The Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma-Ray Observatory has performed several observations of the galactic plane and galactic center region to measure the distribution of galactic 511 keV positron annihilation radiation. Preliminary analysis of data collected during the observation of the galactic center region over the period 13-24 Jun. 1991, indicates the presence of a 511 keV line and positronium continuum superimposed on a power-law continuum. The line of flux was found to be (2.7 +/- 0.5) x 10(exp -4) gamma/sq cm sec, with a positronium fraction of (0.9 +/- 0.2). The 3(sigma) upper limit to daily variations in the 511 keV line flux from the mean during the observation interval is 3 x 10(exp -4) gamma/sq cm sec. If all of the observed annihilation radiation is assumed to originate from the x-ray source 1E 1740.7-2942, the corresponding 511 keV line flux would be (3.0 +/- 0.6) x 10(exp -4) gamma/sq cm sec. The 3(sigma) upper limit for 511 keV line emission from the x-ray binary GX1+4 is 6 x 10(exp -4) gamma/sq cm sec. Results from the galactic plane observations at galactic longitudes of 25 degrees (16-21 Aug. 1991) and 339 degrees (6-11 Sep. 1991) suggest that the emission is concentrated near the galactic center. The observations and the preliminary results are described

Grabelsky, D. A.

Johnson, W. N.

Jung, G. V.

Kinzer, R. L.

Kurfess, J. D.

Purcell, W. R.

Strickman, M. S.

Ulmer, M. P.

English

NASA Technical Reports Server

N92-2!933
OSSE Observations of Galactic 511 keV
Annihilation Radiation
W. R. Purcell, D. A. Grabelsky_ W. N. Johnson_ G. V. Jung_
R. L. Kinzer_ J. D. Kurfess_ M. S. Strickman_ M. P. Ulmer*
ABSTRACT
The Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton
Gamma-Ray Observatory has performed several observations of the Galactic plane
and Galactic center region to measure the distribution of Galactic 511 keV positron
annihilation radiation. Preliminary analysis of data collected during the observation
of the Galactic center region over the period July 13-24, 1991, indicates the presence
of a 511 keV line and positronium continuum superimposed on a power-law contin-
uum. The line flux was found to be (2.7-t- 0.5) x 10 -4 7 cm-2 s-l, with a positronium
fraction of (0.9 _- 0.2). The 3a upper limit to daily variations in the 511 keV line
flux from the mean during the observation interval is 3 x 10 -4 7 cm-2 s-'l. If all of
the observed annihilation radiation is assumed to originate from the X-ray source
1E 1740.7-2942, the corresponding 511 keV line flux would be (3.0 ± 0.6) x 10 -4
7 cm-2 s-1. The 3a upper limit for 511 keV line emission from the X-ray binary
GXl+4 is 6 x 10 -4 7 cm-2 s-1. Results from the Galactic plane observations at
Galactic longitudes of 25 ° (August 16-21, 1991) and 339 ° (September 6-11, 1991)
suggest that the emission is concentrated near the Galactic center. The observations
and the preliminary results are described.
INTRODUCTION
While positron annihilation radiation has been observed from the Galactic center region
by numerous balloon and satellite-borne experiments since the 1970s_ the location and
distribution of the emission has not yet been identified (see Lingenfelter and Ramaty
1989 for a recent review). The situation is further complicated by the possibility that the
511 keV annihilation line may vary in intensity by as much as _-, 10 -3 7 cm-2 sec-1, leading
to the suggestion that the observed emission is composed of two separate sources: a steady
state diffuse Galactic component and a time variable point source near the Galactic center
(Lingenfelter and Ramaty 1989). The source of the diffuse Galactic component is thought
*Northwestern University, Evanston, IL
!Naval Research Laboratory, Washington DC
Universities Space Research Association, Washington DC
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Table 1: OSSE Galactic Center / Galactic Plane Observations
Galactic Position Background Observation On-Source
Longitude Angle Offset Angle Interval Time (sec)
0 ° 90 ° ±10 ° 13-24 July 105 / 105
25 ° 90 ° 4-10 ° 16-21 Aug 5 x 104
339 ° 90 ° 4-4.3/-8.5 6-11 Sept 5 x 104
to be the fl+-decay products from radioactive nuclides produced by supernovae, novae
or Wolf-Rayet stars (Clayton 1973), while 7-7 interactions in the vicinity of an accreting
black hole has been suggested as a possible source of time variable annihilation emission
(Lingenfelter and Ramaty 1982). At this time, however, few details about of the source(s)
of the positrons is known. One of the scientific goals of the OSSE team is to map the
distribution of positron annihilation radiation and to search for time variability from the
Galactic center region.
The OSSE instrument consists of four separate, nearly identical detectors. The primary
detecting element of each detector is a large area NaI(T1)-CsI(Na) phoswich crystal, pro-
viding an effective area of 450 cm 2 at 0.5 MeV. The phoswich is actively shielded and
passively collimated. Tungsten collimators provide a field-of-view which is 3.8 ° × 11.4 °
(FWHM), with the long direction of the collimators oriented parallel to the spacecraft
Y-axis. Each detector has a separate elevation control system which provides indepen-
dent positioning of the detectors about an axis parallel to the spacecraft Y-axis. During
source observations, periodic background measurements are performed by offset-pointing
the detectors from the target. A detailed description of the OSSE instrument and its
operation can be found in Johnson et al. (1989).
ANALYSIS METHOD
Table 1 provides a description of the OSSE observations of the Galactic center and Galactic
plane used to generate the preliminary results reported here. The position angle represents
the angle between the long direction of the OSSE field-of-view and Galactic North. For
the observations described here, the long direction of the OSSE collimator was oriented
parallel to the Galactic plane to maximize the response to a diffuse Galactic distribution.
The background offset angle represents the Galactic latitude at which the background
observations were performed. The Galactic center observation included additional source
pointings at Galactic latitudes of b = +1.5 ° and ±3.0 °. The on-source time for the
Galactic center observation indicates the total on-source time for both the b = 0 ° and the
b = 4-1.5 °, 4-3.0 ° observations. The Galactic plane observation at l = 339 ° included an
432
100.00
>
I0.00
7
I .00
'_ 0.I0
0.01
m
• I
' I
i I | I i ¢ i . . . • |
0.1 1.0
Energy (MeV)
Figure 1: The OSSE Galactic center (b = 0 °) spectrum summed from all four detectors
over the period 13-24 July, 1991. The fitted function consists of a power law, a photopeak
line and a term approximating the positronium continuum. See text for details.
additional source pointing at the target GX339-4.
The instrumental background is measured by offset-pointing the OSSE detectors from
the source position. Source and background measurements are typically alternated at
two minute intervals, with backgrounds being sampled on each side of the source position
along the instrument scan plane. The background offset angles for the Galactic center
and Galactic plane observations were selected to minimize possible residual response to a
diffuse latitude distribution. For the analysis described here, data having low live-times,
telemetry errors or in which the detector zenith angle was greater than 100 ° were excluded.
Background estimation and subtraction was performed for each detector separately. In
order for a source observation to have been included in the analysis two background mea-
surements were required, one before and one after the source observation. For these source
observations, the background spectrum was estimated by linearly interpolating in time,
channel-by-channel, between the two background spectra. The individual background-
subtracted spectra were then summed by detector and by day to perform the spectral
analysis.
The summed background-subtracted spectra for the Galactic center b = 0° observation is
shown in Figure 1. This spectrum shows strong evidence for a 511 keV line and positron-
433
5z*--'N
v 4
?
3
O
--, 2
X
0
_1448 8450 8452 8454 8456 8458 8460 8462
Time (TJD)
Figure 2: The fitted daily 511 keV line flux for the Galactic center b = 0 ° observations.
ium continuum. The position of the line is consistent with an energy of 511 keV, and the
line width is consistent with the instrumental resolution. For the spectral studies reported
here, the spectral fits were performed in count space over the energy range 0.2 - 1.5 MeV.
The fitting model consisted of a power law, a photopeak line fixed in energy at 511 keV
and in width at the instrumental resolution, and a term approximating the positronium
continuum. The positronium continuum component consisted of a triangle having a max-
imum at 511 keV and decreasing linearly with energy and which was smoothed by the
instrumental resolution. The intensity of the positronium continuum component was a
free parameter in the spectral fits. The spectral fits were performed in count space and
did not include folding the model through the instrument response.
PRELIMINARY RESULTS
Using the fitting model described above, the summed background-subtracted spectra for
the Galactic center b = 0° observation results in a fitted 511 keV line flux of (2.7 4- 0.5) x
10 -4 3' cm-z s-1 and a positronium fraction of (0.9 4- 0.2). The spectral fit is shown
in Figure 1. Time variability of the 511 keV line flux was investigated by summing the
background-subtracted spectra for each day of the observation interval separately. The
resulting line fluxes are shown in Figure 2. No significant time variability of the line flux
is observed. The 3_r upper limit to daily variations from the mean is 3 x 10 -4 7 cm-2 s-1.
434
_q
7-.
T
>(
X
_T
4
3
4
90 60 30 0 -30 -60 -90
Galactic Longitude (degrees)
3
2
x
0
B
g g •
_',,\
II !
m
',,%
-6 -4 -2 0 2 4 6
Galactic Latitude (degrees)
Figure 3: The fitted 511 keV line flux for the Galactic plane (top) and Galactic center
latitude scan (bottom) observations. The curves represent the expected OSSE responses
for several Galactic distribution models, fitted simultaneously to the observed longitude
and latitude data shown. The thin solid line represents the CO model of Leising and
Clayton (1985), the thick solid line represents the Nova model of Leising and Clayton
(1985), the thin dashed line represents the observed CO distribution (Dame et al. 1987),
the thick dashed line represents a model following the Galactic visual luminosity (Bahcall
and Soneira 1980), and the dotted line represents the response for a point-source located
at 1=0 °, b=O °.
435
The results of fitting the summed background-subtracted spectra for the Galactic plane
(l = 0 °, 25 ° and 339 °) and the Galactic center latitude scan (b = 0°, +1.5 ° and :t:3.0 °) are
shown in Figure 3. Also shown in this figure is the expected OSSE responses for severM
Galactic distribution models, fitted simultaneously to the observed latitude and longitude
data. As can be seen, the data are sharply peaked in longitude near the Galactic center,
consistent with the reported GRIS observations (Gehrels et aI. 1991). The Galactic center
latitude scan suggests that the emission is centered near the plane and may be slightly
broadened compared with the expected point source response.
From the Galactic center latitude scan shown in Figure 3, it can be seen that the bulk
of the emission can not be from the X-ray binary GXI+4. If the ,-- lo- excess observed
for the b = +3 ° observation is assumed to be due to emission from GXI+4, the 30- upper
limit to the 511 keV flux is 6 x 10 -4 7 cm-2 s-i. If the observed Galactic center flux of
(2.7 -t- 0.5) x 10 -4 3' cm-2 s -1 is assumed to originate entirely from the X-ray source 1E
1740.7-2942, the corresponding 511 keV line flux would be (3.0 :t: 0.6) x 10 -4 3" cm -2 s -1.
If a point source of annihilation radiation with a 511 keV line flux of 2 x 10 -3 3" cm -2
s -1 was active during the Galactic center observation interval, its position (10-) must be
outside of tile region bounded by l = +8 °, b = +4 °.
If all of the observed emission is assumed to originate from a diffuse Galactic source,
the OSSE data are inconsistent (> 2.5_) with a distribution following the CO model of
Leising and Clayton (1985). Further, the fitted OSSE intensities for the models following
the observed CO distribution (Dame et al. 1987) and the Novae model of Leising and
Clayton (1985) are only _ 50% of the 511 keV intensity reported by SMM (Share et al.
1990), suggesting either that the distribution does not follow these models, or that the
SMM observations included a time-variable component which was not active during the
OSSE observations. The latter is not yet required, however; the fitted OSSE intensity
for a model following the Galactic visuM luminosity is consistent with the reported SMM
flUX.
CONCLUSION
The preliminary results described here are based on four weeks of OSSE observations of
the inner Galactic plane. The observation of the Galactic center region over the period
July 13-24, 1991, indicates the presence of a 511 keV line and positronium continuum
superimposed on a power-law continuum, with a line flux of (2.7 + 0.5) x 10 -4 3' cm -2 s -1
and a positronium fraction of (0.9 + 0.2). No evidence for daily variations in the line flux
was observed during this period, with a 30. upper limit to daily variations from the mean
of 3 x 10 -4 3' cm -2 s -1. The Galactic longitude observations at l = 25 ° and 339 ° suggest
that the 511 keV line emission is concentrated near the Galactic center.
Five further OSSE observations of the Galactic center with various collimator position
angles are scheduled for the remainder of the GRO Phase 1 viewing plan which will
436
provide an additional 10 weeks of data. These observations will allow OSSE to monitor the
Galactic center for variability of the annihilation radiation and to measure the distribution
of the emission within _ 5 ° - 10° of the Galactic center. Simultaneous observations of
the Galactic center region with SIGMA are scheduled which will aid in separating point
source contributions from the diffuse component. Ten additional weeks of Galactic plane
observations are also scheduled which will further aid in measuring the diffuse distribution.
REFERENCES
Bahcall, J. N., and Soneira, R. M., 1980, Ap. J. Suppl., 44, 73.
Clayton, D. D., 1973, Nature Phys. Sci., 244,137.
Dame, T. M., et al., 1987, Ap. J., 322, 706.
Gehrels, N., et al., 1991, Ap. J. (Letters), 375, L13.
Johnson, W. N., et al., 1989, in Pro<:. of the GRO Science Workshop, ed. W. N. Johnson,
p. 2-22.
Leising, M. D., and Clayton, D. D., 1985, Ap. J., 294, 591.
Lingenfelter, R. E., and Ramaty, R., 1989, Ap. J., 343, 686.
Lingenfelter, R. E., and Ramaty, R., 1982, in The Galactic Center, ed. G. R. Riegler and
R. D. Blandford (New York:AlP), p. 148.
Share, G. H., et al., 1990, Ap. J. (Letters), 358, L45.
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OSSE observations of galactic 511 keV annihilation radiation

OSSE observations of galactic 511 keV annihilation radiation

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