It is shown that Al26 is a very good candidate for producing a detectable gamma-ray line, and that this line is not only intense but also very narrow. By examining the chart of nuclides for other radioactive isotopes which could produce hiterto unnoticed gamma-ray lines following nucleosynthesis, it is found that for mass numbers less than 60, the isotopes Na22, Al26, K40, Ar42, Ti44, Sc46, Mn54, Co56, Co57, Co58, Co60 and Fe60 are the only ones with sufficiently long half lives (70) days to produce gamma rays in optically thin regions

Lingenfelter, R. E.

Ramaty, R.

English

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https://ntrs.nasa.gov/search.jsp?R=19770010037 2020-03-22T10:26:27+00:00Z
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DECEMBER 1976
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26AC-A GALACTIC SOURCE OF
GAMMA RAY LINE EMISSION
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R. RAMATY
R. E. LINGENFELTER
CODDARD SPACE FLIaHT CENTER --
GREENBELT, MARYLAND
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26Al - A GALACTIC SOURCE OF GAMMA RAY-LINE EMISSION 	 6
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R. Ramaty
NASA/Goddard Space Flight Center
	 i
Laboratory for High Energy Astrophysics
Greenbelt, Mai;yland 20771
and
R. E. Lingenfelter*
Department of Astronomy and
Department of Geophysics & Space Physics
University of California
Los Angeles, California 90024
i
*This research was supported in part by National
Science Foundation Grant AST 76-08178.
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ABSTRACT
S
I
A detectable gamma-ray line at 1.809 MeV results from
the decay of 26 Alin the interstellar medium if this isotope
is synthesized in supernovae with abundance ti10 -3
 relative
to 26 Mg. The expected intensity from the direction of the
galactic center is 'L10-4photons cm 2 sec- lrad-1 , and the line
width is t3 keV. This intensity is comparable to the inten-
sities of the other strongest gamma-ray lines resulting from
processes of nucleosynthesis at 0.847 MeV from 56 Fe, 1.156
MeV from 44	 60 and 1.173 and 1.332 MeV from 60Ni. But the
width of the line from 26 Aldecay is an order of magnitude
smaller than that of either the 0.847 or the 1.156 MeV lines
and hence this line should be much more easily observable
with high resolution detectors.
Subject headings--gamma rays - nucleosynthesis.'
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The purpose of the present Letter is to point out that
observable gamma-ray line emission is produced from the beta r
decay of 26 Alin the interstellar medium.	 26 Aldecays with "$
a half1ife of 7.4x10 5 yrs by positron emission or electron )f
capture into excited states of 26 Mgwith ensuing line emission
at 1.809 MeV (1008)	 and 1.130 Mev (48).	 The isomeric state
of 26A1 with a ha7E life 6.4 seconds, however, beta de-
cays directly to
	 .3e ground state of 26 Mgand hence
does not produce prompt gamma rays.	 But excitation of 26A1
4
to its isomeric state is extremely unlikely in the interstel-
lar medium.
	
For details of the nuclear data see Endt and
van der Leun (1973) and Nuclear Data Group (1973).
26A1 is believed to be produced by explosive carbon
burning in supernovae (Arnett 1969) with a yield relative
to 26 Mgof about 10-3 (Schramm 1971 and private communicaticn
1976).	 The importance of the decay process 26A1-F26Mg as a
source of line emission can be assessed by comparing it with
the other decay chains resulting from nucleosynthesis, which
Clayton (1973) suggests should produce the most intense gamma
ray lines;
	 56Ni_r56Co^56Fe, 44 Ti-}44 5c-44
 Ca and 60Fe^60Cc+60Ni.
These chains, with their relevant half lives, yields per
supernova, photon energies, and photons per disintegration
are listed in Table 1.	 The yields of 56 Ni, 44 Ti and 60 Fe
per supernova are consistent with the solar system abundances
(Cameron 1973) of the final decay products of these isotopes
3(56 Fe, 44 Caand 60 Ni), and with the assumption that 1% of
the 60 Nioriginates from 60 Fe.	 (This value is the minimum
contribution of 60 Feto 60 Niestimated by Clayton 1973.) )
The half lives of the radioactive isotopes are of crucial
importance for the detectability of the lines.	 Because nucleo-
synthesis takes place in dense material, gamma ray lines can
be observed only if significant expansion occurs before the
decay of the nuclei, so that the overlying matter is optically
thin to the emitted photons. 	 For gamma rays from the long-
lived isotopes 26 Al, 44 Ti and 60 Fe, the emitting region is
1
definitely transparent. 	 For the lines of 56Co one expects
1
that the medium could just become transparent in 77 days pro-
vided that in the supernova explosion, bone solar mass is
ejected with a velocity of about 10 4 km/sec.	 But for gamma
rays from the short-lived isotopes 56 Ni(6.1 days)	 and 48V(16
days), which were previously suggested as potential line sour-
ces, the emitting region is opaque, and hence lines from these
isotopes are not included in Table 1.
The half life of the isotope also effectively determines
the width of the lines.	 For times of the order of the half
lives of 44 Ti and 56 Ccwe expect expansion velocities of about
6000 km/sec	 (Chevalier 1977), and hence line widths 	 (FWHM) of
about 40 keV at an MeV.	 on the other hand, for the lines of
26 Aland 60 Fe, expansion velocities after 10 5 yrs should be
only tens of km/sec so that galactic rotation velocities are
D
Fr
f
4
more appropriate giving a FWHM <3 keV.
We proceed now to estimate the fluxes of the lines listed
in Table 1 from various astrophysical sites. Because of its
short lifetime it is unlikely that the decay chain 56	 56 Fe
can be observed from a galactic supernova. The Virgo cluster,
however, with 2500 galaxies (Allen 1973) and an average super-
nova rate of 1 every 50 years per galaxy (Tammann 1974) presents
an essentially continuous source of 56 Ccdecay lines. For a
distance of 19 Mpc the flux of the 0.847 MeV line is about 10-4f
photons cm 2 sec- 1 , where f is , the fraction of the line photons
which escape from the supernova. Clayton et al. (1969) have
estimated that f could be close to unity, but clearly the value
of this parameter depends strongly on the total ejected mass,
on the radial dependence of the composition, and on the velocity
of the ejecta. The ejection velocity could be determined from
the width of the line; for 6000 km/sec it would be about 35 keV.
Because of its longer half life, the decay chain 44 Ti* 44sc-}44Ca
could possibly be observed from a galactic supernova remnant.
For-a hypothetical remnant at the galactic center with an age
equal to the 44 Ti mean life, and for Cassiopeia A with an age
of 300 years, and distance of 2,8 kpc (van den Bergh and Dodd
1970), the 1.156 MeV line fluxes are about 10 -4 and 4 x 10-5
photons cm 2 sec- 1 , respectively. For an expansion velocity
of 6000 km/sec, the width of this line is about 45 keV.
While the short lived isotopes discussed above lead to
v
5essentially point sources of gamma-ray lines, the much longer
lived 26 Aland 60 Fe, are likely to produce diffuse emission
from the interstellar medium. With a supernova rate of (30
years) -1 (Gunn and Ostriker 1970; Seiradakis 1976) in the
galactic volume of 4 x 10 66 em3 , the emissivities of the 1.809
MeV line from 26 Aldecay and of the 60 redecay lines given in
Table 1 are both about 10 -25photons em-3 sec-l . This value is
comparable to the local emissivity of photons of ene gies greater
than 100 MeV due to cosmic ray interactions with the interstel-
lar gas of average density 1 H atom cm -3 (e.g. Stecker 1976).
If the sources of nucleosynthesis have a similar spatial dis-
tribution to that of high energy gamma-rays, then the observed
flux of >100 MeV gamma-rays of ti10 -4 photons cm 2 sec-lrad 1
(Thompson et al. 1976) from the disk in the general direction
of the galactic center would imply a comparable flux for both
the 1.809 MeV line from 26A1 decay and from each of the 60 Fe
decay lines. This flux might be somewhat smaller if unresolved
pulsars contribute si gnificantly to the observed >100 MeV gamma
ray flux (Higdon and Lingenfelter 1976). In any event, a flux
of 'b10 4 photons cm- 2 sec- lrad-1 in the 1.309 MeV line is compar-
able to the most intense fluxes expected from 56Cc and 94 Ti
decay lines. Furthermore because of their much narrower widths
(FWHM <3 keV), the 1.809 MeV line and the lines at 1.173 and 1.332
MeV should be much easier to observe With high resolution detectors.
26 Alcould also be produced by spallation processes (Schramm
'
	 _
	 11 ,.
i
6
1971). But, if such processes were the principal source of
26 Al, other lines such as the 1.779 MeV line from 28 Siand the
1.369 MeV line from 24 Mgshould be at least an order of magni-
tude more intense (Lingenfelter and Ramat_ 1976; Ramaty et al.
1977), unless the spallation occurs only on time scales much
less than 106yrs.
In summary, we have shown that 26 Alis a very good candi-
date for producing a detectable gamma-ray line, and that this
line is not only intense but also very narrow. By examining
the chart of nuclides for other radioactive isotopes which could
produce hitherto unnoticed gamma-ray lines following nucleo-
synthesis, we find that for mass numbers less than 60, the iso-
topes 22Na, 26A1 40K 42Ar, 44 Ti 46Sc 54Mn, 56Co 57Co, 58Co,
60 Ccand 60 Feare the only ones with sufficiently long half
lives (>70)days to produce gamma rays in optically thin re-
gions. The line at 1.275 MeV from 22 Na(2.6 y) has already
been discussed by Clayton and Hoyle (1974) and by Clayton (1975),
while the lines from 26 Al, 44 Ti, 56 Co, 60 Cc and 60 Fewere dis-
cussed in the present Letter and the references therein.
From the abundances of the daughter products of 42 A (33 y),
46 Sc(84 d), 54 Mn(313 d), 57 Co(271 d) and 58 Cc(71 d), we
see that gamma-ray lines from these isotopes should have in-
tensities significantly less than those of 56 Coand 44 Ti decay
lines, The solar system abundance of 40K is sufficiently low
to preclude its decay line at 1.460 MeV from becoming a ser-
c
r,
7ious candidate for detection. Thus the lines from decay of
26 Al, 44 Ti, 56 Ccand 60 Feare the most intense lines resultingy	
from processes of nucleosynthesis and hence have the best pro-
spects of detection. Among these, the line from decay of
26A1, because of its very narrow width, is a very promising
candidate.
Acknowledgements. We wish to acknowledge valuable discus-
sions with C. E. Fichtel, B. Kozlovsky, D. N. Schramm and
B. J. Teegarden.
REFERENCES
Allen, C. W., 1973, Astrophysical Quantities, Athlone Press,
London, 3rd ed.
Arnett, W. D., 1969, Ap. J.' , 157, 1369.	 t
Bergh, S. van den, and Dodd, W. W., 1970, Ap, J,, 162, 485.
Cameron, A. G. W., 1973, Space Sci. Rev., 15, 121.
Chevalier, R. A., 1977, Ann. Rev. Astron. and A'strophys., 15
in press.
Clayton, D. D., 1973, Gamma Ray Astrophysics, ed. F. W, Stecker
and J. I. Trombka, NASA SP 339, p. 263.
Clayton, D. D., Colgate, S., and Fishman, G. J., 1969, Ap. J.,
155, 75.
Clayton, D. D., 1975, Ap. J., 198, 151.
,
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ii
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t
Clayton, D. D., and Hoyle, F., 1974, Ap. J. Letters, 187, L101.
Endt, P. M. and van der Leun, C.., 1973, Nuc. Phys., A214, 1. 	 J'
Gunn, J. E., and Ostriker, J. P., 1970, Ap. J., 160, 979.
Higdon, J. C., and Lingenfelter, R. E., 1976, Ap. J., 208, L107.
Lingenfelter, R. E., and Ramaty, R., 1976; The Structure and-
Content
-
of the Galaxy and Galactic Cosmic Rays, ed. C. E.
Fichtel and F. W. Stecker, NASA X-662-76-154, p. 264.
Nuclear Data Group, 1973, Nuclear Level Schemes A = 45 through
A = 257 from Nuclear Data Tables, Academic Press, New York.
Ramaty, R., Kozlovsky, B., and Lingenfelter, R. E., 1977, in
preparation.
Schramm, D. N., 1971, Astrophys. Space Sci., 13, 249.
Seiradakis, J. H., 1976, The Structure and Content of the Galaxy
and Galactic Cosmic Rays, ed., C. E. Fichtel and F. W.
Stecker, NASA X-662-76-154, p. 299.
Stecker, F. W., 1976, The Structure and Content of the Galaxy
and Galactic Cosmic Rays, ed. C. E. Fichtel and F. W.
Stecker, NASA X-662-76-154, p. 357.
Tammann, G. A., 1974, Supernovae and Supernova Remnants, ed.
C. B. Cosmovici, Reidel, Dordrecht, p. 155.
Thompson, D. J., Fichtel, C. E., Hartman, R. C,, Kniffen, D, A „
Bignami, G. F., Lamb, R. C. Ogelman, H., Ozel, M. E., and
Tumer, T., 1976, The Structure and Content of fhe Galaxy
and Galactic Cosmic Rays, ed. C. E. Fichtel and F. 14. Stecker,
NASA X-662-76-154, p. 1.
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TABLE 1
Decay Chain Half Life Nuclei/Supernova
Photon Energy
(MeV)	
_
Photons/Dis-
integration
S6Ni-*56Co-	 Fe 77.3 d 3 x 10 59 0.847 1
].238 0.70
2.598 0.17
1.771 0.16
1.038 0.13
94Ti_F44Sc+99Ca 47 y 6 v. :0 51 0.068 1
0.078 1
1.156 1
60Fe+60Co+60Ni 3 x 10 5 y 5 x 10 50 0.059 1
1.173 1
1.1332 1
26A1-,26Mg 7.4 x 10 5 y 4 x 10 50 1.809 1
1.130 0.04
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Al-26: A galactic source of gamma ray line emission

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