CR interactions with photons in space

1.6.1. CR nuclei interactions with space photons

When propagating in space, the CR nuclei interact with photons of various energies, first of all with the universal microwave radiation at a temperature of space), and the infrared and light quanta of radiation from stars. In this case the photon energy Eph in the center of mass system CR particle - background photon will be where Epho and E are the energies of photon and particle in the laboratory coordinate system, M is the rest mass of particle, P = v/c, and a is the angle between the directions of the photon and particle motion in laboratory coordinate system. These interactions are most significant for the high energy particles and give rise to the following two important effects. First, the interactions with the universal microwave radiation may be the main reason for constraining the primary CR spectrum at the high energy side; second, the interactions of high energy heavy nuclei with photon emission from the Sun will result in photodisintegration of nuclei (into a photo-nucleon and residual nucleus), in generation of correlated cascades when observed on the Earth (Gerasimova and Zatsepin, 1960). The energy loss in interactions with the microwave universal radiation at T ~ 2.7 °K becomes significant only at E > 1019 ■iG20 eV.

Puget et al. (1976) studied the following effects of interactions of CR nuclei of high energy with photons: 1. Compton interaction; 2. Pair production in a field of a nucleus (generally, electron-positron pairs); 3. Photo-splitting of nuclei; 4. Photoproduction of hadrons. In the system of the rest of a nuclei of CR the process 1 takes place at every energy Eph ; the process 2 occurs at the minimum energy

Eph = 2mc2 = 1 MeV; for the process 3 the resonance increase of a cross section takes place at the photon energy Eph from 15 to 25 MeV, and the process 4 occurs with the minimum energy Eph ~ 145 MeV. The data were presented for the effective cross sections for the listed processes in a relation to CR nuclei from deuterium to iron. The analysis and estimates of expected density of photons of about 2.7 °K (with very high density about 200 cm 3 in galactic and intergalactic

various energies Epho in the Galaxy and in intergalactic space were made. Based on these data the calculations of the efficiency of these processes depending on the energy of various CR nuclei and their importance in forming the energy spectrum and chemical composition of primary CR in the range of super-high energies were carried out. Puget et al.(1976) draw the conclusion that as no cut-off of the spectrum up to the energies (1 ■ 2)x1020 eV was revealed, this undoubtedly gave the evidence against the model of uniform filling of Metagalaxy by CR of high energy (independently of a nature of high-energy CR, whether they are nuclei or protons). The obtained results give evidences in favor of local origin of super high-energy CR (in the Galaxy or in a local group of galaxies). New result was obtained concerning protons: it was shown that in the case of meta-galactic origin of CR, a cutoff must take place not only for nuclei but as well for protons (at the energy ~ 5x1019 eV owed to photo-producing mesons). The discussed effects are of substantial importance in a propagation of super-high energy CR, in forming their spectrum and chemical composition.

1.6.2. CR electron interactions with the photon field

In contrast to the loss of nucleus energy the electron energy loss (in eV/sec) in interactions with photon emission may be very significant (Ginzburg and Syrovatsky, M1963):

2(E/mc2 ) if E << mc2 (mc2/Epho ) {mc2/Epho j^n^EEpho/m2c4) if E >> mc2(mc2/Epho)

where Wph = VphEpho is the mean density of the photon emission energy. The effects of the interaction of electrons with the photon field must be taken into account both in the processes of propagation of electrons of CR and in the problem of their origin. In particular, including of these effects and bremsstrahlung losses (see above, Section 1.8.2) results in the conclusion that high energy electrons, observed near the Earth, certainly cannot be of extragalactic origin and cannot come from such remote distances as the region of the center of the Galaxy.

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