Accretion and merger shocks in clusters of galaxies are potential accelerators of high-energy protons, which can give rise to high-energy neutrinos through pp interactions with the intracluster gas. We discuss the possibility that protons from cluste
r shocks make a significant contribution to the observed cosmic rays in the energy range between the second knee around 10^17.5 eV and the ankle around 10^18.5 eV. The accompanying cumulative neutrino background above PeV may be detectable by upcoming neutrino telescopes such as IceCube or KM3NeT, providing a test of this scenario as well as a probe of cosmic-ray confinement properties in clusters.
More than a dozen blazars are known to be emitters of multi-TeV gamma rays, often with strong and rapid flaring activity. By interacting with photons of the cosmic microwave and infrared backgrounds, these gamma rays inevitably produce electron-posit
ron pairs, which in turn radiate secondary inverse Compton gamma rays in the GeV-TeV range with a characteristic time delay that depends on the properties of the intergalactic magnetic field (IGMF). For sufficiently weak IGMF, such pair echo emission may be detectable by the Gamma-ray Large Area Space Telescope (GLAST), providing valuable information on the IGMF. We perform detailed calculations of the time-dependent spectra of pair echos from flaring TeV blazars such as Mrk 501 and PKS 2155-304, taking proper account of the echo geometry and other crucial effects. In some cases, the presence of a weak but non-zero IGMF may enhance the detectability of echos. We discuss the quantitative constraints that can be imposed on the IGMF from GLAST observations, including the case of non-detections.
