The pair annihilation radiation in gamma-ray bursts is seen as broad lines with extended hard wings. This radiation is suggested to escape in a collimated beam from magnetic polar regions of neutron stars

Aptekar, R. L.

Golenetskii, S. V.

Guryan, Y. A.

Ilyinskii, V. N.

Mazets, E. P.

English

NASA Technical Reports Server

0G I.I-3
7
ANNIHILATION RADIATION IN COSMIC GA_-RAY BURSTS
R.L.Aptekar, S.V.Golenetskii, Yu.A.Guryan, ¥.N.Ilyinskii,
E .P.Mazets
A.F.Ioffe Physical-Technical Institute, Academy of Sciences
of the USSR, 194021 Leningrad, USSR
The pair annihilation radiation in gamma-ray bursts is
seen as broad lines with extended hard wings. This ra-
diation is suggested to escape in a collimated beam
from magnetic polar regions of neutron stars.
Cosmic gamma-ray bursts are widely believed to be gene-
rated on strongly magnetized neutron stars. The strongest
support for this hypothesis comes from the observation in
the gamma-ray burst spectra of absorption and emission fea-
tures which are supposed to be cyclotron and gravitationally
redshifted annihilation lines, respectively
(_azets et al., 1981, 1982; Teegarden and °° ..................180q 7g
Cline, 1980; Hueter and Gruber, 1982). How- + v\
ever the origin and emission mechanism of _. +v-,2
gamma-ray bursts remain unclear (Lamb,1984; >_ kLiang, 1984; Woosley, 1984 ). This emphasi-
zes the need ins comprehensive study of _°_ //i__
the spectral behavior of bursts. ^4
The Konus experi ent on the Venera 11
to 14 spacecraft carried out in 1978 =_0' / \\_
through 1983 has revealed over 350 gamma-
ray bursts. Several tens of events exhibit
in their spectra emission features. In many Q" ................
cases the statistical accuracy of measure- o o2 0'E, KEV
ments was high enough to provide more accu-
rate information on the spectral shape of. Pig.1
these features.
The new data show that the emission features represent
very broad asymmetric lines with extended hard wings. This
means that the gamma-ray bursts under study are made up es-
sentially of two radiation components. The first (softer)
,e".........., ....., ,11,,_,,,,_........,H......., ........,.- component is character-
_\ \ 20,7g_ \ ized by an exponential
\ falloff with increasing\
.>,0' _ 3 +_ photon energy. Thespectrum drops steeply
_ below 1 MeV. This com-
_,_'_ / \\\_ / ponent is similar to
I the continuum spectra
" of the bursts without
_'_" / _ _ emission features.The second (har-
der) component exhibits
o' ........ ................... ........ : ....... ........ a low-energy cutoff be-I01 I0 2 I0 3 ,r I0 2 I0 3 102 I0 3
E,KEV low N300 keV. The po-
Pig.2 sition of the spectral
.. 
O  1 1 3 
 
l'iI I TI  DI TI   I  MM -RAY RSTS 
.L. ptekar, S. . olenetskii, u. . uryan, V.N.Ilyinskii, 
. azet  
.F.loffe Physical-Technical Institute, cade y of Sciences 
of the SSR, 194021 eningrad, USSR 
The pair annihilation radiation in ga ma-ray bursts is 
seen as broad line,s ith extended hard ings. his ra-
i ti  i  s est  t  s e i  a lli ated ea  
fro  agnetic lar re i ns f eutr n st rs. 
os ic a ma-ray ursts r  idely elie e  t  e e e-
r t   str l  agnetized eutr n st rs. e str est 
support f r t is hypothesis co es fro  t e servati n i  
t  ma-ray rst ectr  f sorption  issi n -
t res hich are supposed t  be c cl tr  and ravitati nally 
redshifted annihilation lines, respectively (Mazets et al., 1981, 1982; Teegarden and 10° .----r,....,."..,,,..--,le---,-0~'"'79rmr--~ 
Cline, 1980; Hueter and Gruber, 1982). How- ,,+ V-II 
ever the origin and emission mechanism of 
10-1 a ma-ray ursts re ain clear ( a b,1984; ~ 
iang, 1984; oosley, 1984 ). his e phasi- ~ 
zes the need in a comprehensive study of ~~10-2 
t e s ctral ehavior f ursts. ?' 
he onus experi ent on the enera 11 ~ 
to 14 spacecraft, ca.rried out in 1978 ~ 10-~ 
t r h 1983 as re ealed er 350 a a-
r  rsts. everal t  f ents hibit 
in their spectra e ission features. In any 11i·";--L-..L.U.U""::-'----'--'-U""'"-;'-~ 
f ~ ~ s t  t ti ti al r cy 0 easure- E,KEV 
ents as igh enough t  provide ore accu-
rate information  t e ectral s ape f. 
 t res. 
Fig.1 
The ne  data sho  that the e ission features represent 
very broad asy etri,c lines ith extended hard ings. This 
eans t at the ga ma-ray ursts under study are ade up es-
ti llY f t  r i ti n ponents. e i t ( ft r) 
10-
1 
11 1-1,."111 111111111 II component is character-
',- 2 ",2 O'l79 r, ized by an exponential 
\ 
\ 
ll ff it  i r si g 
photon energy. The 
s tr  r s t l  
l   eV. his -
ponent is similar to 
the continuum spectra 
f  rsts it ut 
emiseio,n features. 
he  r-
10-s10L 1-'-'-J..LWJ
IO
'-=-2-'-'-J..l.WJ
IO
"-=J-J.{I'-II J..J.' 'JJJ.' "I'''--:~2:-'-'....L' .w' 'JJ.L" II"-:'~ 3:-"lfJL .l.J.W"-:102::-,-,-~w...1 o~J !eI ~w: ~~~~ ~n ~ u~~~ b~! ~ 
E, KEV lo  '" 300 ke . he -
F1~.2 sition of the spectral 
https://ntrs.nasa.gov/search.jsp?R=19850025592 2020-03-20T17:55:13+00:00Z
OG 1.1-3
8
,o°: . ......., ........, ... ,o': .., ....., ,.. ....., ,,, maximum varies from _-350
250582 80982 to 450 key for different
event s. This narrow regi-
_ _ o° on corresponds to the
_ gravitational redshift of
_62 ; _ the pair annihilation ra-
_\ _6 diation produced near the
o_
/_ _ o surface of a neutron
__ _ star. The hard wing of
_3 _ the feature extends to-
ward high energies by ap-
,_ ........ , ........, ,,, ,o__........, .... _...,,_ ,_ proximately a power law
0_ 0_ 03 0 02 0_ with a slope of -2.5 to
E KEy E ,EV --3.5. It is highly proba-
Pig.3 Fig.4 ble that the hard tails
in the gamma-ray burst
spectra revealed in the S_ data (Riager et al., 1982) ori-
ginate in such features (Mazets et al., 1983).
In the present report we are going to submit only
a few observations of annihilation features as well as some
considerations concerning their possible interpretation.
The results obtained will be treated in more detail else-
where.
We have fitted the shape of the photon energy spectrt_
having an emission feature with a sum
e(
/dE :AE- exp(-E/k )+B
EP*(kmc2)# (I).
The first term of this expression describes satisfactorily
the spectrum of a continuum without the absorption and emi-
ssion features (Mazets et al._ 1982). The second term is a
simple model for the hard radlation component representing
a bro_ad line with an extended hard wing. This functlon_/=grows
as E % passes through a maximum at E_=K_/(#-_)I/#kmc 2,
and falls off after this as E _r J. The parameter k permits
shifting the distribution along the energy axis. A and B are
dimensional normalization factors.
Consider examples of such representation of the gamma-
ray burst spectra with clearly pronounced features. Pig.1
shows a photon spectrum of the 18 April 1979 event. The ex-
perimental points exhibit a complete agreement between the
measurements performed on the two spacecraft. The solid li-
ne at energies above 50 keV corresponds to relationship (1).
The two radiation components are shown separately by dashed
lines, The parameter adjustment was performed by minimizing
the sum _(N_obs-Njc)2/g2(Njobs ) where NjobsiS the experi-j u
mental value of the photon flux in channel j of the spect-
rum, and Njc is the corresponding value calculated by Eq.(ll
The _2 test (_q_p2 =23 for the number of the degrees of fre-
edom q-p =24) showed this approximation to be satisfactory.
V) 
.. 
::E 
-I 
10 
~,62 
en 
z 
o 
~ 
o 
I 
a. 
-3 
10 
G 1. 3 
 
25 0582 80982 
Fig.) i .4 
axi um varies from ~350 
to 450 keV for different 
events. This narro,w regi-
on corresponds to the 
vitational shift ,f 
the pair annihilation ra-
i ti n r uced ar t  
surface of a neutron 
star. The hard wing of 
t  t re t ds -
ard high energies by ap-
proximately  power law 
ith a sl e f - .  t  
-).5. It is highly proba-
ble that the hard tails 
i  the ga ma-ray burst 
spectra revealed in the SMM data (Rieger et al., 1982) ori-
i ate i  s c  .features ( azets t l., 83). 
In the present report we are going to submit only 
 f  servations f nihilation t res  ell  e 
considerations concerning their possible interpretation. 
e r sults tained ill e tr ted i  ore etail ls -
here. 
We have fitted the shape of the photon energy spectrt~ 
ving  i sion t re ith   
0{ 
dN/dE = OexP(-E/kT)   (J E 2 {J 
 +(k  ) ( 1). 
The first ter  of this expression describes satisfactorily 
the spectrum of a continuum ithout the a,bsorption and e i-
ssio,n features ( azets et ale p 1982). The second term is a 
si ple odel for the bard rad~ation co ponent representing 
a broad line with an extended hard wing. This function grows 
as E- cX, passes through a maximum at Emax=[ot /(,P-«)] '1kmc2, 
and falls off after this as E-<? -o(). The parameter k permits 
s ifting t e istribution al ng t e energy xis.  and re 
i ensional ormalization ctors. 
onsider e a ples f s ch r presentation f t e a ma-
ray urst s ectra ith learly pronounced f atures. Fig.1 
s  hoton ectrum ·f t e 8 pril 79 vent. he x-
erimental oints xhibit a c plete greement et en t e 
easurements erf r ed n t e t o acecraft. he olid -
ne at energies above 50 ke  corresponds to relationship (1). 
he t o r diation c po ents re s n separately y ashed 
li es. he parameter adjustment as perfor ed by inimizing 
the sum ~(NjObS-Njc)2/v2(NjObS) where NjobsiS the experi-
ental v~lue of the photon flux in channel j o,f the spect-
rum, and N. is the corresponding value calculated by Eq. ( 1 ). 
The)( 2 tes~c(1'2 =23 for the number of the degrees of fre-· 
<\ ~q-p 
edo  -p  24) showed t is approximation t  e satisfactory. 
@ OG 1.1-3
Fig.2 displays three spectra of the
2 April 1979 event measured consecutive- _o"__'"" '_'o_'_'8...... _
ly with a 4 s accumulation time. In ad- • _._ ,.o.,
ditien to a strong annihilation compo- ,o-_ H_H
nent, the spectra contain also an inten- _ \±
se cyclotron feature. Both the continuum
and the features exhibit a strong and _,_
independent variability. The hard compo-
nent in the energy spectra can evolve on
a very fast time scale. Pig.3 shows two o_
spectra of the 25 May 1982 gamma-ray
burst. The strong annihilation llne is
seen only in the first spectrum measured 0_
with an accumulation time of 1 s. 0 0_ 0_ 0_E, KEV
It is remarkable that the annihila- _ig.5tion component is present also in some
very short gamma-ray bursts. Fig.4 presents a spectrum of
the 8 September 1982 event which lasted only for 100 ms.
The above examples represent only a small fraction of
the available data on the bursts exhibiting this pattern of
spectral behavior.
The spectral range of our measurements extends only as
far as2 MeV. Therefore we cannot provide direct evidence
which would show that the hard wing can extend up to several
MeV and even higher. There is. however, a remarkable obser-
vation supporting our suggestion. Fig.5 presents a spectrum
of the 25 _arch 1978 event measured on HEAO-I up to about
10 MeV (Hueter, 1984). One can see here both types of spect-
ral features. Our approximation of this spectrum with the
two-component model yields a result which does not differ
from the other cases considered.
In conclusion consider a possible explanation for such
a spectral behavior of the annihilation radiation from a
pair plasma assuming its temperature to vary within a fairly
"broad range, k_ O.1-_ MeV. In Pig.6 constructed from the
data of Ramaty and M4szaros (1981) the solid lines corres-
,_,3........, ....., ........pond to annihilation spectra for a pla-
,_. 8 sma with the temperature varying from
;I_,_3×I0K
,_,*//_',,0'K 3X108 tO 3X10I0 K. The dashed line is a
//l_,3xmO,K result of superposition of these spect-a. While being schematic, i resul
%_ _,_ _', nevertheless exemplifies clearly the
'//_ possibility of formation of an annihl-
'O'°K lation spectrum with a maximum around
",, mc2 and a hard power-law wing. This si-
_I_ ,o'% tuation can apparently become realized
_I_*16'7 in a plasma with a spatially nonuniform
and/or rapidly varying temperature.
As follows from the data obtained,
16'8---_-_-_-_'........' .....the line energy flux S1 is comparableI0 I I I0 I0_
ENERGY(MeV} with the continuum energy flux Sc and
Fig.6 even may exceed it, i.e. S1/Sc = 1 to 4.
• 
9 G  
i .  i l s t  ctr  ,f t  
2 April 1979 event measured consecutive-
ly with a 4 s accumulation time. In ad-
di tion to a strong annihilation compo- 10-2 
nent, the spectra contain also an inten- ~ 
se cyclotron feature. oth the continuu  ~ 
and the features exhibit a strong and ~103 
i e e dent riabilit . e r  c po- ~ 
nent in the energy spectra can evolve on ~ 
 r  t tim  l . Fi .   tw  "-10" 
s ctr  f t e 25 ay 1982 ga ma-ray 
rst. e str  nibjlation li  i  
seen only in the first spectru  easured 105,-:--,-.LU.L=::--~'-1.WL::c-L'-.u.Aul 
it   ulation im  f   10' 10:,KEV 10' 10" 
It is remarkable that the annihila-
ti  ponent i  r s t l  i  s  Fi .  
very s rt ga ma-ray ursts. i .  resents a spectru  f 
t   t ber  t hich l t  l    s. 
e a e e a ples r r sent l  a s all fr ti  f 
t  ail ble t   t  ursts hibiti g t i  att r  f 
ectral havior. 
The spectral range of our easure ents extends only as 
f r as 2 e . herefore e cannot r i e irect e i e ce 
hich ould s  t t t  r  i   t   t  s eral 
e  a  e e  i er. here i  ever, a r arkable ser-
vation supporting our suggestIon. ig.5 presents a spectru  
f t   March  t easured  O-1  t  ut 
10 e  ( ueter, 1984). ne can see here both types f s ect-
r l feat res. ur approxi ation f t is spectru  ith t e 
t - ponent odel i l s  sult hich es t i r 
o   t er s si ered. 
I  clusi n nsi er a ssi l  planati n f r s  
a s ectral ehavior f t  nihilati n r iati n fro  a 
air plas a assu ing its te perature to vary ithin a fairl  
broad range, kT O.1-~ eV. In Fig.6 constructed fro  the 
t  f a aty  eszaros ( ) t  li  li  rres-
O::\uJ 
." -0 
pond to annihilation spectra for a pla-
sma with the temperature varying from 
3x108 to 3X1010 K. The dashed line is a 
result of superpo;si tion of these spect-
ra. hile being sche atic. t is result 
nevertheless exemplifies clearly the 
possibility of formation of an annihi-
l ti  s ectr  it  a axi u  ar d 
N ~ 1017 
c2 and s. hard po er-la  ing. his si-
t ati n ca  a parently beco e r li d 
in a plasma with a spatially nonuniform 
a d/or ra i l  ar i g te perature. 
As follows from the data obtained. 
liil~OL._I..I...Ll.LlllllI---l-I-Ju.uw.IO~.J..UWII02 the line energy flux 3  is comparable 
ENERGY (MeV) wi th the continuum. energy flux S c and 
Fig.6 even may exceed it, i.e. Sl/3c = 1 to 4. 
lO OG 1.1-3
We believe that such a ratio can be accounted for by diffe-
rent directional patterns of these two components.The loca-
tion and temperature of the regions responsible for the li-
ne and continuum radiation are apparently different.The ma-
gnetosphere of a neutron star is opaque to hard radiation.
Therefore the parers formed in the (_) and (_) processes
above the surface of an isotropicaIly emittlng continuum
source should apparently accumulate in the star' s magnetic
polar regions before annihilation. Here a window should
exist in the opaque magnetosphere within a solid angle are-
und the star's magnetic moment through which the annihila-
tion radiation from the stored plasma will escape in a col-
limated beam (cf, e.g., Katz, 1982).
The annihilation lines are observed in about 10-15% of
the gamma-ray bursts. One may suggest that this frequency
corresponds to a difference between the directional pat-
terns of the annihilation and continuum emission. Accordin-
gly, the actual ratio for the energy emitted in the two
components will drop down to SI/S c _ 0.1 - 0.4.
Reference s
Hueter G.J., Gruber D.E., 1982. In: Accreting Neutron
Stars, ed. W.Brinkmann & J.Tr_muer, MPE report 177,
Garching, 213 •
Hueter G.J., 1984. In: High Energy Transients in Astrophy-
sics, ed. S.Woosley, AIP Conf. Proc. No 115, NY, 373.
Katz J.I., 1982. Astrophys. J. _, 371.
I_mb D.Q,, 1984. In: High Energy Transients in Astrophysics,
ed. S.Woosley, AlP Conf. Proc. No 115, NY, 512.
Liang E.P., 1984. Ibid. 597.
Mazets E.P., Golenetskii S.V., Aptekar R.L. et al., 1981.
Nature 2__, 378.
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We believe that such a ratio can be accounted for by diffe-
r t ir ti al tt r s f these two co ponents. he lo -
tio  and te perature o,r the regions res nsible f r the li-
ne a.nd continuu  ra iati  are apparently iffere t. e a-
gnetosphere of a neutron star is opaque to hard radiation. 
,!herefore the pairs formed in the ('t.r) and (r fJ) processes 
above the surface of an isotropica:Lly emitt~ng continuu  
source should apparently accu ulate in the star's agnetic 
l r s r  nihilati n. er   i  l  
exist in the opaque agnetosphere ithin a solid angle aro-
 t  r'  agnetic o ent thro  hi  th  ihila-
tion radiation fro  the stored plas a ill escape i  a col-
li at   ( f, . ., atz, 2). 
 nihilati n lin  r  s r ed i  t -15  f 
the ga ma-ray bursts. ne ay suggest that t is frequency 
corresponds to a difference bet een the directional pat-
 f  nihilation  t n  issi n. ccordin-
l , t  t al r ti  f r t  r  itt  i  t  t  
components will drop down to Sl/Sc ~ 0.1 - 0.4. 
eference  
ueter .J., ruber . ., 1982. I : ccreting eutron 
st rs, ed. .Brinlanann &: J.TrU uer, PE re rt , 
arching, 3. 
Rueter .J., 1984. I : igh nergy ransients i  strophy-
i , . . osley, l  onf. r c. o , , . 
a,tz J .I., 1982. strophys. J. 260, 371. 
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ed. S. oosley, lP onf. Pro,c. o 115, Y, 512. 
i g .P., 4. i . 7. 
azets .P., olenetskii s.v., ptekar . . t l., 1981. 
ature 290, 378. 
Mazets .P., olenetskii . ., I1 i skii . . et l., 1982. 
strophys. pace ci. §g, 1. 
azets .P., olenetskii . ., uryan u. . et l., 1983. 
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.L.Burns, . .Harding  . amaty, lP on!. roc. 
o 101, , ~6. 
amaty ., Meszaros ., 1981. strophys. J. 250, 384. 
ieger B., Reppin ., Y~nbach G. et al.,1~82. In: ccreting 
eutron stars, ed. .Brink ann & J. Tru per, PE report 
77, arching, 29. , 
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Annihilation radiation in cosmic gamma-ray bursts

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