The different models aiming to describe UHECR observations have different consequences when applied to the transition from galactic to extra-galactic CR, therefore this theme of investigation has obtained an increasing interest in recent years. In general the physics of galactic CR is well understood in the framework of the standard model, which is based on the paradigm of Super Nova Remnant (SNR) as sources of galactic CR and Diffusive Shock Acceleration (DSA) as the mechanism of particles acceleration. In the standard model of galactic CR the observation of the knee at energy 2 × 1015 eV in the spectrum fixes the maximum energy of protons at the source and therefore, in a rigidity dependent scenario, the maximum energy for iron nuclei, the end of galactic CR, will be around energies 1017 eV.
In figures 8, 9 we plot together the fluxes of galactic and extra-galactic CR assuming the two different models dip (figure 8) and disappointing (figure 9) for UHECR and using the flux of galactic CR as computed in [10], that takes into account the scale distribution of SNR in the galaxy. The experimental data of figures 8, 9 are those of HiRes [6] and Auger [7] on the UHECR side and, on the galactic side, the data of all particles spectrum of Kascade [11] and an average of the fluxes measured by different experiments as presented in [12].
The matching of galactic and extragalactic fluxes in the case of the dip model (figure 8) gives a very good description of the experimental data, reproducing in an extremely accurate way the spectra observations in the intermediate energetic regime where the transition is supposed to stand. The case of the disappointing model gives a less accurate description of the experimental data (figure 9) with a slightly suppression of the theoretical flux in the transition region not seen experimentally.
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Figure 8. Transition from galactic to
extra-galactic CR in the case of the dip model, with
Emax = 1021 eV and
|
 |
Figure 9. Transition from galactic to
extra-galactic CR in the case of the disappointing model assuming a
mixture of protons, He and Fe nuclei at the source with
Emaxp = 4 × 1018 eV and
|
The less good agreement with experimental data obtained, in the
transition region, with the disappointing model respect
to the dip model is a direct consequence of two important facts: (i) the
quite flat power law injection index
(
g = 2.0) of the disappointing model, (ii) the
quite low maximum energy associated to iron nuclei
(EmaxFe ≃ 1017 eV) in the
standard model of galactic CR. These two facts produce a lacking of
particles in the transition region not seen experimentally, to avoid
such circumstance one should assume a larger maximum energy for galactic
iron. Only in this way in-fact it is possible to fill the gap in the
transition region making the theoretical spectrum compatible with
observations.
Let us conclude this section emphasizing the importance of the study of the transition between galactic and extra-galactic CR. In general, the transition is the superposition of the low energy tail of UHECR with the high energy tail of galactic CR and it is supposed to stand in the energy range 0.1-10 EeV. As discussed above the informations obtained at these energies on galactic CR involve the maximum acceleration energy of particles and their chemical composition. Therefore, assuming the standard model of galactic CR, the study of the transition can unveil the whole picture of the origin of galactic CR. Moreover, the low energy tail of UHECR can give key informations on the (possible) existence of the pair-production dip and on the propagation of UHECR in intergalactic magnetic fields [15]. Therefore, the experimental studies in the transition region are of paramount importance in this field of research, with the mass composition measurements being probably the most important task [15].