A binary X-ray source, an extended extragalactic X-ray source and several nearby stars were surveyed for X-ray emission. The energy spectrum and time structure of X-ray flux from the binary source, Her X-l, was investigated in the range from 0.15 to 6 KeV. This source was observed at a binary phase of 0.18 with the system near elongation normal to the line of sight. Intense pulsations were observed in optical emission lines near this binary phase. The spectrum and angular distribution of X-ray emission from the X-ray source in the Virgo Cluster of Galaxies, near M 87, was also observed. In addition, the stars Alpha Leo, Zeta Her, and Epsilon Vir were investigated. Epsilon Aur and Alpha Aur were also scanned. These stars were studied since there is increasing evidence that such objects may be transient sources of soft X-rays

Acton, L. W.

Catura, R. C.

English

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https://ntrs.nasa.gov/search.jsp?R=19760015048 2020-03-22T15:14:23+00:00Z
'A	 ,l
	
(NASA-u-146799) CESERVA_ION OF SOFT X-RAYS 	 N76-22136
FROM COSMIC SOURCES final Report (Lockheed
Missiles and Space Co.) 13 p HC $3.50
	
CSCL 03H	 Unclas
G3/93 17837
Ti INAI REPORT
Contract NASv-2660
a
Observation of Soft X-Rays Prom
Cosmic Sources
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31 March 1975	 3
R. C. Catura
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Lockheed Palo Alto Research Laboratory
3251 Hanover Street
Palo Alto, California 94304
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INTRODUCTION
The purpose of this program was to continue the investigation of spec-
tra and angular structure of soft x-ray sources which was begun under
Contracts NASw-1388 and NASw-2414. The rocket payload flown or NASA-17.012
was refurbished, modified, and flown on 17.013 under this contract.
During the flight of Aerobee 17.013 observations were made on a
binary x-ray source, an extended extragalactic x-ray source and several
nearby stars were surveyed for x-ray emission. These observations and their
F
objectives are discussed in the following paragraphs.
F
The energy spectrum and time structure of x-ray flux from the binary
source, Her X-1, was investigated in the range from 0.15 to 6 keV. This
source was observed at a binary phase of 0.18 with the system near Tonga-
tion normal to the line of sight. Intense pulsations have been observed in
optical emission lines near this binary phase. 'The`urpose of the rocket
observations is to confirm the soft x-ray flux reported by the Naval Research
Laboratory and investigate its intensity and pulsation phase relative to
flux at higher energies. These observations were made at a different binary
phase than those of NRLy_in particular,.at a phase when line of sight x-ray
absorption is l i kely to be `a minimum.
The spectrum and angular distribution of x-ray emission from the x-ray
source in the Virgo Cluster of Galaxies, near M 87, was also observed. This
measurement was made with greatly improved sensitivity resulting from longer
1
observing time with a detector of lower background than that used for the
observation of this source during the flight of 17.08. The purpose of the
observation is to investigate the angular structure of this source, now
in dispute, and study the relationship between-the x-ray emission, the active,_
galaxy M 87, and the cluster of galaxies.
Several stars were observed for x-ray emission during a routine part
of the ACS program when a star tracker "locks on" these stars to calibrate
€ the gyro-system.
	
These stars are a Leo, S Her, and g Vir.	 A scan was also
made over e Aur, whose companion star may be a black hole, and a Aur.	 Evi-
-
dense for x-ray emission from a Aur was obtained from previous observations f
#with this payload.on the flight of 17.012.	 The 'purpose of these observations d
is to investigate nearby stellar objects for x-ray emission, since there is
increasing evidence that such objects may be transient sources of soft x-rays.
MERIMFENT PAYLOAD
s
A major part of the instrumentation used for observations under this
contract has been flown previously on NASA-Aerobees 17.08 and 17.012.	 A
,a
schematic view of the payload for 17.013 is shown in Figure 1. 	 The primary
a
instrumentation consists of a pair of gas flow proportional counters
`r
which view space through
	
set of x-ray reflectors. _These reflectors are,a
parabolic in shape along the lengths shown and are linear in a dimension.^
thenormal to
	
plane of the figure.
	 The surface of these mirrors is made k
from polished Kanigen, a form of chemically deposited nickel.	 In this
s
view, a plane wave of x-rays entering from the left undergoes ?,single
4
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REPRODUC1Br'i
ORIGIiQAL PAGL ISPQOR Er
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Yreflection by the mirrors and is focussed on an aperture in the focal
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plane.	 Three apertures are used in this system, one which lies on the
'
optical plane and two others which are slightly off this plane.
	
The fields
e
} of view of these apertures, when projected through the reflector system
onto the sky, are 0.1° by 80 for that on the optical plane and0.250 by 80
for those which are off the plane. 	 Behind each aperture is a gas flow pro-
ortional counter which detects the x-rays. 	 The	 rims
	
mode of acquiringP	 Y	 p ary	 	
data with this system is by slowly scanning these fields of.view across
an x-ray source and registering the counting rates in the detectors as
the image of the source sweeps across the apertures.	 By utilizing rate-
1
w integrating gyros, the rocket attitude control system (ACS) was able to
provide a stable scan rate of .02 0 sec
-1
 during data acquisition. 	 The
star tracker allows the ACS to position the start of a scan on the sky to
within a few tenths of a degree. 	 A 35 mm camera photographs the star field i
^ every 1.5 sec to provide aspect data.
Since the x-ray efficiency of the reflector system decreases rapidly
r,
i
at energies above 2 keV it was necessary to provide an auxiliary detector
$ for observing the spectrum of Her X-1 at higher energies.	 For this purpose,
a proportional counter wlth a 1 micron polypropylene window was added to
{}j
(
{E, the payload as shown in Figure 1. 	 This detector has an area of approxi-
mately 100 cm2 and has its field of view collimated to 20 FV M in two
dimensions.
	
A gas anti.coincidence system reduced the background rate<
in this proportional counter to 0.3 count sec -1 in the range from 0.-(5
to 6 keV as measured on the ground. 	 Anticoincidence counters are indicated
in Figure 1 by chasbers with an X at their center.
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x
In addition to the collimated proportional counter the payload from
17.012 was modified in the following ways for the flight of 17.013.
a) The proportional counter system at the reflector focus was re-
designed to increase the angular interval ever which x-rays were detected
and to improve the anticoincidence efficiency for reducing background count-
ing rates. The detector system on 17,012 had two proportional counters
using different gases which were isolated by a pressure tight wall. These
detectors had apertures which were 0.1 0 and 0.30 wide. In the new design
the wall, its pressure seal, and the detector with a 0.30 aperture were
replaced by two detectors, each having a 0.25 0 aperture. The time available
for observing a source as it sears across the focal plane was thereby increased
by — 50%. Background rejection efficiency was increased by surrounding the
x-ray detectors as completely as possible with gas anticoincidence counters.
b) Previously, the proportional counters at the reflector focus
operated at 400 Torr, requiring complicated gas flow systems because of the
sub-atmospheric pressure. Also, because the detectors used different gases,
two of these flow systems were required. Since reliability of gas flow
systems and thin polypropylene yrindows operating at slig':- ly above atmospheric
pressure have greatly improved, this redundancy was no trit. ,ger required.
The system was redesigned for 17.013 so that the detectors used a common
gas and operated at 760 Torr. Gas was flowed through the detectors until
just before launch when they were sealed-off. During flight a demand-type
regulator supplied gas to compensate for any pressure drop. This greatly
simplified the gas system at a considerable saving in weight.
5
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c) A 32-channel pulse height analyzer (PEA) was used to digitize
the pulse amplitudes from the detectors at the reflector focus. Outputs
from the three detectors were fed through a common amplifier into the PEA.
Each x-ray pulse generated an eight bit word,where the first five bits
were the digitized pulse amplitude and the remaining three bits identified
the detector in which the event had occurred. For each evert, this eight
bit word was shifted to a, register which was sampled approximately every
50 microseconds by a POM encoder. Output of the encoder was transmitted
to the ground for later de-coding. On 17.012 the data were transmitted
in an analog manner. This system was retained on 17.013 for monitoring the
detector operation in routine checks and for redundancy.
d) Piducial markers placed on the camera photographs for calibration
purposes were improved by replacing cross-shaped apertures in front of light
emitting diodes with precise circular apertures .006" in diameter.
e) Detector energy calibration with a radioactive source was initiated
just before each slow data taking scan in addition to the beginning and end
of the flight.
f) The Bragg crystal spectrometer and a photoelectric aspect sensor
flown on 17.012 were not flown on 17.013.
6
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PRELTMVMRY RESULTS FROM 17.01;')
Aerobes 17.013CG was successfully launched from White Sands Missile
Range on 3 February 1975 at C2:16 MST. The payload reached a peak altitude
of 253 lzi and provided a time above 120, km of 342 sec. All of -the instru-
ments in the experiment payload performed properly and acquired useful
data.
Preliminary data from the three detectors at the focus of the x-ray
mirror';tnd from the collimated proportional counter are shown for the entire
flight in Figure 2 on the following page. Detector 1 is behind the 0.10
aperture while detectors 2 and 3 are behind 0.25 0 apertures.. The counting rates
in each of the detectors have been summed for 3 sec intervals and plotted
as histograms. The mirror detector data refer to the energy range from .5
to 2 keV, -.chile those from the collimated detector range from .75 to 6 keV.
The sequence events which occurred during the flight are indicated along
the top of the figure. Labels such as "to Her %-1" indicate a sequence of
attitude control systerL (ACS) maneuvers required to position the payload
axis for data taking scans and "Cal" indicates the duration of a period
of energy calibration of the detectors .
 with a radioactive source. Data
	
f^
have not been included for the "Cal" periods. A period of "noisy data"'
1
is indicated in the plot for mirror detector 2 when the analog data output
malfunctioned briefly. The PCM data appears to be clean and does not have
this difficulty.	 i
TO HER X-1	 CAN OVER HER X-1 2 TO VIRGO UIQ' I s	 141	 1	 1
40
MIRROR
DETECTORI
20
0
NOISY
DATA40
MIRROR
DETECTOR 2
20
0
40 —
MIRROR
DETECTOR 3
E^ 20 —
0
0
60	 COLLWATEDDETECTOR
40
20 IFLLIJ,
0 100	 120	 140	 160	 180	 200	 220	 240	 260
TIME AFTER LAUN
C^
OUGLNAL P-4'^GL 'S
I-JP
kD7U;^ 'T FRAME
TO -.1
ccSCAN OVER VIRGO	 a AND aAUR -` SCANS OVER a AND a AUR 0
CRAB
NEBULA
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129
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3
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280	 300	 320	 340	 300	 3B0	 40U
	
420	 440	 400
H(SEC)
y
a
A 1.0° scan over Her X -1 was performed at a rate of 0.02° sec-1.
The ACS positioned the payload axis too clos& to the source at the beginning
of this scan so the source was already in the fieldof view of detector 3,
as indicated by the high counting rate at about 175 sec. As the scan pro-
ceeds, the image of Her X-1 moves successively into detectors 2 and 1 as
indicated by the sequential increase in count:=,ng rates registered by these
detectors. Data in detector 2 is particularly noisy between — 200 and 250 sec,
however PCM data confirms the increased counting rate in detector 2 near
i l
	
190 sec as being a real signal from Her X-1. The counting rate in the colli-
mated detector is modulated very little throughout this period because of
its relatively broad field of view (20 FWHM) compared to the apparent motion
of the source during :e ecan from -0.5° to + 0.50 relative to the payload
axis. Also, because of the broad collimation, Her X-1 remains in the field
of view of this detector during a roll maneuver at the end of this slow scan.
A scan of 1.60 was performed over the Virgo x-ray source, again at a
rate of 0.020 sec-1, but in the opposite sense to that over Her X-1. Con-
sequently, this source appears in the three mirror detectors in the opposite
sense, with the counting rates peaking at times of approximately 270, 280
and 295 sec in detectors 1, 2 and 3 respectively. Details of the source
extent and its angular structure must await further analysis. A substan-
tial decrease in counting rate occurs iil the collimated detector throughout this
scan since the apparent source motion proceeds from approximately 0° to 1.6°
relative to the payload axis.
9
REP tO IICILME Y OF TAT:
ORIGINTAL PAGL IS POOR {
During the maneuver to e and a Aur the payload axis passed sufficiently
near the crab rebu_:i to be observed by the collimated detector but appears
in the mirror detectors only as a slight increase in counting rate by way
of scattering off the reflectors. Two scans in opposite sense, 60 in length
and at a rate of — .15° sec -1 were made over E and [x Aur. Ho signal is appar-
ent from these objects in the preliminary arita of Figure 2. The staxs Ce Leo,
S Her and e Vir were in the field of view of detector 1 near times of 125, 158,
and 245 sec. Dais_, no signal is apparent at these times in this preliminary
reduction of data. Final analysis will decrease the noise level somewhat
and if no signal is detected, allow at least upper limits to be placed on
x-ray emission from these objets.
Preliminary reduction of data from the collimated detector has been
completed for the observation of Her X-1. Figure 3 shows a preliminary
light curve for this source in two energy intervals where the data have been
folded with the period 1.24 sec. Modulation of the x-ray emission at this
pulsation period is apparent and appears to diminish somewhat at low energy.
Fil m from the 35 mm camera, which photographed the star field, has
been developed and is of high quality. Reduction of these photographs
will provide the necessary aspect information for analysis of the flight
data.
r^
10
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M
104
8C
H 6Cz
^j 4C
2C
C
8C
r 6Cz
4C
2(
1.5-6keV
BKG
0.75-1.5keV
BKG _,
I
	
7I	 I	 I	 I	 I	 I	 I
0	 2	 4	 6	 8	 10	 2	 4	 6	 8	 10
1.237 SEC LIGHT CURVE BINS
Figure 3 X-ray light curves of H er X-1 1
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COTICLUSIOT?S MM RrC0!-2,7T1DATIONS
h%	 X-ray reflector, that are built in the future should focus in both
6_	 !
dimensions to produce an image of an x-ray source. Tilhile singly focussing
r
optics have some advantage over collimated proportional counters, they
r	 suffer from the fa.^t that in acauiring data their field of view must be
f
scanned over a source. This is very inefficient for measuremer_ts frith 	 1
d
good angular resolution since only a small fraction of the observationi
time is spent collecting data from a given part of the source. iben the
modest improvement in observing time obtained on this flight by increasing
the anulex aperture of the detectors at the reflector focus has proven
to be very beneficial. Imaging optics, however, nan acquire data with
high resolution from each part of an extended source region for the full
t3
observing time.
B)	 Reliability of gas flow systems and thin polypropylena detector windows
operating at atmospheric p,ecsures have been established on this flight
and on other solar rocket flights. The simplicity and low cost of these
systems are of distinct advantage and t., eir use is recommended. Also,
there is no danger of applying high voltage destructively to the detector.
while D. is accidentally evacuated, as there is in the case of sub-atmospheric
counters utilizing flow into a vacuum system. 	 i
12
c	 r


Observation of soft X-rays from cosmic sources

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