Dip in UHECR and Transition from Galactic to Extragalactic Cosmic Rays

Abstract

The dip is a feature in the diffuse spectrum of UHE protons in energy range $1\times 10^{18} - 4\times 10^{19}$ eV, which is caused by electron-positron pair production on CMB photons. Calculated for power-law generation spectrum with index $\gamma_g=2.7$, the shape of the dip is confirmed with high accuracy by data of Akeno - AGASA, HiRes, Yakutsk and Fly's Eye detectors. The predicted shape of the dip is robust: it is valid for the rectilinear and diffusive propagation, for different discretenesses in the source distribution, for local source overdensity and deficit, for source inhomogeneities on scale $\ell \lsim 100$ Mpc etc. Below the characteristic energy $E_c \approx 1\times 10^{18}$ eV the spectrum of the dip flattens for both diffusive and rectilinear propagation, and more steep galactic spectrum becomes dominant at $E < E_c$. The energy of transition $E_{\rm tr} < E_c$ approximately coincides with the position of the second knee $E_{2kn}$ observed in the cosmic ray spectrum. The critical energy $E_c$ is determined by the energy $E_{\rm eq} = 2.3\times 10^{18}$ eV, where adiabatic and pair-production energy losses are equal. Thus, position of the second knee is explained by proton energy losses on CMB photons.