Abstract
Active galactic nuclei are extragalactic sources, and their relativistic hot-plasma jets are believed to be the main candidates of the cosmic-ray origin, above the so-called knee region of the cosmic-ray spectrum. Relativistic shocks, either single or multiple, have been observed or been theorized to be forming within relativistic jet channels in almost all active galactic nuclei sources. The acceleration of non-thermal particles (e.g. electrons, protons) via the shock Fermi acceleration mechanism, is believed to be mainly responsible for the power-law energy distribution of the observed cosmic-rays, which in very high energies can consequently radiate high energy gamma-rays and neutrinos, through related radiation channels. Here, we will focus on the primary particle (hadronic) shock acceleration mechanism, and we will present a comparative simulation study of the properties of single and multiple relativistic shocks, which occur in AGN jets. We will show that the role of relativistic (quasi-parallel either quasi-perpendicular) shocks, is quite important since it can dramatically alter the primary CR spectral indices and acceleration eciencies. These properties being carried onto gamma-ray and neutrino radiation characteristics, makes the combination of them a quite appealing theme for relativistic plasma and shock acceleration physics, as well as observational cosmic-ray, gamma-ray and neutrino astronomy.
Highlights
Cosmic Rays (CRs) are subatomic particles and radiation of extra-terrestrial origin
From the tables above, one notices that (i) the spectra by each shock in every shock-set become gradually flatter, following on one hand the findings of single relativistic shocks discussed in the previous section, and on the other hand the trends reported in e.g. Melrose and Pope (1993) and Gieseler and Jones (2000) for multiple shocks. (ii) The flatness of the spectra is more evident for the consecutive quasi-parallel shocks than for the quasi-perpendicular ones, following, as expected, the individual shock effects we discussed in the previous paragraph. (iii) The acceleration in the case of quasi-parallel shocks seems very efficient and comparable to single shock acceleration
What one can in generally conclude from the above brief discussion, is that the extragalactic TeV sources, which can not be observed as candidates of ultra-high energy CRs, may emit high energy neutrinos implying the hadronic nature of the cosmic rays
Summary
Cosmic Rays (CRs) are subatomic particles (e.g. protons) and radiation of extra-terrestrial origin Their energies range between a few eV to hundreds of PeV. CRs above energies of ∼ 1017eV, almost certainly originate from extragalactic sources such as Active Galactic Nuclei (AGN) and their relativistic jets. It is the particle shock acceleration in the superalfvenic jet plasmas, which is believed to be the main mechanism responsible for the production of the nonthermal CRs We will discuss the primary particle shock acceleration mechanism in AGN jets, in the context of comparative test-particle simulation studies, for relativistic single and multiple quasi-parallel and quasiperpendicular shocks. Near-perpendicular shocks strongly appear to give the steepest spectra compared to quasi-parallel shock cases, and are the less efficient accelerators, see e.g. Niemec and Ostrowski (2004), Stecker et al (2007), etc
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