Data from the SAS-3 satellite were used in a search for X-ray pulsations from the direction of the galactic center. No periodic X-ray behavior was detected in the frequency interval 0.6 Hz to 0.0006 Hz and energy range 2.5 - 35 keV. For periods less than 60 sec, the upper limit to the amplitude of any pulsation in the 2.5 - 10 keV band is approximately .0017 cts/sq cm/s. This corresponds to a pulsed fraction of approximately 1.3 percent of the total GCX flux. Somewhat higher limits apply for longer periods and for energies greater than 10 keV

Cordova, F. A.

Garmire, G. P.

Lewin, W. H. G.

English

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'r
A SEARCH FOR X-RAY PULSATIONS
FROM THE GALACTIC CENTER
by
France A. CSrdova and Gordon P. Garmire
California Institute of Technology
Pasadena, California 91125
and
Walter H. G. Lewin
Massachusetts Institute of Technology
Cambridge, Massachusetts 02138
FINAI. TECHNICAI. REPORT
NASA Grant NSG 5197
"Search for Periodic Variations in the X-Ray Flux
From the Galactic Center"
Principal Investigator: Gordon Garmire
November 1978
(TTASA-CR-157930;	 A SEARCH FOB X-RAY
1179-11966
PULSATIO NS F p r)M THE GALACTIC CENTER Final
Rp port (Californi a Inst. of Tech.) CSCL HC	 Unclas
A02/MF A01	 G3/89 37190
Dec
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ABSTRACT
A search hes been made for X-ray pulsations from the direction of
the galactic center using data from the SAS-3 satellite. No periodic
X-ray behavior was detected in the frequency interval 6 x 10 -1 to
6 x 10 -4 liz and energy range 2.5 - 35 keV. For periods 4 60 sec, the
upper limit to the amplitude or any pulsation in the 2.5 - 10 keV band
Is — 1.7 x 10
-3
 cts cm -2
 s -1 . This corresponds to a pulsed fraction
of ^ 1.3% of the total CCX flux. Somewhat higher limits apply for
longer periods are for energies greater than 10 keV.
'	 ^^ {rte' I	 `^ "+^	 rte' _ s	 F-1
-	 -	 -	 - - - -	
-	 •^''.rs ^ . cwt► ! ^^t^ ^ ^
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I.	 INTRODUCTION
X-ray emission was first detected from an extenCed (^- 2°) region
In the direction of the galactic center by Kellogg et al. (1971). Withi	 —
the poor spatial resolution of the Uhuru detector used for this obser-
vation, it was not possible to determine whether the source was truly
diffuse, or whether discrete sources contributed to at least part of
the GCX flux. Discrete sources were later discovered in this region--
two transient sources (Eyler et al. 1975; Ariel -V Group, 1975), and at least
three burst sources (Lewin et al. 1976). Since all of these sources
were short-lived in nature, the question remained open as to whether
discrete sources could account for the steady GCX flux.
A particularly interesting possibility was that the burst sources
t	 might have steady components whose total flux was observed as GCX (Lewin
t
F.t al. 1976). A subsequent survey of this region during an NRL rocket
flight is consistent with this proposal. Cruddace et al. ( 1977) were
able to resolve GCX into a minimum of 4 discrete sources, and noted
that each of the burst error circles overlapped at least one of the
error boxes of these steady sources.
The next interesting question was the determination of the nature
of the discrete GCX sources. Our objective: was to look for periodic X-ray
behavior from GCX. There was a considerable amount of SAS-3 data al-
ready in hand because of the large amount of time the satellite pointed
in the direction of the center of the galaxy watching for bursts. While the
rather large field of view (1.7 0 MM) of the SAS-3 Horizontal Tube detector
ti	 would preclude association of any pulsations with one of the known discrete
UKiG isAL YAV^' ^^
OF 
PWR QUALITY
I	 ^
3
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ORIGINAL PAGE J.
OF POOR QUAD hP
GCX sources, it was still valuable to search for pulsations Just to
identify the kind of sources present at, or near, the Galaxy's center.
If one or more pulsations were present, the large SAS-3 data hase
extending over three years would allow a more detailed study of these
objects.
There is evidence that tran31ent sources may arise frequently
at the galactic center (Cruddace et al. 1977), and we know of two
transients, A1118-61 and A05355+26 which exhibit long period pulsations
of 405s and 104s, respectively (Ives at al. 19751 Rosenberg et al.
1975). Thus it was important for this study that each GCX observation
he for a long enough duration to detect periods ,several minutes long,
and that there would be several observations spaced widely apart, to
accommodate the detection of a pulsating transient source.
With these objectives in mind, we made a preliminary search for
pulsations from the galactic center using SAS-3 data from long pointings
at GCX. In the following sections, we describe the data reduction
procedures and summarize the results.
I1. THE EXPERIMENT AND DATA REDUCTION
The data used for this analysis were taken with the Horizontal 'Tube
system (~ 1.7 0 FWHM) of the SAS-3 X-ray observatory (Lewin et al.
1976b). The argon-filled proportional counters in this system had a
geometrical area of 115 cm  and covered the energy range 2.5 - 10 keV,
while the xenon-filled counter had an area of 80 cm  and spanned the
interval 8 - 35 k.:V.
4
1 ^ 1 ^ 1'l.1	 ! FTT
t
^R tint W ^^ f W ALI I
'	 The sampling length used in the analysis was 0.8314 s, thus
r,	 restricting the search to periods greater than — 1.7 s. Although the
i
data used here were from relatively long observations (a 6.3 h data
block in 1976 Feb and a 18.9 h block on 1977 Feb), the analysis is
probably insensitive to periods longer than 30 minutes because of
severe source modulation due to spacecraft pointing jitter. Drifts
often amounted to a cumulative drift of — t o from the nominal pointing
direction. (These drifts were corrected frequently.) The orbital
period of — 94 minutes and its harmonics are clearly present in the
	 t
power spectra (Figure 1).
A typical block of data is broken up with laps due mainly to earth	
l
occultations and passage through regions of high charged particle
fluxes. Segments of noninterropted data might be anywhere from several
minutes to — 30 minutes long. 1'he average count rate per bin of 0.8314 s
i^
was calculated for each uninterrupted data segment and a weighted mean 	 1
was derived for the entire data block from these individual averages.
Then this weighted mean was subtracted from each data bin. Individual
bad data points were rejected by requiring that each datum be less than i
5a greater than the average value of the previous 3 data values. Values
of the bins during the gap intervals were set to zero; thus the entire 	 4
I
data block had a zero average count rate. The data were then analyzed
using the Cooley-1'ukey fast Fourier transform algorithm. The power
spectra were obtained by squaring the complex Fourier coefficients.
1	 ^
The upper limits to the pulsed flux were derived using; the fact
w
.; _;^
^.I
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t,
that the fluctuations In the power level of a given frequency bin ar.
distributed exponentially. Thus, the 902 confidence criterion implies
that the power in any bin must exceed the local mean power times
en(N/0.1), where N is the number of points in the power spectrum. The
local mean power wa y s derived by summing the power spectrum over 128
bins and fitting the resulting spectrum to a polynomial, using the least
squares fitting technique. Pit , mean power at any frequency was assigned
from this smoothed fit. Since this method is less sensitive to fluctu-
ations in the local mean power, it provides a statistically more re-
liable calculation of the ratio of the power at any frequency to Oe
local mean power than using a short section of nearby power points as
a reference. To test the validity of this approach, a sinusoidal "tracer"
signal was added to the data and transformed. The tracer amplitude com-
puted from the power spectrum was in close agreement with the amplitude
of the original signal.
III. RESULTS
The 90% confidence upper limits to pulsations from CCX are summarized
for both detectors in Fig. 2. Fcr periods less than a few minutes the
power spectra were well-behaved, i.e., the distribution of power points
was exponential. The upper limits increase rapidly for longer periods.
Because of spacecraft pointing jitter, the data is insensitive to periods
longer than — 40 minutes. In Fig. 1, we show only the low frequency
i	 portion of a 2 16 point power spectrum to illustrate how noisy the low
frequency data is because of the presence of many harmonics of the orbital
i
r
j'
in
ORIGINAL PAGE i;
and i ►oninertial periods of the spacecraft. Particularly obvious in Fig. 1
are a large number of very high harmonica of the orbital period. To
check that these were due to periodicities in the "window function", i.e.,
the pattern of data and gaps in the sequence of orbits used its input to
the transform, and not to real periodicities in the data, we Fourier
transformed the window function itself. This was accomplished by assigning
a value of 1 to all the real data values, and a value of 0 to all the gaps.
p'
	
The same pattern of high harmonics resulted from this transform as from
the transform of the real data. The periodicities in the window function
are due to earth occultations, South Atlantic Anomaly crossings, and
transmission gaps. The fact that the power is so high in these harmonics
simply means that the data covering many hours of observation cannot be
fit to the simple mean value we used to pad the gaps. The fluctuations in the
background are definitely in excess of Poisson statistics, but this is
not confined to the GCX region. SAS-3 observations of other areas of
the sky show similar noisy background behavior (Doty, private communi-
cation). We should point out that the high frequency (P < a few minutes)
portion of the power spectrum waa not affected by the background noise,
as was proved when we again transformed the original data, this time
fitting each uninterrupted data stream (4 30 minutes long) to a low-
order polynomial. The high frequency upper limits were the same using
both reduction techniques. of course, with the latter technique, we
were insensitive to periods greater than a few minutes.
The major contributor to the GCX background is the cosmic ray
background. We attempted to mt-asure this background using earth
8occultation data,	 The X-ray sky background	 (non-cosmic)	 is not known,
-2	 -1
but is probably less than 0,01 ct cm a per detector.	 Subtracting
the cosmic ray background only, we calculate that the flux from GCX
i•
changed between the two February observations (- 1 year apart) 	 from
0.10 to 0,14 cts cm -2
 e	 1	 in the argon detector,	 and	 from 0.04 to 0.07
cts cm-2 a -1 in the xenon detector.	 We note that the increase in flux
from GCX in	 1977 February over the 1976 February measurement might 1
t	 ;	 have been due to the presence of a variable source or a transient t
't	 source which had steady emission, 	 an well as burst activity.	 The
SAS-3 Observatory detected a two - fold increase in the number of bursts
t	 in 1977 February, as compared to the observation one Year before 	 ( Lewin,
`	 Hoffman,	 and Doty 1977).
The hest upper limits range from —
 1.6 x 10
-3
 cts cm-2 6
-1	 for
-2	 -2	 -1
periods :; 20 s to — 1.0 x 10
	
cts cm
	 s	 for periods of order 20-30 ^
minutes in the energy range 2 . 5 - 10 keV.	 Tlrese correspond to pulsed
fractions of	 the total GCX flux of	 1.17 and	 7.0%,	 respectively,	 At
higher energies (i.e., 	 in the 8 - 35 keV band)	 these limits range from
2.3 x 10 -3 cts cm-2
 a-1 , or 3.3% of the total GCX flux for periods
6000 s,	 to 6.7 x 10-3 cts cm-2 s -1 , or 9.5% of the total GCX for longer
i
periods.	 Only the pulsed ,amplitude of	 the orbiral	 period of
	 the space-
craft and the first few harmonics of this period exceeded these upper ►`.}
limits. s
i
I
:.1
It is fruitful, as a guide for future observations, to ask what is
the upper limit to the pulsed fraction from a discrete, stead-state,
source in GCX based on the results quoted here. As an example, the flux
from GCX-3, one of the 4 discrete sources in GCX discovered by
,1
1t11(;jN AAA.	 tali l : 1:)
or r(K% ^u^l'Y
1
IV. StrMARY
9
et al. (1977), comprised ,-• 17% of the total GCX flux measured in their
observation. Hen e, our upper limit implies that the pulsed fraction
of this source in the regime 2.5 - 10 keV mist be no greater than
for periods from — 2 - 60 s. Most of the pulsating galactic sources
(Pop. I) have pulsed fractions between 20 and 1001.
Since the search for pulsations from the direction of galactic
center was begun, numerous SAS-3 pointings of GCX have been carried
out, and the data from these observations will be similarl y searched
for periodic phenomena. However, from this initial study, which draws
on data taken during long observations separated by one year, it is
observed that there are no steady short period pulsations to a limit
of — 1.3j of the total GCX flux in the range 2.5 - 10 keV, and 3.3$
in the range 8 - 35 keV. For longer periods, this upper limit becomes
several times higher.
AC KNOW LFIXNENTS
The authors thank Richard Brenner of Caltech for his advice
concerning the FFT analysis, and Lynn Cominskv and Dr. John Doty
of MIT for their help with various aspects of the data reduction.
This work. was supported by National Aeronautics and Space Administra-
tion grant NSG 5197.
ti	 _
I
E'
11
11
I/	
1
10
REFERENCES
Ariel V Grw.., University of Birmingham. 	 1976, I.A,U, Circular No, 2934.
Cruddace, R. G., Fritz, G., Shulman, S., and Friedman, h. 	 1977,
Eyles, C. J., Skinner, G. K., Wfllmore, A. P., and Rosenberg, F. D.
1975, Nature, 257, 291.
Ives, J. C., Santord, P. W., rnd Bell-Burnell, S. J. 	 1975, Nature, 254,
578.
Kellogg, E., Gursky, It.,  Murray, :.., 'rannanbwim, H,, and Giacconi, R.
1971, &i._J,_ (Letters), 169, L99.
Lewin, W. H. G., Noff—an, .1. A., Doty, J., Hearn. D. R., Clark, G. W.,
lernigan, .1. G., Li, F. K., McClintock, J. E., and Richardson, J.
191Ga, M.N.R,A.S„ 177, 83.
%ewin, W. H. G., Li, F. K., Hoffman, J. A.. Doty, J., Buff, J.,
CIZA , G. W., and kappaport, S.	 1976b, M.N.R,A.S., *77, 93.
Lewin, W. H. G., Hoffman, J. A., and Doty, J. 	 1977, I.A.U. Circular
No, 2934.
Rosenberg, F. D., Eyles, C. J., Skinner. G. K., and WiIImore, A. P.
1975, Nature, 256, 628.
AL
11
ORIGINAL PAGE It
I o l 11(4)R 01 1 A I ,TTV
FIGURE CAPTIONS
Fig. 1. The low frequency portion of the 1977 Feb GCX power spectrum
•
(2.5 - 10 keV) showing the difficulty of extracting periods
40 minutes owing to spacecraft pointing jitter. Several of the
harmonics of the orbital period P o of 94.3 minutes and nontnertial
I ?V.	 period 
P 1 
of 101 minutes are specifically denoted, while the
numerous harmonics at higher frequencies are simply indicated with
markers. Because of the nature of the discrete fast fourier trans-
form algorithm, the entire power spectrum has only 16 poin:d, or
sra-half the number of input data. Each datum represents a
T - 0.6314 s integration. rhtis the frequency f - harmonic
No./(2 17 x t) Ilz.
Fig. 2. The depvnJunce of the upper Emit to the pulsation amplitudh-
on the period being searched for the 1976 Feb and 1977 Feb observa-
tions (Fig. 2a and 2b, respectively). Tabulated in the upper right-
hand corner are the dates and duration of each observation and the
total GCX flux in cts s -1 per energy interval. 	 I
t
TO
k
 \i
0
	
0
0
	
0
	
0
	
0
	
b
83m
od
J
O
F
	
t
-A
L
A
DD
LLJ
co2QZ
00
u
the
Z02
8=8
OWGINAL PAGE IS
OF 1'( )( IR OIIJALITY
a
1.2 UT 1977 FEB 6.6 ;-7.592
+ Energy (keV) GCX flux (ctss')
1.0 • 2.5-10 15.8± 3.0
0 8-35 5.3± 1.2
0.8
co
U
z n.6
— -o-
W
U 0.4 —o--Z — -+
W
0.2 --^ -- _	 --♦---
Z
OU
o 1.2
O b UT 1976 FEB 8.881 - 9.144
v  1.0 Energy (keV) GCX flux (cts s'1U) — —
^= c 2.5-10 11.6± 1.2
2 0,8 0 8-35 3.2 ± 0.4J
rz
0 0.6
CL
0.4 - ^^—t----^—_
# 0.2
I O -4	 10-3	 10-2	 10-1
FREQUENCY (Hz)
Figure 2
i
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A search for X-ray pulsations from the galactic center

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