Cosmic Rays

In this talk I present resent results on high energy cosmic rays, including energy spectra, composition and the study of arrival directions of cosmic ray primaries. Some related results in gamma-ray astronomy and selected recent advances in theory are also covered. c ⃝ Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. The question of origin of high energy cosmic rays belongs to the list of one of the most interesting and important problems in particle astrophysics. Without exaggeration one can characterize resent years of research in this field as a quest for cosmic ray sources. In this talk I'll concentrate on this particular topic as well as on related problems of the cosmic ray spectrum and the mass composition, covering both the area of the Ultra-High Energy Cosmic Rays (UHECR) and recent related results in the gamma-ray astronomy. All related studies in this area of research are centered, one way or another, around Greisen, Zatsepin and Kuzmin (GZK) effect. And here is how and why. 2. Ultra-high energy cosmic rays 2.1 Propagation of the ultra-high energy cosmic rays 1. Greisen-Zatsepin-Kuzmin cutoff. Immediately after the discovery of the relict Cosmic Microwave Background Radiation (CMBR), Greisen, Zatsepin and Kuzmin [1] have realized that the highest energy protons should catastrophically loose energy in photo-production of pions on this universal background. This process limits the distance to farthest sources of observed rays to be roughly 100 Mpc and should lead to the cutoff in the energy spectrum. The findings of Greisen, Zatsepin and Kuzmin are based on solid fundamental physics which involves precisely measured cross-sections in a GeV energy range (in the center of mass reference frame) and on validity of Lorentz transformations. The question of whether the GZK-cutoff is present or not in the cosmic ray data is of fundamental importance since the absence of the GZK cutoff would be inevitable signal of the new physics. On the other hand, observational confirmation of the cutoff in the highest-energy part of the cosmic ray spectrum would signify that the UHECR propagation length at high energies becomes substantially shorter than the characteristic length at which the Universe becomes homogeneous. Since the matter distribution is non-uniform being averaged over the scales of a few hundred Mpc, one generally expects the UHECR flux to be anisotropic, showing both point sources and flux variations at large angular scales. That is why a …