during this scan revealed
-4-
a strong periodicity in the x-ray intensity at a frequency 
consistent with the scan rate, which was determined inde-
pendently from telemetered engineering data. The proba-
bility that random fluctuations were responsible for the 
-3 periodicity is 10 ; equivalent to a 3.45 sigma deviation 
on the normal distribution curve. Possible instrumental 
effects which could cause an apparent modulation of x-ray 
intensity were investigated and found to be negligible 
The same analysis was applied to data taken during the 
remaining 5 hours, 24 minutes of the flight when the in-
strument could be pointed toward and away from the source, 
but the collimators could not be stabilized. No evidence 
of significant periodicity was found in these other data. 
The Fourier coefficients computed from these other data 
followed the predicted probability law very closely which 
indicates that the statistical analysis is valid and that 
the probability quoted above is realistic. The observed 
periodicity is therefore attributed to a modulation of 
the Cyg XR-1 intensity by the collimators. 
The amplitude of the periodic component indicates 
that the source diameter is less than 1.4 arc minutes. 
The result is actually consistent with the hypothesis that 
Cyg XR-1 is a point source, but the limited observing 
time and relatively poor statistics introduce too much 
uncertainty to justify setting a smaller limit than 1.4 
arc minute at this time.
-5-
Given sufficient observing time, this instrument is 
capable of measuring diameters of high energy x-ray sources 
down to 0.1 arc minutes. Flights are planned for both 
Cyg XR-1 and the Crab Nebula. 
The author is indebted to Prof. George Clark for the 
original concept of this work, Prof. Herbert Schnopper for 
his advice and support and to other members of the staff of 
the M.I.T. Center for Space Research - especially Messrs. 
T. Dawson and E. Mangan for their help in carrying out the 
experiment.
Franklin W. Floyd* 
M.I.T. Center for Space Research 
REFERENCE 
(1) S. Bowyer, E. T. By.ram, T. A. Chubb, and H. Friedman, 
Science, 146, 912 (1964) 
(2) M. Oda, H. Bradt, G. Garmire, G. Spada, B. V. Sreekantan, 
H. Gursky, R. Giacconi, P. Gorenstein, J. R. Waters 
Ap. J. (Letters), 148, L5 (1967) 
(3) A. Sandage, P. Osmer, R. Giacconi, P. Gorenstein, 
H. Ginsky, J. Waters, H. Bradt, G., Garmire, B. V. Sreekantan, 
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J. R. Waters, A. Sandage, P. Osmer, and J. V. Peach, 
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* Presently with M.I.T. Lincoln Laboratory
-6-
REFERENCE (continued) 
(6) 0. J. Eggen, K. C. Freeman, and A. Sandage, Ap. J. 
(Letters), 154, L27 (1968) 
(7) R. C. Haymes, D. V. Ellis, G. J. Fishman, S. W. Glen, 
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A limit on the angular size of Cyg XR-1

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