Subatomic Physics

600 Nuclear and Particle Astrophysics
spectrum is similar to Fig. 19.11 for energies above 1 GeV, but is somewhat steeper
above 100 GeV because of the electromagnetic interaction during propagation. The
electron spectrum thus provides a sensitive test of propagation models. (52)
Two more facts concerning the energy spectra are important for the discussion
of the origin of cosmic rays. One is the isotropy of cosmic rays; the other is their
constancy over a long period of time. Measurements in outer space indicate that the
cosmic-ray flux is essentially isotropic for energies � 10 15 eV. The time dependence
of the intensity over long periods has been studied by looking at the abundance
of nuclides created in moon samples and meteorites. The cosmic ray intensity has
been approximately constant over a period of about 10 9 y.
The experimental evidence discussed above implies that it is likely that there are
several sources of cosmic rays. (53) For energies below ≈ 10 15 eV the source should
have the following properties: (50) The total produced energy must be of the order
of 10 49 ergs/y in our galaxy; the cosmic rays must be isotropic and constant during
at least 10 9 y. The primary spectrum must include heavy elements up to about
Z = 100 but less than about 1% antihadrons. For cosmic rays of high energies the
sources are likely ultra-galactic.
Figure 19.12: Sketch of cross section through our galaxy.
Where do cosmic rays originate?
To sharpen this question,
it helps to draw a cross
section through our galaxy, as
in Fig. 19.12. Cosmic rays can
be produced in the inner radiation
disk, in the galactic
halo, or they can flow into the
galaxy from the outside. (54)
Most experts believe that the
cosmic rays below 10 18 eV
originate in our galaxy. (54,55)
A favored hypothesis is that supernovae and neutron stars produce cosmic rays
with the correct properties. (55) There is even some, not-universally accepted, evidence
of the production of cosmic-ray nuclei in the expanding shock wave of a
supernova explosion. (56) In our galaxy, a supernova appears about every 40 y and
one supernova is believed to produce between 10 51 and 10 52.5 erg of energy. A recent
bright extragalactic one observed in 1987, and called SN 1987a, has been studied
52 D. Casadei and V. Bindi,Astrophysical Journal 612, 262 (2004).
53 See however A. Dar, A. De Rujula, hep-ph/0606199.
54 T.K. Gaisser, T. Stanev, Nucl. Phys. A777, 98 (2006); V. Berezinsky, A. Gazizov, and S.
Grigorieva Phys. Rev. D 74, 043005 (2006).
55 H. Bloemen in Interstellar Processes, (D.J. Hollenbach and H.A. Thronson, Jr, eds.) Reidel,
Dordrecht, 1987, p. 143; P.K. MacKeown and T.C. Weeles, Sci. Amer. 252, 60 (November 1985).
56 R. Enomoto et al., Nature, 416, 823 (2002); Y.M. Butt et al., Nature 418, 499 (2002).