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Recent evidence from the Pierre Auger Observatory suggests a transition, at 5 EeV-10EeV in the composition of Ultra High Energy Cosmic Rays (UHECRs), from protons to heavier nuclei such as iron. I consider here the implications of the…
The origin of ultra high energy cosmic rays promises to lead us to a deeper understanding of the structure of matter. This is possible through the study of particle collisions at center-of-mass energies in interactions far larger than…
The nature of ultrahigh-energy cosmic rays (UHECRs) at energies >10^20 eV remains a mystery. They are likely to be of extragalactic origin, but should be absorbed within ~50 Mpc through interactions with the cosmic microwave background. As…
For several decades, the origin of ultra-high-energy cosmic rays (UHECRs) has been an unsolved question of high-energy astrophysics. One approach for solving this puzzle is to correlate UHECRs with high-energy neutrinos, since neutrinos are…
The sources of ultra-high energy cosmic rays are not yet known. However, the discovery of anisotropic cosmic rays above 57x10^18 eV by the Pierre Auger Observatory suggests that a direct source detection may soon be possible. The…
The all-sky survey in high-energy gamma rays (E$>$30 MeV) carried out by the Energetic Gamma Ray Experiment Telescope (EGRET) aboard the Compton Gamma-Ray Observatory provides a unique opportunity to examine in detail the diffuse gamma-ray…
We have calculated the probability that the clustering of arrival directions of ultra-high energy cosmic rays (UHECRs) is consistent with a finite number of uniformly distributed proton sources. The case of a continuous source distribution…
The origin of highest energy cosmic rays is yet unknown. An appealing possibility is the so-called Z-burst scenario, in which a large fraction of these cosmic rays are decay products of Z bosons produced in the scattering of ultrahigh…
While several arguments can be proposed against the existence of particles with energy in excess of $(3-5)\times 10^{19}$ eV in the cosmic ray spectrum, these particles are actually observed and their origin seeks for an explanation. After…
The flux of Ultra High Energy Cosmic Rays (UHECRs) at $E>10^{18.5}$ eV is believed to arise in plasma shock environments in extragalactic sources. In this paper, we present a systematic study of particle acceleration by relativistic shocks,…
We consider various classes of persistent extragalactic astrophysical sources which have been suggested in literature as possible emitters of ultra-high-energy cosmic rays (UHECR). We compare the strength of the claimed correlations by a…
Ultrahigh energy cosmic rays (UHECRs) are believed to originate from astrophysical sources, which should trace the large scale structure (LSS) of the universe. On the other hand, the magnetic field in the intergalactic space (IGMF), which…
The IceCube experiment has recently reported the first observation of high-energy cosmic neutrinos. Their origin is still unknown. In this paper, we investigate the possibility that they originate in active galaxies. We show that hadronic…
Anisotropy in the arrival direction distribution of ultrahigh-energy cosmic rays (UHECRs) produced by powerful sources is numerically evaluated. We show that, taking account of the Galactic magnetic field, nondetection of significant…
Auger data show evidence for a correlation between ultrahigh-energy cosmic rays (UHECRs) and nearby starburst galaxies. This intriguing correlation is consistent with data collected by the Telescope Array, which have revealed a much more…
The quest for the origin(s) of ultra-high-energy cosmic rays (UHECRs) continues to be a far-reaching pillar of high energy astrophysics. The source scrutiny is mostly based on three observables: the energy spectrum, the nuclear composition,…
Ultra High Energy Cosmic Rays (UHECRs) hit the Earth's atmosphere with energies exceeding $10^{18}$ eV. This is the same energy as carried by a tennis ball moving at 100 km/h, but concentrated on a sub-atomic particle. UHECRs are so rare…
Relativistic jets of radio galaxies (RGs) are possible sources of ultra-high-energy cosmic rays (UHECRs). Recent studies combining relativistic hydrodynamic simulations with Monte Carlo particle transport have demonstrated that UHECRs can…
We investigate the dependence of composition, spectrum and angular distributions of ultra-high energy cosmic rays above 10^19 eV from individual sources on their magnetization. We find that, especially for sources within a few megaparsecs…
Active galactic nuclei (AGNs) have been one of the most widely discussed sources of ultrahigh-energy cosmic rays (UHECRs). The recent results of Pierre Auger observatory (PAO) have indicated a possible composition change of UHECRs above…