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Sources of high energy cosmic rays
Cosmic rays are charged particles that arrive to earth from outer space. They are composed of protons or light nuclei (e.g., iron nuclei), and have been observed up to enormous energies, of 10^20 eV and higher. For comparison, this energy is more than 3 orders of magnitude the maximum energy (in the center of mass) achieved in LHC.
The main question is the origin of these events. At such high energies, particles can interact with the cosmic microwave background (CMB) and lose their energy; therefore, the sources are confined to ~100 Mpc (the so called "Greisen-Zatsepin-Kuzmin [GZK] cutoff"). Furthermore, assuming electromagnetic (Fermi- type) acceleration, the requirement that the particles are confined to the source while being accelerated can be translated to a low limit on the source luminosity. Thus, only very few sources are luminous enough to enable acceleration of particles to such high energies.
Given the uncertainty in the composition of these objects, together with Murase & Meszaros, we proposed a new type of objects: radio quiet AGN's as a valid source. While being less luminous than radio loud AGNs, these are much more numerous, and show evidence for weak jets on pc scale, which could serve as natural acceleration sites.
Furthermore, we have investigated recent claims that Cen A, the most nearby AGN is the source of these cosmic rays. In collaboration with Loeb, we have derived a new way of constraining the ability of this object to accelerate cosmic rays, by using the broad-band spectra. We found that the core region is excluded, while the giant radio lobes are plausible acceleration sites.
Pe'er, A., Murase, K., & Meszaros, P. (2009), "Radio-quiet active galactic nuclei as possible sources of ultrahigh-energy cosmic rays" Phys. Rev. D., 8013018
Pe'er, A., Loeb, A. (2012), "Constraining sources of ultra high energy cosmic rays using high energy observations with the Fermi satellite" J.C.A.P., 03, 007