Our balloon-borne germain gamma-ray telescope was flown over Alice Springs, Australia, on 1984 November 20 to search for the 511 keV positron annihilation line from the Galactic Center. The measured line flux at Earth was (0.6 + or - 4.4) x 0.001 ph/sq cm/s indicating that the source was still in a low or off state

Leventhal, M.

Maccallum, C. J.

English

NASA Technical Reports Server

0G2.5-5218
SEARCH FOR POSITRON ANNIHILATION LINE AND CONTINUUM
RADIATION FROM THE GALACTIC CENTER
C. J] MacCallum
Sandla Laboratories, Albuquerque, NH 87185
M. Leventhal
AT&T Bell Laboratories , Murray Hill, NJ 07974
ABSTRACT
Our balloon-borne germanium Y-ray telescope was flown over
Alice Springs, Australia, on 1984 November 20 to search for
the 511 keV positron annihilation llne from the Galactic
Center. The measured llne flux at Earth was (0.6 + 4.4)
x10 -4 ph cm-2 s-I indicating that the source was still in a
"low" or "off" state.
I. Introduction] A series of 12 balloon and satellite experiments
dating back to 1970 and involving slx different scientific groups has
established the existence of a powerful, compact, and variable source of
511 keV positron annihilation llne radiation within a few degrees of the
Galactic Center (GC). Surprisingly, the source was observed to "turn
off" rather abruptly at the beginning of this decade (see reference I
for a review of the observations). Three previous balloon flights of
the joint Bell/Sandia gamma-ray astronomy group's hlgh resolution Ge
telescope have played an important role in discovering the source and
confirming Its variability (2,3,4). We report here the results of our
fourth flight which indicate that the'sourcewas still In an "Off" or
"low" state in fate'1984.
2. Experlment. The instrument was essentially the same as that described
originally by Leventha !, MacCallum, and Watts (5). It is built around a
single large (-200 cc), hlgh-purlty Ge detector operated at cryogenic
temperature and surrounded by ~200kg of NaI in active antlcolncldence.
The entrance aperture In the NaI shield defines the field of view, "15 °
FWHM at 511 keV. The energy resolution of the system in the laboratory
has been improved to 2.1 keV FWHM at 511 keV by changes in electronics.
The flight took place over Alice Sprlngs, Australia on 1984 November
20. Observations of the GC direction (RA = 265.6°, decl = -29.0 °) were
made at a mean atmospheric depth of 3.3 g/cmsq. Data were accumulated in
alternate -15 minute target-background pair segments wlth the telescope
maintained at the same zenith angr_ebut rotated 180 ° in azimuth for the
background measurements. Consequently, the background observations swept
over the celestial sphere, paying no special attention to any particular
point. A total of 7.5 hours of useful GC data a_d background data were
accumulated in this fashion] For each segment, data were accumulated
into 8192 energy channels each of width 0.798 keV. There was a gain
drift totalling about two channels at 511 keV during the course of the
• 
18 OG 2. -5 
S CH F R P SI N A NIHILATI N LI  ND CONTI M 
RADIATI N F  T E GALACTIC E TER 
. J. acCall  
andia aboratories, lbuquerque, NM 5 
. eventhal 
AT&T Bell Laboratories~ Murray Hill, NJ 07974 
BS T 
Our balloon-borne germanium Y-ray telescope was flown over 
lice Springs, ustralia, on 1984 ovember 20 to search for 
the 511 keV positron annihilation line fro  the alactic 
enter. he easured li  ,fl  t arth as ( .  ± . ) 
x10-4 ph cm-2 s-1 indicating that the source was still in a 
o  r ff  t t . 
1. t uction.  ri s f  all n  tellit  eri ents 
dating back to 1970 and involving six d~fferent scientific groups has 
esta lis ed t e e iste ce f a powerful, co pact, and ariable s rce of 
511 keV positron annihilation line radiation ithin a fe  degrees of the 
alactic enter ( ). urprisi gly, t e s rce as observed t  "t r  
off" rather abruptly at the beginning of this decade (see reference 1 
for a revie  of the observations). Three previous balloon flights of 
the joint Bell/Sandia gamma-ray astronomy group's high resolution Ge 
tel~scope have played an i portant role in discovering the source and 
confirming its variability (2,3,4)~ We report here the results of our 
fourth flight which indicate that the"source'was still in an "off" or 
lo   i  l  ' 984. 
2. Experiment. The instrument was essentially the ,same as that described 
originally by Leventhal, MacCallum, and atts (5). It is built around a 
single large (-200 cc), high-purity Ge detector operated at cryogenic 
temperature and surrounded by -200kg of NaI in active anticoincidence. 
The entrance aperture in the NaI shield defines the field of view, -15 0 
FWHM at 511 keV. The energy resolution of the system in the laboratory 
has been i proved to 2.1 keV FWHM at 511 keV by changes in electronics • 
The flight took place over lice Springs, ustralia on 1984 ove ber 
. bservations f th   ir ti  (RA • ~ ; l = -2 . 0 ) ere 
, ade at a ean at ospheric depth of 3.3 g/c sq. ata ere accu ulated in 
alter ate -15 inute target-background pair seg ents ith the telescope 
aintained at the same zenith angle but rotated 180 0 in azimuth for the 
background measurements. Consequently, the background observations swept 
over the celestial sphere, paying no special attention to any particular 
point. A total of 7.5 hours of useful GC data and background data were 
accumulated in this fashion. For each segment, data were accumulated . 
into 8192 energy channels each of width 0.798 keV. There was a gain 
drift totalling about two channels at 511 keV during the course of the 
https://ntrs.nasa.gov/search.jsp?R=19850025649 2020-03-20T17:53:23+00:00Z
2 14 0G 2.5-5
flight, which was monitored by means of three calibration lines from a
weak onboard 207Bi source that were visible in every segment spectrum.
3' Results. The data were searched for a possible astronomical llne
superimposed on the instrumental line at 511 keV, using the maximum
likelihood method advocated by Cash (6) applied to Poisson-distributed
variables. With only "5 counts/channel/segment, Poisson statistics
seemed likely to be more reliable than Gaussian statistics Further-
more, the determination of parameter error bounds stands on firmer
theoretical footing in the Cash method, which is no more difficult to
implement than the more familiar "minimum chl-square" methods. (Never-
theless, in this case, we found a least-squares weighted estimate based
on Gaussian statistics gave much the same answer.) For each of 26
segments, a set of 30 channels centered on 511 keV were analyzed under
the assumption that a uniform instrumental continuum background plus an
instrumental line at 511 keV were Present , at the same strengths in both
segments of a target-background pair. In addition, an astronomical llne
was assumed to be present in each target segment, at a constant strength
(modified appropriately for atmospheric attenuation). All lines were
assumed to have the same width and to be centered on 511 keV. The
various parameters were then determined by the maximum likelihood
method, and "one-sigma" error bounds were determined for the astro-
nomical line strength by finding that value for it that increased the
Cash statistic C ( C = -2_nP, P = maximum likelihood probability
maximized wrt all other parameters ) by unity. The result for the 511
-4 -2 -I
llne flux was (0.6 ± 4.4)xi0 ph cm s . The error bound is slightly
worse than our 1981 result, largely due to a small decrease in the
effective area of our detector (now = 10.3 cmsq at 511 keV).
Figure I shows two spectra in the neighborhood of 511 keV. One is
the sum of all target segments, divided by total llve time; the other is
the same for background segments. A rough energy calibration has been
performed simply by centering on the peak channel of each segment. The
comparison is illustrative only, in that no consideration is given to
variations in atmospheric attenuation or background rates, but the
comparison clearly illustrates the lack of a visible astronomical
signal.
Continuum radiation from the Galactic Center direction was clearly
detected below ~250 keV. This can be seen in Fig, 2, which is the same
as Fig. I but centered at a lower energy, "110keV. First indications
are that the continuum spectrum is softer than when measured in 1977-81,
but the detailed analysis is still in progress.
4 Discussion. When fully on the GC positron source was truly a
remarkable object, calling for ~1043 annlhilatlons/sec (corresponding to
"1038 ergs/sec) if isotropic and located at the GC distance of -10 Kpc.
Nothing like this has been seen anywhere else in the sky. It was easily
the brightest gamma-ray object in the Milky Way in annihilation
radiation alone. Because of its unique nature it is tempting to locate
it within the central parsec of the Galaxy. If so. it would amount to
 O   
flig t, hich as onitored by eans of t ree cali rati  lines from a 
eak onboard 207 i source t at ere isi le i  every seg ent spectru . 
3. esults. he ata ere searc e  f r a ssi le astro ical li  
ri osed  t  in tru ental li  t 1 , i  th  a i  
ik ih  et  ocated    li   isson-distributed 
ri l s. ith l  -5 unts/ annel/segment, iss  t ti ti s 
e  li l  t   or  li l  t  au si  ti t s. rt er-
ore, t e eter inati n f ara eter rr r s sta s  fir er 
t reti al ti  i  t  as  et od, hi    or  i i lt t  
i l ent t  t  or  f ili r " i im  i- ar " et ods. ( ever-
t eless, i  t is case, e found a least-s ares eighted esti ate based 
 aussian t ti ti s  uc  t  sa  er.) r  f  
seg ents, a set of 30 channels centered on 511 ke  ere analyzed under 
t  ass pti n t t a if r  i stru ental c tinu  ac r nd l s a  
instru ental line at 511 keV ere present, at the sa e strengths in both 
seg ents of a target-background pair. In addition, an astrono ical line 
as assu ed t  be present in each tar et seg ent, at a constant stre t  
( odifi  propriately f r t ospheric )~ ll li  er  
u      idt     t    . e 
ari s r eters er   t i ed   a i  k h d 
ethod, a  " e-si a" rr r s ere eter i ed f r t  str -
ical  t t   i i  t t l    t t i d t  
as  t tistic  (   ~    axi u  lik lih  r bability 
axi ized rt ll t er ara eters) by it . he r s lt f r t e 511 
li e flux as (0.6 ± 4.4)Xl0- ph c - s- 1• he error bound is sli tl  
orse t  r  ult, l l   t   all cr ase i  t  
f ti e r  f r et ct r (   ~  s  t  ). 
i ure 1 s s t  ectra i  t  i or ood f  . ne i  
t   f ll t et ents, i i   t t l li  ti ; t  t er i  
t e sa e f r background seg ents.  rough energy cali rati  has been 
rf r ed i ply  nteri g  t   annel f  ~  
co parison is ill strati e only, in that no consideration is given to 
ariati s i  at ospheric atte ati n r background r t s, t t e 
parison l rl  illu tr t s t  l  f  i i l  stro ical 
al. 
ontinuu  ra iati n fro  the alactic enter . irecti n as clearl  
detected belo  -250 ke . his can be seen in Fig. 2, hich is the sa e 
as i . 1 ut t r d t a l er e er y, - e . irst i i ati ns 
are t at the continuu  spectru  is s fter than hen easured in 1977-81, 
t t  etail  alysis i  till i  r gress. 
4. iscussion. hen f ll  on t e GC sitr  s rce as tr l  a 
re arkable bject, calli  for -1043 a nihilati ns/sec (corresponding t  
-
38 r s/s c) if is tr ic a  l cate  t t e C ista ce f -  pc. 
othing li  t i  as  s  here l  i  t  s . t as sil  
the brightest ga ma-ray object in the ilky ay in annihilation 
r iati n l e. ecause f it  ni e at re it i  te pting t  l ate 
it ithin t  ntral ars c f t  alaxy. If s , it ould a ount t  
• 
2"18 OG 2.5-5
about 0.1% of the total bolometric luminosity of the nuclear region and
would obviously be an important clue to the nature of thls source (7).
|0-
_|"
i
._ mmtgt Fig.I Channel
Io •_ FF2]bkg spectra around 511keV for sum of tgt
segments and foro
16 sum of bkg segments.
No astronomical
signal Is apparent.
I@
0
CHANNEL RELATIVE TO PEAK CHANNEL
,,o. Eatgt
bkg Fig. 2 Channel4
spectra around 110
too, keV for sum of tgt
segments and for
• sum of bkg segments.
_a. The Galactic Center
continuum is evident.
o
Considerable theoretical effort has gone into modeling the source
of positron annihilation radiation. Various black hole models are
favored (8,9,10), although supernova (11) and pulsar (12,13) models have
also been considered. It would appear that only the black hole models
can easily and naturally account for the large flux, rapid variability,
.. 
• 
2 5 OG 2.5-5 
about 0.  of t e t tal bolometric luminosity of t e nuclear region and 
would obviously be an i portant clue to t e nature of this source (7). ' 
c 
!) 
0 
z 
::; 
.. 
z 
z 
c 
z 
~ 
i 
0 l' 
10 
, 
0 
F tgt 
rz2J bkg 
~.e,~."'f.I!tt C)"'C\.~",..,e, 
CHANNEL RELATIVE TO PEAK CHANNEL 
~tgt 
f72J bkg 
Fig. 1 Channel 
spectra around 511 
k Y f r s  f t t 
s ents and f r 
s  f  ents. 
o astr omical 
i al i  pparent. 
Fig. 2 Channel 
ectra ound  
Y or  f t 
ents  r 
 f g ents. 
 alactic enter 
t n   i nt . 
onsiderable the retical eff rt has gone into odeling the source 
of positron annihilation radiation. arious black hole odels are 
favored (8,9,10), although supernova (11) and pulsar (12,13) odels have 
als  been c si r . It l  ap r th t l  the blac  l  odels 
can easily and naturally account for the large flux, rapid variability, 
216 OG 2.5-5
and the absence of other nuclear gamma-raY lines In these models the
variability arises from changes in either the rate of accretion or the
dynamics of the interaction between an emitted positron beam and a
stopping cloud. In recent years considerable support for the presence
of such an object at the GC has come from the more conventional branches
of astronomy (14,15,16,17,18). However this is sure to remain a contro-
versial issue for years to come.
It does not seem possible to predict with any certainty when the
positron annihilation radiation might reappear. We intend to monitor
the Galactic Center periodically in the future with a more sensitive
gamma-ray telescope in the hope of once again detecting this source.
5. Acknowledgements. We wish to thank A. Huters, P. Stang, N. Corlls,
L. Gillette and D. Sayers for their outstanding technical support, and
the men of the NSBF balloon launching team led by R. Kubara for a
good fllght.
References.
I. MacCallum, C. J. and Leventhal, M., in Posltron-Electron Pairs in
Astrophysics, edited by Burns, M. L., Harding, A. K. and Ramaty, R.,
AIP Conference Proceeding No. I01, p. 211 (1983).
2. Leventhal, M., MacCallum, C. J. and Stang, P. D , Ap. J (Letters)
225, LII (1978).
3. Leventhal, M., MacCallum, C. J., Huters, A. F. and Stang, P. D.,
Ap. J. 240, 338 (!980).
4. Leventhal, M., MacCallum , C. J., Huters, A. F and Stang, P. D.,
A_ '.J. (Letters) 260, LI (1982).
5. Leventhal, M., MacCallum, C. J. and Watts, A , AP- J" 216, 491
(1977)'
6. Cash, W., AP" J- 228, 939 (1979).
7. Lingenfelter, R. E. and Ramaty, R., in The Galactic Center, edited
by Riegler, G. R. and Blandford, R. D., AIP Conference Proceeding No.
83, p. 148 (1982).
8. Lingenfelter, R] E] and Ramaty, R. in Posltron-Electron Pairs in
As_trophysics, edited by Burns, M. L. Harding, A. K., and Ramaty, R., AIP
Conference Proceedings No. 101, P. 267, (1983)
9. Kardashev, N. S. Novlkov, I. D., Polnarev, A. G. and Stern, B' E.,
ibld, p. 253.
10. Burns, M. L., ibld, p. 281.
11. Colgate, S. A., ibld, p. 273.
12. K. Brecher andMastichiadis, A., ibid, p 287
13. Sturrock, P. A. and Baker, K. B., Ap. J. 234, 612 (1979)
14. Lo, K. Y. and Claussen, M. J., Nature 306, 647 (1983).
15. Hall, D. N. B., Klelnmann, S. G., and Scoville, N. Z., Ap. J.
(Letters) 262, L53 (1982).
16. Becklln, E. E., Gatley, I. and Werner, M. W., _ 258, 135 (1982)
17. LacY, J. H., Townes, C. H., GebSlle, T. R. and Hollenbach, D. J.,
Ap. J. 241, 132 (1980).
18. Kellerman, K. I., Shaffer, D] B., Clark, B. G., and Geldzahler, B.
(Letters) 214, L61 (1977)J., Ap. J.
218 OG 2.5-5 
and the abs nce f other nuclear g mma-ray lines. In these models the 
variability arises from changes in either the rate of accretion or the 
dynamics of the interaction b tween an emitted positron beam and a 
stopping cloud. In recent years considerable support for the pres nce 
of such an object at the GC has come from the more conventional branches 
of astr nomy ( 4, 5, 6, 7,18). However this is sure to remain a contro-
versial issue for years to come. 
It does not seem possible to predict with any certainty when the 
positron annihilation radiation might reappear. We intend to monitor 
the Galactic Center periodically in the future with a more sensitive 
g mma-ray telescope in the hope f once again detecting this source. 
5. Acknowledgements. We wish to thank A. Huters, P. Stang, N. Corlis, 
L. Gill tte and D. Sayers for their outsta ding technical support, and 
th  men of the NSBF balloon launching team led by R. Kubara for a 
good flight. 
References. 
1. MacCallum, C. J. and Leventhal, M., in Positron-Electron Pairs in 
Astrophysics, edited by Burns, M  L., Harding, A. K. and Ramaty, R., 
AlP Confer nce Proceeding No. 101, p. 211 (1983)~ 
2. Leventhal, M., MacCallum, C  J. and Stang, P. D., Ap. J. (L tters) 
225, Lil (1978). 
~ Leventhal, M., MacCallum, C  J., Huters, A  F. and Stang, P. D., 
Ap. J. 240, 338 (1980). 
4. Leventhal, M., MacCallum, C  J., Huters, A. F. and Stang, P. D., 
AE  J. (L tters) 260, Ll (1982). 
5. Leventhal, M., MacCallum, C  J. and Watts, A., Ap. J. 216, 491 
(1977) • 
6. Cash, W., Ap. J. 228, 939 (1979). 
7. Li genfelter, R. E. and Ramaty, R., in The Galactic Center, edited 
by Ri gler, G  R. and Blandford, R  D., AlP Confer nce Proceeding No. 
83; p. 148 (1982). 
8. Li genfelter, R. E. and Ramaty, R. in Positron-Electron Pairs in 
AstrophYSics, edited by Burns, M. L. Harding, A. K., and Ramaty, R., AlP 
Conference Proceedings No. 101, P. 267, (1983). 
9. Kardashev, N. S. Novikov, I  D., Polnarev, A. G. and Stern, B. E., 
ibid, p. 253. 
~ rns, M  L., ibid, p. 281. 
11. Colgate, S  A.:-Ibid, p. 273. 
12  K. Br cher and Mastichiadis, A., ibid, p. 287~ 
13. Sturrock, P~ A. and Baker, K. B.,~ J. 234, 612 (1979). 
14. Lo, K. Y. and Claussen, M. J., Nature 30 , 647 (1983). 
15. Hall, D  N  B., Klei mann, S  G., and Scoville, N  Z., Ap  J. 
(Letters) 262, L53 (1982). 
16. Becklin,  E., Gatley, I~ and Werner, M W~, Ap. J: 258, 135 (1982) 
17. Lacy, J. H., Townes, C  H., Geballe, T. R. and Hollenbach, D  J., 
Ap  J. 241~ 132 (1980). 
18. Kellerman, K. I., Shaffer, D. B., Clark, B. G., and Geldzahler, B. 
J., Ap  J. (L tters) 214, L61 (1977). 


Search for positron annihilation line and continuum radiation from the Galactic Center

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