The galactic Low Mass X ray Binary GX1+4 was detected by the coded aperture hard X ray gamma ray SIGMA telescope during the Feb. to April 1991 observations of the galactic center regions. The source, whose emission varied during the survey of a factor greater than 40 pct., reached a maximum luminosity in the 40 to 140 energy range of 1.03 x 10(exp 37) erg/s (D = 8.5 kpc), thus approaching the emission level of the 1970 to 1980 high state. Two minute flux pulsations were detected on Mar. 22 and on Mar. 31 and Apr. 1. Comparison with the last period measurements shows that the current spin-down phase of GX1+4 is ending. Concerning the proposed association of this source with the galactic center 511 keV annihilation emission, upper limits were derived

Barret, D.

Churazov, E.

Denis, M.

Gilfanov, M.

Jourdain, E.

Laurent, PH.

Lebrun, F.

Paul, J.

Roques, J. P.

Salotti, L.

English

NASA Technical Reports Server

N92-2i899
DETECTION AND PERIOD MEASUREMENT8 OF GXI+4
AT HARD X-RAY ENERGIES WITH THE SIGMA TELESCOPE
Ph Laurent, L. Salotti, F. Lebrun, J. Paul
Service d'Astrophysique, CEN Saclay, 91191 Gif sur Yvette Cedex,
FRANCE
M. Denis, D. Barret, E. Jourdain, J.P. Roques
Centre d'Etude Spatiale des Rayonnements, 9 ave du colonel
Roche, 31029 Toulouse Cedex, FRANCE
E. Churazov, M. Gilfanov, R. Sunyaev, A. Diachkov,
N. Khavenson, B. Novickov, I. Chulkov, A. Kuznetzov
IKI, Space Research Institute, Profsoyouznaya, 84/32, Moscow,
117296, USSR
ABSTRACT
The galactic Low Mass X-ray Binary GXI+4 has been detected by
the coded aperture hard X-ray/gamma-ray SIGMA telescope during the
February/March/April 1991 observations of the galactic center
regions. The source, whose emission varied during the survey of a
factor greater than 40 %, reached a maximum luminosity in the
40-140 energy range of 1.03 1037 erg s "I (D=8.5 kpc), thus
approaching the emission level of the 1970-1980 high state. 2
minute flux pulsations have been detected on March 22 and on
March 31/ April I. Comparison with the last period measurements
shows that the current spin-down phase of GXI+4 is ending.
Concerning the proposed association of this source with the
galactic center 511 keV annihilation emission, upper limits have
been derived.
1 INTRODUCTION
GXI+4 is the X-ray pulsar nearest to the galactic center.
It was discovered first at high energies (E>20 keV) by the MIT
Balloon Group in 1970 [i] and later identified in the X-ray band
(2-10 keY) with the source 4U1728-24 [2]. X-ray pulsations of
about 2 minutes period were interpreted as the spin signature
of a highly magnetized neutron star accreting matter from a normal
companion. During all the 1970's GXI+4 maintained an average X-ray
emission level of i00 mCrab that, for d=8 kpc, gives a typical
luminosity of 103z-1038 erg s "I, while the pulsation period decreased
steadily with a spin-up rate of dP/dt = -2.6 s yr "I, the fastest
among the known pulsars. As pointed out by the discoverer the
energy spectrum stretches deeply to high energies (kT=30-50 keV
for thermal bremmstrahlung models or, alternatively, _ = -2.7 for
a power law) making GXI+4 the hardest pulsar and one of the
brightest sources of the galaxy also for E>30 keV.
In the early 1980's, however, GXI+4 faded away. Deep
exposures performed by EXOSAT in 1983 and 1984 failed to reveal
any signal down to an upper limit of 0.4 mCrab (2-7 keV band)
[3]. It was only in 1987 that the source was again detected by
209
https://ntrs.nasa.gov/search.jsp?R=19920012656 2020-03-17T13:16:27+00:00Z
GINGA, but at the weak level of 3 mCrab [4]. GINGA and the
X/gamma HEXE instrument onboard the MIR space station observed
GXI+4 in 1987,1988 and 1989 [5][6][7][8] showing that, at X-ray
energies, the source has brightened up to a level of about 15
mCrab and that the pulsation period had unambiguously reversed the
spin up to a spin down at the quite steady rate of dP/dt =
+1.4 s yr "I. The HEXE luminosity in the 20-100 keV energy band
was 1.0 I03z erg s "I on October 31, 1987, assuming a distance of i0
kpc.
In 1990, SIGMA has detected the pulsation of GXI+4,
confirming the spin down phase but at a slower rate (dP/dt =
+0.9 s yr'1)[9].
McClintock and Leventhal [i0] made the hypothesis that this
source could be the compact source responsible for the variable
positron/511 keV galactic center line [11][12]. However, no
decisive argument was given to rule out another hard source,
IE1740-2942, and the authors deferred the final word to
observations carried out by imaging/high resolution gamma ray
instruments.
In this paper we discuss the observations of GXI+4 at hard
X/soft gamma energies (0.035-1.3 MeV) performed by the coded
aperture SIGMA telescope during the winter/spring 1991 survey of
the galactic center.
2 THE SIGMA TELESCOPE
The CEA/CESR/CNES imaging telescope SIGMA is the main
payload of the Soviet scientific satellite GRANAT in orbit
since December 1989. The detector is an Anger camera which
measure energies and interaction points of photons in the
0.035-1.3 MeV energy range. The optical system is provided by an
URA coded mask placed 2.5 m in front of the detector.
This arrangement features a Full Sensitivity Field of View
(FSFOV) of 4.7°x 4.3 ° . In a wider external zone (ii.5°xi0.9 ° at
half sensitivity), named Partially Coded Field of View (PCFOV),
the incomplete shadowing progressively reduces the instrument
sensitivity to zero.
During a typical SIGMA observation, lasting approximately
one day, the telescope records three series of 'spectral' images
(124 x 116 3.30 arcmin pixels) in 95 energy channels which cover
the entire energy range varying with the energy resolution. In
addition, the total camera counts are stocked every 4 s in four
energy bands. Dead time information is also known every 4 s.
More details about the instrument and the data collection
modes can be found in Paul et al. [13] while the in-flight
performances are described by Mandrou et al. [14].
210
3 OBSERVATIONS
Figure 1 shows the strong
variability of the Galactic Cen-
ter region during Spring 90,
Winter 91, and Spring 91 [15].
Contour levels are drawn every
0.i cts/s starting at 0.25
cts/s.
The first image shows the
well known source IE1740.7-29.42
and the source GRS 1758-258,
discovered by GRANAT. GXI+4 is
not visible.
In the second image taken
in Winter 91, appears only the
GRANAT source. IE1740.7-29.42
has faded away and GXI+4 is not
yet visible.
GXI+4 dominates the third
image taken in March/April 91.
IE1740.7-29.42 is still below
the SIGMA detection limit and
GRS 1758-258 is detected at the
same flux level as it was
observed in Spring and Fall
1990.
In order to better deter-
mine the source position as
detected by SIGMA, the Spring 91
observations In the range 43-77
keY have been fitted with the
SIGMA/URA point spread function
having as additional free para-
meters the amplitude and the
background level. Best fit
equatorial coordinates are :
_1950=262.26 ± 0.06 deg.
61950=-24.74 ± 0.06 deg.
where the errors indicate the
95% confidence level. This
position is fully compatible
with the optical position.
A simple X 2 analysis
C5)
J.8
a.8
8.8
(c
4.8
?..8
8.8
8.8 358.8 3S6.8
8.8 398.8 3S6.8
8.8 _$8.8 3S6.e
Figure i SIGMA observations of
the Galactic Center region in
the 40-110 key energy band :
performed including all positive (a) Spring 90. (b) Winter 91.
and negative observations in the
40-77 keY range reveals that (c) Spring 91.
211
during the Spring 91 survey the source was variable at a very high
(99.95%) confidence level; taking into account only the positive
observations the confidence drops to 95%. The mean positive count
rate during the positive detections (0.27+/-0.03 cts/s) compared
to the upper limits derived in the other cases suggests a
variation of more than 40% of the hard flux.
An average raw energy spectrum of the source has been
obtained regrouping the net source counts in i0 channels
covering the band 40-140 keV. This spectrum has been fitted
folding successively a power law and a thermal bremmstrahlung
model with the detector response function [16]. The best fitting
slope and electron temperature was -3.5 (-4.5:-2.8) and 28.4
(18.7:46.7) keY respectively. Errors are given at the one sigma
confidence level.
The average flux in the 40-77 keV energy band was 125 mCrab
on March/ April 1991. Assuming a distance of 8.5 kpc, the source
luminosity reached on March 22 the level L = i.i ± 0.3 1037 erg s "I
in the 40-140 keV energy interval.
No statistically significant evidence of a peaked
annihilation/511 keV emission has been found, at the source
position, in individual observations of the survey nor in their
sum which totals a dead-time corrected exposure time of
about 2.3 l0 s s. This enables to put an upper limit of 1.9 10 .3
phot. cm "2 s "I (90% confidence level) for a 511 keV emission from
GXI+4.
4 THE PULSE PERIOD AND PROFILE
In order to measure the source pulsation we have used the
epoch folding method applied on 4 s total camera counts computing
the X 2 statistic against the hypothesis of uniform distribution.
The four accumulation energy bands have been held fixed at 40-77,
77-155, 155-290 and 290-630 keV throughout the whole survey.
As the spacing between statistically independent periods (_p
= P2/Ts, T s total time of observation) was, in all the cases,
substantially greater than the period shift expected from the
last measured period derivative [9] we have not used this term in
the folding procedure.
We have detected the pulsation of GXI+4 in the first energy
band, in two sets of observation.
212
The results are summarized in the following table :
Date Time base
(l_ss)
Epoch of
observation
(JD-2440000)
Period (s)
03/22/91 5.6 8338.59 115.06±0.03
03/31/91 & 19.6 8347.87 115.086±0.005
04/01/91
The light curve relative to the second period detection is
traced in Fig. 2. It reveals a smooth profile with a broad
single hump. The better defined March 31/ April 1 curve appears
asymmetric about the peak position with a sharp drop on the right
side. Contrary to previous detections [8][17] our results do not
show any evidence of a double peak pulse confirming that the
high energy profile is variable. The ratio of the modulated
flux to the modulated+steady component for the March 22
observation was 0.54+/-0.15 while for the March 31/April 1
observation it was 0.67+/-0.14. These values indicate an highly
modulated flux as already reported in the review of Frontera and
Dal Flume [18] at the same energies and by Kendziorra et al. [19].
G)41+'1 - P=I, 15.08& +/- 0.005 s. - 31.,_'03 0[/04/1991
76.72 , ,
76.51
,_ 76.29
00 76.08
75.86
I
I75.65
0.0 0.5 1.0 1.5 2.0
Pulse Phase
Figure 2 Light curve of GXI+4 during March 31 /April 1
observation.
It has been reported by some authors [20][21][22] that for
E>20 keV there are significative differences between odd and even
pulses and that the true pulsation has a period twice the 2
minute period. In this respect we have folded our data at the
213
double of the periods reported into 20 bin light curves. Then,
the two halves of the light curves have been compared by means
of a standard X 2 Test. Reduced X 2 (i0 d.o.f.) values were 1.51
and 0.57 for the March 22 and the March 31/April 1 respectively,
i.e. values compatible with the null hypothesis.
5 PERIOD HISTORY
Since 1986 GXI+4 has showed a clear spin down behavior.
Taking into account the Ginga and the Kvant measurements in
1986-1989 an average spin down rate of dP/dt =4.5 10 .8 s s I has
been derived with little deviations from the linear trend [8].
However the two GRANAT points in 1990 and 1991 (Barret et al. [9],
and this paper) have significantly changed the scenario showing
that the spin down rate is unambiguously lowering. In fact,
comparing our March 31/April 1 measurement with the point obtained
by Barret et al. [9] on August, 27th 1990, an average spin down
rate of dP/dt =2.6 +/-0.3 10 .8 s s "I is deduced, a value which
further lower the last dP/dt (3.1 10 .8 s s-l) measured by Barret
et al. [9]. The complete period history of GXI+4 is shown in
Figure 3.
140.0
135.0
130.0
w 125.0
m
o
.m
k 120.0
@,,
115.0
II0.0
105.0
I I I l
R
%
x X
X
X
x
I I
0.0 2000.0 4000.0 6000.0
,@, ÷
e
!
8000.0 I0000.0
Figure 3 Period history of GXI+4.
214
6 CONCLUSIONS AND DISCUSSION
The winter/spring 1991 SIGMA survey has shown the strong
variability of the Galactic Center region. For instance, the
40-77 keV flux from GXI+4 has varied by more than 40%. At maximum
level the source luminosity reached i.i 1037 erg s -I (40-140 keV)
assuming a distance of 8.5 kpc. Similar luminosity levels have
been reported during the '70 when the source was in the high
state.
The average energy spectrum has been found softer than
expected and looks more similar to the steeply falling spectra
typical of other known hard X-ray binaries (GX304-1, GX301-2,
4U1626-67) rather than to previously recorded spectra from GXI+4.
Given the large error bars, our plasma temperature can be made
consistent with the temperature of 41+/-8 keV measured by the
AIT/MPI group [19] but a decrease is found if we compare our value
with the more recent HEXE measurement [8].
We have obtained an upper limit for the annihilation emission
coming from GXI+4 (1.9 10 .3 phot cm -2 s -I) that does not
rule out this source as the compact galactic center annihilation
emitter, since reliable 511 keV fluxes from the compact
component lower than 10 .3 phot cm 2 s -I have been recorded
[23][24][25][26]. A more compelling limit (0.6 10 .3 phot cm "2 s "I)
was given by the OSSE instrument onboard the GRO satellite (this
conference).
We have detected the 2 minute pulsation period in the
40-77 keV range on two occasions (considering the 03/31-04/01
observations as a single detection). Folded light curves reveal
a broad single hump. There is no indication of the double peak
structure previously detected at hard energies. Marginal
evidence has been found concerning the differences between
alternate pulses. Our result (dP/dt = +0.8 ± 0.09 s yr "I) confirms
the spin down trend observed by GINGA and during the first SIGMA
survey of the galactic center but at a slower rate [27][9]. The
pulsation history clearly suggests that the current spin down
phase of GXI+4 is ending.
The evolution we observed in GX 1+4 could be due to an
increasing accretion rate as it is expected from a "fast rotator"
with a magnetic field of = 1012 Gauss [28]. As the luminosity of
the source reaches the values obtained during the seventies, it
seems reasonable to think that GXI+4 will end soon its spin down
trend and perhaps go back to a spin up period.
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216


Detection and period measurements of GX1+4 at hard x ray energies with the SIGMA telescope

Detection and period measurements of GX1+4 at hard x ray energies with the SIGMA telescope

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