The UCSD/MIT Hard X-Ray and Low Energy Gamma-Ray Experiment aboard HEAO-1 scanned the galactic center region during three epochs in 1977 and 1978 from 13 to 180 keV. The results are presented from the scanning epoch of 1978 September. Twenty-two known 2 to 10 keV source positions were necessary for an acceptable fit to the data. The spectra of the 16 strongest, least confused sources are all consistent with power laws with photon spectral indices ranging from 2.1 to 7.2. Acceptable fits to thermal bremsstrahlung models are also possible for most sources. No one source in this survey can be extrapolated to higher energy to match the intensity of the gamma-ray continuum as measured by HEAO-1 large field of view detectors, which implies that the continuum is a composite of contributions from a number of sources

Jung, G. V.

Knight, F. K.

Matteson, J. L.

Rothschild, R. E.

English

NASA Technical Reports Server

217 OG 2.5-7
SPECTRA AND POSITIONS OF GALACTIC GAMMA-RAY SOURCES
F. K. Knight
MIT Lincoln Laboratory
Lexington, MA 02173 USA
and
J.L. Matteson, G.V. Jung, and R.E.Rothschild
CASS, Univ. of California, San Diego
La Jolla, CA 92093 USA
ABSTRACT
The UCSD/MIT Hard X-Ray and Low Energy Gamma-Ray
Experiment aboard HEAO-I scanned the galactic center
region during three epochs in 1977 and 1978 from 13-
180 keV. We present the results from the scanning
epoch of 1978 September. Twenty-two known 2-10 keV
source positions were necessary for an acceptable fit
to the data. The spectra of the 16 strongest, least
confused sources are all consistent with power laws
with photon spectral indices ranging from 2.1 to 7.2.
Acceptable fits to thermal bremsstrahlung models are
also possible for most sources. No one source in this
survey can be extrapolated to higher energy to match
the intensity of the gamma-ray continuum as measured
by HEAO-1 large field of view detectors, which implies
that the continuum is a composite of contributions
from a number of sources.
INTRODUCTION
The galactic center region (i.e. within about 20 ° of the
galactic center) contains more than 30 2-10 keV x-ray sources
brighter than about i0 UFU1. Observations of this region between
I0 and 200 keV are hampered by coarse angular resolution and its
attendent problems of source confusion. Previous observations
did, however, continually find bright source(s) near the
galactic center2 s. The HEAO-I UCSD/MIT Sky Map4 provided a
sensitive survey of this region in four broad energy bands
covering 13-180 keY. This Sky Map obtained close to 30 sources
within 20 ° of the galactic center at a typical sensitivity of 20
UPU.
Wide field of view (_i0 ° FWHM) observations extending to 1
MeV and above have detected variable continuum emission from a
region centered on the galactic center, as well as variable 511
keY emissions e. Between the Fall of 1979 and the spring of 1980
both components varied by a factor of two or moreV as seen by
HEAO-3. This variability implies that the dominant source(s)
must be _3" in extent at a distance of i0 kpc. The source(s),
however, of this emission have yet to be localized above 200 keV,
and it is unclear which, if any, of the 1 keV EINSTEIN sourcese
• 
217  . -7 
TRA  SITIONS  LACTIC MA-RAY RCES 
. . night 
I  incoln aboratory 
exington,  02173 S  
d 
J. . atteson, . . J g, a  .E.Rothschild 
S, niv.  alifornia, an iego 
a la,  093  
BSTRACT 
he CSD/MIT ard -Ray and o  nergy a ma-Ray 
xperiment oard EAO-l s nned t  alactic nter 
re ion ring t ree epochs i  1977 and 1978 fro  -
0 . e resent t  r sults fr  t e ning 
och f 78 eptember. enty-two own - 0  
s urce ositions ere ecessary f r a  cceptable it 
 t e ata. he ectra f t e  t ngest, ast 
nfused s urces re ll nsistent ith er l s 
ith hoton ectral i ices ging  .   . . 
cceptable fits t  t ermal remsstrahlung odels r  
l o o sible r ost urces. o e urce i  t is 
s rvey ca  e xtrapolated t  i her e ergy t  atch 
t e i tensity f t e a ma-ray c ntin um as easured 
by EAO-l lar e fi ld f vie  etectors, hich i plies 
t at t e c ntin um i  a c mposite f ntributions 
  ber f urces. 
ODUCTION 
he alactic center region (i. . ithin about 20 0 of t e 
alactic enter) ontains ore t an 30 - 0 ke  -ray s urces 
brighter than about 10 UPUl. bservations of this region bet een 
10 and 200 ke  are ha pered by c arse angular resolution and it  
tt ndent roblems f urce onfusion. revious bservations 
id, owever, ntinually fi d right s urce(s) ear t e 
galactic center2 3. The HEAO-l UCSD/MIT Sky ap4 provided a 
sensitive survey of this region in four broad energy bands 
covering 13-180 keV. his Sky ap obtained close t  30 sources 
ithin 20 0 of the galactic center at a typical sensitivity of 20 
PU. 
ide field of view (~10° PWHM) observations extending to 1 
eV d bove ave etected ariable ontin um ission   
gion entered n t e alactic enter, s ell s ariable 11 
keV e iss ionS 6. Between the Pall of 1979 and the spr ing of 1980 
both components varied by a factor of t o or ore' as seen by 
HEAO-3. This variability i plies that the dominant source(s) 
ust be ~3" in extent at a distance of 10 kpc. The source(s), 
owever, f t is ission ave et t  e l calized a ove 00 eV, 
and it is nclear hich, if any, f t e 1 ke  INSTEIN s urces 8 
https://ntrs.nasa.gov/search.jsp?R=19850025650 2020-03-20T17:53:31+00:00Z
. 218
OG 2.5-7
are associated with the gamma-ray emission.
We present here further analysis of the HEAO-I UCSD/MIT
data base uslng a finer binning structure retaining full
spectral resolution. By applying this analysis to the region
within 40 ° of the galactic center only, we can achieve slightly
better sensitivity than the HEAO-I Sky Map and can measure the
detailed spectrum of each source along with its six-month
variability. We compare each spectra, or group of spectra, to
that measured at the same time by the higher energy HEAO-I
instruments, with the goal of associating the flux from
individual sources with the variable gamma-ray flux.
INSTRUMENT AND OBSERVATIONS
The detectors used in this analysis are the 13-180 keV Low
Energy Detectors (LEDs) of the Hard X-Ray and Low Energy Gamma-
Ray Experiment aboard HEAO-I9. They were i00 cm2 Nal/Csl
phoswich scintillation detectors, oriented perpendicular to the
spin axis, which pointed withln 0.5 ° of the Earth-Sun line. Their
fields of view were defined by passive and active shielding to be
1.6°x20 ° FWHMwith the planes of the slat collimators inclined at
+60 ° (LED i) and -60 ° (LED 2) to the scan plane. During the 16
month lifetime of HEAO-I, the galactic center was within the LED
fields of view for three, 30 day epochs in the Fall of 1977,
Spring of 1978, and again in the Fall of 1978.
ANALYS IS
The scanning data were accumulated into 1-day sums for each
LED binned along the scan plane of the satellite. The bins were
0.4 ° wide spanning +40" from the galactic center. The photons in
any one bin, thus, came from a region on the sky with the width of
the bin and +20 ° along the detector's instantaneous slat
direction. In order to maximize sensitivity, many of these 1-day
accumulations were summed to produce a time-averaged counting
rate versus scan angle for each epoch for each LED.
In order to determine which sources were contributing a
significant amount to the data, the data were summed from 13-80
keY in each bin. Using the known positions of each source and the
aper tur e response, the sour ce intens it ies and over all
background rates were fit to both detector data sets +
simultaneous ly.
[n order to extract spectra of these sources, the results of
the fitting were used to determine those angle bins that contain
events from a single source, and less than 20% contamination from
other sources on the list. Similarly, regions devoid of response
to any source on the list were selected for background
determination. The full 64 channel spectra of scurce and
background regions were used with standard spectral
8 
O  .  
are ass ciated it  t e ga ma-ray e ission. 
e resent ere f rt er alysis f t e AO-l SD/MIT 
data base using a finer binning structure retaining full 
spectral resolution. By applying this analysis to the region 
within 40° of the galactic center only, we can achieve slightly 
etter s nsitivity than t e EAO-l Sky ap and can easure t e 
etailed s ctrum f h s rce l g ith it  si - onth 
variability. We compare each spectra, or group of spectra, to 
that easured at the sa e ti e by the higher energy HEAO-l 
instru ents, ith the goal of associating the flux fro  
individual sources ith the variable ga ma-ray flux. 
N MENT  SERVATIONS 
he etectors s d i  t is alysis r  t  - 80 ke  o  
Energy Detectors (LEDs) of the Hard X-Ray and Low Energy Gamma-
ay xper iment aboard EAO-19. hey ere 100 c 2 aI/Cs I 
phoswich scintillation detectors, or iented perpendicular to the 
in is, hich ointed ithin .  f  arth-Sun . heir 
fields of vie  ere defined by passive and active shielding to be 
. °x20  HM with  l nes f  t lli ators li ed t 
+60° (LED 1) and -60 0 (LED 2) to the scan plane. uring the 16 
onth lif ti e f EAO-l, t e alactic center as ithin t e LE  
fields of vie  for three, 30 day epochs in the all of 1977, 
s t: ing f 78, a d ain i  t e Pall f 78. 
ALYSI  
he sca ning ata e:re accumulated i t  I- ay su s f r each 
 i ned l ng t e s an lane f t e tellite. he i s ere 
0.40 ide spanning ±40° fro  the alactic center. The photons in 
any e in, t us, ca e fro  a region on t e sky ith t e idth f 
t e in d ±20° l ng t e etector's i t ntaneous lat 
direction. In order to aximize sensitivity, many of these I-day 
accumulations were summed to produce a time-averaged counting 
r te ersus s an ngle f r ach e och f r ach ED. 
In order t  determine hich sources ere contributing a 
ignificant ount  e ata, e ata ere med  -80 
ke  i  each in. sing t e known OSitions f each s urce a d t e 
pert r  sponse, t e ur  i tensiti s d verall 
ackground tes ere it  oth etector ata ts 
i ultaneousl . 
In order t  extract spectra of t ese sources, t e results of 
the fitting ere used to determine those angle bins that contain 
events fro  a si gle source, and less t an 20  contamination fro  
other sources on the list. Similarly, regions devoid of response 
t  any source on t e list ere selected f r background 
determination. The full 64 channel spectra of se urce and 
background regions ere used ith standard spectral 
• 
219 OG 2.5-7
deconvolution programs to determine various spectral
parameters. If spectra from both detectors were available, a
simultaneous fitting of both spectra took place; otherwise, a
single detector spectrum was fit. The results presented in Table
1 assume a power law model only.
RESULTS AND DISCUSSION
Twenty-two, known, 2-10 keV sour ce pos it ions wet e
necessary to fit the LED data, and are listed in Table i. This
list of source positions used to fit the present data agrees with
the HEAO-I Sky Map list with a few exceptions. The present list
contains the SAS-3 and Cos-B source 1715-321, the Ariel-5 source
1712-337, and the HEAO-I A-1 source 1815-121, which are not in
the Sky Map. On the other hand, all Sky Map sources not in the
present list were either highly confused or not significant4.
The spectral indices shown in Table 1 are the result of
fitting individual sources to a power law model. All fits were
acceptable, except for GX 17+2 and OX 9+9, and most sources could
also be fit acceptably by a thermal bremsstrahlung model.
No one source in this survey is intense enough to equal the
gamma-ray flux from this region, which was also measured by the
HEAO-I Medium Energy Detectors s in the Fall of 1978. The
"galactic center" source, 1742-294, is brightest at i00 keV, but
is at least a factor of three below the i00 keV gamma-ray flux.
Extrapolating the 13-165 keV fit to 1742-294 to higher
energies reveals that its power law index must decrease to less
than -3 above -250 keV in order not to exceed observations (see
Figure i). This may indicate that a high temperature thermal
model or a Comptonization model would be more appropriate for
1742-294. If we sum the spectra of the three brightest sources
• within 5° of the galactic center (1742-294, GX 1+4, and GX 5-1),
as shown in Figure i, nearly all of the gamma-ray flux above i00
keY can be accounted for. This may not apply at other times,
however, when the gamma-ray flux was brighter and 1742-294 was
- weaker at i00 keVS.
ACKNOWLEDGEMENTS
This work was supported by the Office of Naval Research and
NASA Grant NAG 8-499. The authors wish to thank Frank Primlni,
Duane Gruber, and Ken Kalata for useful discuss ions and insight,
REFERENCES
1. Forman, W., et. al. (1978), _. J.Suppl.,8a,357.
2. Dennis, B.R., et. al. (1980), _. J.(L,,w8).23_, L49.
2 1 OG 2.S-7 
deconvolution progra s to deter ine vari s spectral 
par eters. If sp t  from bot  det tors w r  av il ble, a 
sim lt neous fittin  of bot  spe t  to k plac ; oth r ise, a 
sin l  det t r spectr  was fit. The res lts prese ted in Ta le 
1 assume a power law model only. 
RESULTS AND DISCUSSION 
T nty-t o, kno , 2-10 keV source po itions w re 
ne ssary to fit th  LED dat , and are liste  in Ta le 1. This 
list of source pos iti s used to f it the pres nt dat  agrees wit  
the H O-l Sky Ma  list wit  a few ex tions. The pre t list 
contains the SAS-3 and Cos-B SOUl:ce l7lS-32l, the Al:iel-5 SOUl:ce 
1712-337, and the HE -l A-I SOUl:ce l81S-12l, which are not in 
the Sk  M . On the oth  han , all Sk  M p SOUl:ces no  in the 
pr nt lis  w r  eith r hig l  co f sed or not sig i  icant4. 
The sp tral indi  show  in T l  1 are the res lt of 
fittin  indi i ual sources to a po er law model. All fits were 
ac table, ex t for GX 17+  and GX 9+ , and m st sour s coul  
als  be fit ac tably by a ther al bre sstrahlung m el. 
No on  sour  in thi  surv  is inten  eno  to eq l the 
ga a-ray flux from thi  regi , w i  was also m sured by the 
H O-l Me i  E r y D t t rsS in the F ll of 197 . The 
-g ctic ce r- SOUl:ce, 174 94, is bri test at 100 keV, bu  
is at lea t a fact r of thre  belo  the 10  keV ga a-ray flux. 
E tra olating the l3-l6S keV f it to 1742-294 to highel: 
en r ies rev ls that its power law inde  m st de r se to less 
tha  -3 above -250 keV in order not to exceed observations (see 
Fig r  1). T is may indi te that a hig  tem erature ther al 
m el or a C ptonization m el w l  be m r  ap priate for 
17 - 94. If we sum the sp tr  of the thre  bri test so r s 
• wit i  5° of the galactic ce ter (1742-294, GX 1+4, and GX 5- ), 
as show  in Fig  1, ne rl  all of the gam a-ray flux abo  100 
keV can be acc nted for. T is ma  not ap l  at oth r tim s, 
ho ever, w  the ga a-ray flux wa  bri ter and 174 4 w  
.. weaker at 100 keVS . 
• 
ACKNOWLEDGEMENTS 
T i  w  w  sup rted by the O i  of N al R arch and 
NAS  Grant NAG 8-499. The authors wish to thank Frank Pri ini, 
D e G er, and K  K l ta for us l disc ssions and inS t • 
REFERENCES 
1. For an, W., et. a!. (19 ), Ap. J. Suppl., 38, 357. 
2. Dennis, B. ., et. a!. (1980), Ap. J. (Letters), 236, L49. 
22O
0G2.5-7
3. Knight, F.K., et. al. (1985), Ap. J.,29o, 557.
4. Levine, A.M., et al. (1984), Ap. d._up_.. 64, 581.
5. Matteson, J.L. (1982), The e_acUc Center, Riegler and Blandford
eds, Am. Inst. of Physics, New York, NY, 109.
6. Leventhal, M. and MacCallum, C.J. (1982), The ee_actlcCenter.
Riegler and Blandford eds, Am. Inst. of Physics, New York, NY,
132.
7. Rlegler, G.R., et al, (1981), Ap. j.(L_terB).248, LI3.
8. Watson, M.G., st al. (1981), Ap. j.,260, 142.
9. Matteson, J.L. (1978), _._AA, No. 78-35.
Table 1. Oalactlc Center Source Ltet Figure 1: Galactic Center Spectra
_gr¢_e Name Spectral Index IO'l- ' ' ' _'"1 ' ' ' ' ....
--4 . HEAO-IUCSD/MI7
1642-455 GX 340+0 4.99+1.31 +
16S7-415 38e pler 2.70+0.25 SEPTEMBER1978
1658-298 3.61+0.39 12- 500 keV
1700-377 2.77.*-0.18 +
1702-363 GX 349+2 3.76+0.27 +
1702-437 .... I0"= -j.
i,, + SUM OF 1742- 294, 6 x 1+ 4,1705-250 Nova Oph ....
j &GxS-I1712-337 3
.20+0.83 _ . (12-m5keY)
1715-321 3,46*0.39
1728-169 OX 9+9 3.67*0.25 '_ +,.1728-247 GX 1+4 2.30+0.07 --
1755-444 .... { +'+,,.fJl".,
1"/42-294 OCX 2. 124"0.04 10"3 GALACTIC CENTER
1743-322 .... T -]-_ REGION
1"/44-265 GX 3+1 _.\ ' \ (SO-5OOkeV)
17_8-250 GX 5-1 3.914"0.13 1742-294 4. __ /J
•,s820sox9+i 4944"0241811-171 GX 13+1 7.19+1.731813-140 GX 17+2 4.164.0 ii
• 0..
1820-303 NGC 6624 3.664.0.19
1815-121 4.614"1.05 x I0"4
1822-371 ....
.b.
m
0"610 I I I L_A ''1100 * ' ' '''_.-_
ENERGY(keV)
110 
O  2.5-7 
. night, P . ., t. l. ( 985), Ap. J., 290, 557. 
4. evine, . ., t al. (1984), Ap. J. Suppl., 64, 581. 
5. atteson, J. . (1982), The aaJactlc Center, iegler and landford 
ds, m. I st. f hys iC8, ew ork, Y, 9. 
6. eventhal, . and acCallum, .J. (1982), The aaJactic Center, 
iegler and landford eds, Am. Inst. of PhYSics, New ork, NY, 
32. 
. iegler, . R., t l. ( 981), Ap. J. (Letters), 248, 13. 
. atson, .G., et l. 981), . J., 50, 2. 
. atteso.n, . . 978), hoc. 111M, o. 8-35. 
Yable . Gala tic enter ource i.t 
19~r~. Ha e pectral Index 
--~-------------------------------
1642-455 ax 340+0 4.99~1.31 
57-415 s 1sr 70~0.25 
58-298 61~0.39 
00-377 7~0.18 
02-363 ax +2 76~0.27 
02-437 
05-250 ova  
12- 37 
15-321 
1728-169 ax 9+9 
28-247 ax 4 
1735-444 
1742-294 acx 
43-322 
1744-265 ax 3+1 
.20:1:0.113 
. 6:1:0.39 
3.67:1:0.25 
2.30:1:0.07 
2.12:1:0.04 
~8-250 ax 5-1 3 91~0.13 
17$8-205 ax 9+1 4.94~0.24 
' - 71 ax 1 9~1.73 
113-140 ax  6~0.11 
' - 03 H  4. 6~0  
815-121 4.61~I.05 
' -371 
----------------------------------
,i9ure 1: Galactic center Spectra 
10-8 
 
O-I O/ IT 
SEPTEMBER 1978 
+ 
2 0 V 
 
t,   F 142-294,611  , 
.... ./ 6Gx5-1 \ t.......--- (12 -165 keY) 
-+ 
'+ ++ 
, t 
i; 
+, t 
+. t 
*++ 
't 
1742-294 , 
(12 -165 keY) \ 
~+ 
\ 
10-a L---'-_'_..L..1...I..U.~--'L.......I-'-........ ~ 
10  1000 
GY ( ) 
• 
• 


Spectra and positions of galactic gamma-ray sources

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