A light curve of gamma-ray continuum emission from point sources in the galactic center region is generated from balloon and satellite observations made over the past 25 years. The emphasis is on the wide field-of-view instruments which measure the combined flux from all sources within approximately 20 degrees of the center. These data have not been previously used for point-source analyses because of the unknown contribution from diffuse disk emission. In this study, the galactic disk component is estimated from observations made by the Gamma Ray Imaging Spectrometer (GRIS) instrument in Oct. 1988. Surprisingly, there are several times during the past 25 years when all gamma-ray sources (at 100 keV) within about 20 degrees of the galactic center are turned off or are in low emission states. This implies that the sources are all variable and few in number. The continuum gamma-ray emission below approximately 150 keV from the black hole candidate 1E1740.7-2942 is seen to turn off in May 1989 on a time scale of less than two weeks, significantly shorter than ever seen before. With the continuum below 150 keV turned off, the spectral shape derived from the HEXAGONE observation on 22 May 1989 is very peculiar with a peak near 200 keV. This source was probably in its normal state for more than half of all observations since the mid-1960's. There are only two observations (in 1977 and 1979) for which the sum flux from the point sources in the region significantly exceeds that from 1E1740.7-2942 in its normal state

Gehrels, Neil

Tueller, Jack

English

NASA Technical Reports Server

N92-2) 935 .I
The Gamma-Ray Continuum Spectrum from
Galactic Center Disk and Point Sources
Neff Gehrels and Jack Tueller
Laboratory for High Energy Astrophysics
NASA / Goddard Space Flight Center
Greenbelt, MD 20771 USA
the
ABSTRACT
A light curve of gamma-ray continuum emission from point sources in the
galactic center region is generated from balloon and satellite observations made
over the past 25 years. The emphasis is on the wide field-of-view instruments
which measure the combined flux from all sources within -20* of the center.
These data have not been previously used for point-source analyses because of
the unknown contribution from diffuse disk emission. In this study, the galactic
disk component is estimated from observations made by the GRIS instrument in
October 1988. Surprisingly, there are several times during the past 25 years
when all gamma-ray sources (at 100 keV) within about 20" of the galactic center
are turned off or are in low emission states. This implies that the sources are all
variable and few in number. The continuum gamma-ray emission below
-150 keV from the black hole candidate 1E1740.7-2942 is seen to turn off in
May 1989 on a time scale of less than two weeks, significantly shorter than ever
seen before. With the continuum below 150 keV turned off, the spectral shape
derived from the HEXAGONE observation on 22 May 1989 is very peculiar
with a peak near 200 keV. This source was probably in its normal state for more
than half of all observations since the mid 1960's. There are only two
observations (in 1977 and 1979) for which the sum flux from the point sources
in the region significantly exceeds that from 1E1740.7-2942 in its normal state.
INTRODUCTION
Recent monitoring of the galactic center region by the GRANAT hard X-ray and
gamma-ray imaging instruments has shown that almost all gamma-ray sources within a few
degrees of the center are variable 1-3. These include 1E1740.7-2942, GRS1758-258, and
GXI+4. Other observations by imaging and scanning instruments over the past 25 years 4-17
give occasional snap shots of the point sources in the region. There are additionally more than
a dozen observations of the galactic center since 1968 by wide (>10 ° FWHM) field-of-view
instruments. These wide-field measurements can, in principle, be used to determine the sum of
the gamma-ray emission from point sources in the region. Efforts to do this in the past 10 were
inhibited by an uncertain knowledge of the diffuse continuum gamma-ray emission from the
galactic disk. In this paper, we use a recent observationl8,19 by the balloon-borne Gamma-Ray
Imaging Spectrometer (GRIS) to estimate the diffuse continuum component. We then subtract
this component from the historical wide-field measurements to obtain the first estimate of the
long-term light curve of point-source flux from the galactic center region.
446
https://ntrs.nasa.gov/search.jsp?R=19920012692 2020-03-17T13:15:12+00:00Z
DIFFUSE DISK SPECTRUM
An estimateof thediffusediskcomponentof the gamma-ray continuum radiation near
the galactic center is derived in the paper by Tueller et al. 18 in this volume. We summarize the
key points and results here. The derivation is based on observations of the galactic center by
the GRIS instrument. GRIS is a high resolution spectrometer 2° with an array of germanium
detectors (I 530 cm 3 active volume) surrounded by a 15-cm-thick active sodium-iodide shield.
The field-of-view is 17 ° FWHM below 100 keV increasing to - 23 ° FWHM in the MeV range.
Observations 2° were made by GRIS on 28 - 29 October 1988 of the galactic center and
of a point in the galactic plane 25 ° west of the center (l = 335 °, b = 0°). The point in the plane
was chosen to just exclude the center from the field-of-view. The spectrum at l -- 335 ° is well
fit by a broken power law with F = 9.99 × 10 -5 (E/100 keV) "2-64 photons cm -2 s "1 keV -1 below
182 keV and F = 4.96 × 10 -5 (E/100 keV) "1"47 photons cm "2 s-1 keV "1 above 182 keV (see
Figure 3 of Tueller et al.18).
We make two key assumptions in this study: 1) GRIS was predominantly measuring
the diffuse disk gamma-ray emission in this observation, and 2) this emission has an
approximately flat distribution in galactic longitude over the central radian of the disk. With
these assumptions the l = 335 ° GRIS measurement can be used as an estimate of the diffuse
disk spectrum at the galactic center. Assumption (1) would be false if point sources near
l = 335 °, b = 0 ° contribute significantly to the measurement. The only source in the region
with a known hard spectrum is GX339-4 in its low X-ray, high gamma-ray state. However,
GX339-4 was observed 21 by Ginga to be in the opposite high X-ray, low gamma-ray state 1.5
months before the GRIS observation. Assumption (1) is therefore probably accurate, although
there is some possibility of contamination from GX339-4 (it is a variable source) or other point
sources. Assumption (2) would be false if the longitude distribution of the diffuse continuum
emission were strongly peaked toward the galactic center. However, the diffuse emission in
the 20 keV to 10 MeV energy range is thought to be due to bremmstrahlung of cosmic ray
electrons 22 and should have a fairly flat distribution over the central radian.
Figure 1 shows the GRIS l = 335 ° spectrum plotted per radian of the galactic plane and
compared with previous observations near the galactic center. If anything, the GRIS spectrum
is lower than the others. However, as will be seen in the next sections, there are at least two
observations of the galactic center by wide-field instruments that see exactly this same
spectrum. The implication is that at these times (23 April 1968 and 22 May 1989) all point
sources in the region were in very low-emission states and that the diffuse spectrum estimated
from GRIS data is approximately correct for the galactic center region.
ANALYSIS METHOD
The 12 galactic-center observations by wide field-of-view instruments used in this
analysis are listed in Table 1. Observations by instruments with fields-of-view greater than 35 °
were not included due to uncertainties in the disk spectrum beyond the central radian. A few
observations by Bell/Sandia and HEAO-I were not included in this initial study due to the
unavailability of detailed spectral data.
447
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Diffuse Galactic Disk Spectrum
- _ r _rr]" T x r_-_ _ ,rTrri_ r r Trlr_l
GRIS ._
(1 = 335 °)
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I k.nVlgne et al
Schonfeld_r et
r
tiEAO I & COS-.B
extrnpolated
I
Ol ] lO
Energy, MeV
O0
Figure 1 - GRIS spectrum at l = 335 °
divided by the -0.3 radian GRIS field-
of-view, compared with previous
measurements of the diffuse gamma-ray
emission from the galactic disk near the
galactic center. Adapted from Harris et
al. 23
The procedure used in our analysis was to digitize the published energy spectrum from
each observation, subtract off the diffuse component and fit the residual. Examples are shown
in Figure 2 for the HEAO-3 observations in 1979 and 1980, where the original and residual
spectra are both plotted. The diffuse component was calculated by multiplying the GRIS l =
335 ° spectrum (per radian of the galactic plane) shown in Figure 1 by the FWHM fields-of-
view of the instruments (Table 1). The residuals are estimates of the sum of the spectra of
point sources within the fields-of-view. We fit the residual spectra up to -150 keV with a
power-law of the form A (E/100 keV) _. Our emphasis in this initial study is on the hard X-ray
spectrum up to 100 keV, so we did not include any data above 200 keV. In this way we also
largely avoided contamination from any positronium continuum emission.
RESULTS
Some surprising results are obtained in this analysis. For several of the observations
(1968.31, 1974.25, 1980.2, 1989.39) the diffuse component was found to dominate the
continuum spectrum. With the diffuse component subtracted off, the residual point-source
component is small, and in two cases (1968.31, 1989.39) actually consistent with zero. For
example, the HEAO-3 March/April 1980 residual spectrum in Figure 2(b) is seen to be less
than one part in ten of the measured spectrum in the hard X-ray band.
The results of the fits (flux at 100 keV, A, and spectral slope, 0¢) to the residual spectra
are listed in Table 1 with the flux at 100 keV plotted in Figure 3. Also shown in the figure for
comparison is the flux at 100 keV for the source 1E1740.7-2942 in its "normal" state observed
by GRIP in 198813 and 198914, Mir/HEXE 16 in 1989, and GRANAT/Sigma 1-3 in 1990.
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, , j Calculated Point Source Flux
from Galactic Center Region
100 keV
+
1 E1740 7-2942
' ,!,
- 1 -" L , _ J _ _ , , I , , J _ I , , .L_.___ L L L ,
1965 1970 1975 1980 1985 1990 1995
YEAR
Figure 3 - Residual total
point-source flux at 100 keV
calculated from wide field-
of-view measurements.
Fluxes are from Table 1.
The 100 keV flux level for
1E1740.7-2942 in
March/April 19901-3 is
shown by the dashed line.
DISCUSSION
In terms of point-source emission, the 12 wide-field observations in Figure 3 can be
grouped as follows:
1) 2 observations (1968.31, 1989.39) are consistent with no point-source emission.
2) 2 observations (1974.25, 1980.2) have low point-source emission of -3 x 10 .5 photons
cm 2 s -1 keV -1. Many sources are candidates for this emission. The flux level is about right
for GRS1758-258 as observed 3 in March/April 1990, 1E1740.7-2942 in its low state as
observed 1 in February 1991 or GXI+4 as observed 33 in the 1970's.
3) 6 observations (1970.90, 1971.89, 1979.29, 1981.89, 1988.33, 1988.83) have 100 keV
flux levels consistent with that of 1E1740.7-2942 in its normal state, although significant
contributions from other sources such as GXI+4 and GRS1758-258 can not be ruled out. The
1988.33 observation by GRIS was just 18 days after an observation by GRIP in which
1E1740.7-2942 was indeed found to be in its normal state and the only strong 100 keV source
in the region.
4) 2 observations (1977.86, 1979.8) have flux levels significantly exceeding that of
1E1740.7-2942 in its normal state. The 1977.86 observation by Bell/Sandia was just 12 days
before an observation 34 by the NRL high-energy X-ray balloon instrument, which scanned the
galactic center region. The scans revealed comparable 100 keV fluxes from 1742-294, which
is very likely the same source as 1E1740.7-2942, and from 1743-322.
We thus conclude that there are few strong point sources of 100 keV emission within
-20" of the galactic center and that all of them are variable. The black hole candidate
1E1740.7-2942 is probably a dominant contributor to the high energy flux for more than half
of the observations.
451
1E 1740.7-2942 Turn-Off
There are 5 observations of 1E1740.7-2942 within a period of two months in March-
May 1989. Two are with imaging instruments (GRIP 14, EXITE17), two with narrow ( < 2 °)
field-of-view instruments (Mir/HEXE 16, POKER 15) and one with the wide-field
HEXAGONE 31 (Table 1). A plot of the 1E1740.7-2942 60 keV flux as a function of time
from these observations is shown in Figure 4. The HEXAGONE data point is the residual
after the diffuse disk flux was subtracted and represents an upper limit for any point source in
the field-of-view. The source turned off in gamma-ray continuum emission in 13 days. This is
the fastest variability ever seen in 1E1740.7-2942 and implies a compact emission region of
size less than 13 light days.
'>
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Q.
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I--
<
X
..J
0
60
1 E1740.7-2942 OBSERVATIONS IN 1989
",, £
',I
80 100 120 140 160
TIME IN 1989 (day of year)
Figure 4 - The flux at 60 keV from
1E1740.7-2942 vs. time for
observations between 20 March and 22
May 1989 b_ Mir/HEXE 16, GRIP t4,
E X I T E POKER 1 5 and
HEXAGONE 31. The HEXAGONE
point is a 1_ upper limit for all sources
in the -19 ° field-of-view. The source
turn-off occurs in 13 days. Note that in
order to have best statistics for all
measurements, this plot is at 60 keV
instead of the 100 keV used elsewhere.
170 keV Feature
The 130-180 keV feature observed 32 in the 22 May 1989 HEXAGONE spectrum of the
galactic center becomes more prominent when the diffuse disk emission is subtracted, as
shown in Figure 5. The point-source spectrum actually has a maximum at -170 keV. This
feature has been interpreted 35 as a Compton backscatter peak from an accretion disk or cloud
surrounding a source of 511 keV positron annihilation photons. Figure 5 shows that on 22
May 1989, the only significant gamma-ray emission from the galactic center region was in two
lines (170 and 511 keV) both probably associated with positron annihilation.
452
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LL
10
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I0
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• HEXAGONE
22 May 1989!/
measured
point-source
residual
I
'\ i
+
4
1
,, 4. 1
100 I o0o
ENERGY (keV)
Figure 5 - HEXAGONE galactic center
spectrum on 22 May 1989 as measured 32
(filled circles) and with the diffuse disk
component subtracted (histogram). The
data points below 130 keV for the residual
spectrum have been averaged to improve
statistics.
ACKNOWLEDGEMENTS
We greatly appreciate discussions with J. Grindlay, T. Prince, G. Ricker and P.
Ubertini concerning 1) the series of balloon measurements just preceeding the HEXAGONE
observation in May 1989, 2) the possible contamination to our diffuse spectrum from
GX339-4, and 3) the 1970's balloon observations of GXI+4. We thank K. Pollock for
assistance in the data analysis and in preparing this paper.
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The gamma ray continuum spectrum from the galactic center disk and point sources

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