How, where and when do cosmic rays reach ultrahigh energies?

Abstract

Understanding the origins of ultrahigh energy cosmic rays (UHECRs) -which reach energies in excess of 10 20 eV -stretches particle acceleration physics to its very limits. In this review, we discuss how such energies can be reached, using general arguments that can often be derived on the back of an envelope. We explore possible particle acceleration mechanisms, with special attention paid to shock acceleration. Informed by the arguments derived, we discuss where UHECRs might come from and which classes of powerful astrophysical objects could be UHECR sources; generally, we favour radio galaxies, GRB afterglows and other sources which are not too compact and dissipate prodigious amounts of energy on large scales, allowing them to generate large products 𝛽𝐵𝑅 without the CRs undergoing restrictive losses. Finally, we discuss when UHECRs are accelerated by highlighting the importance of source variability, and explore the intriguing possibility that the UHECR arrival directions are partly a result of "echoes" from magnetic structures in the local Universe.

Constraining astrophysical sources of intermediate-mass Ultra-high energy cosmic rays (UHECRs)

“…CRs with energy higher than 10 18 eV (1 Exa eV or EeV) are referred to as Ultra-High-Energy Cosmic Rays (UHECRs): In this study, we use UHECR to refer to those with E ≥ 50 EeV. The composition and sources of UHECRs, and the effect of galactic and extragalactic magnetic fields over them remain uncertain [2]. Evidence points to fully ionized nuclei that energetic astrophysical objects have accelerated to very high energies, but determining whether they are dominated by high, intermediate, or light masses remains a challenge.…”

Cosmic ray acceleration by multiple shocks in the jets of Active Galactic Nuclei

“…Besides that, starbursts do not satisfied the minimum energetic requirement to be sources of UHECRs [10,11]. Jets from powerful radiogalaxies, on the other hand, fulfil the energetic requirements, but justifying the presence of atomic nuclei inside them is not trivial [see e.g., [12][13][14][15][16][17][18]. Moreover, the mechanism and efficiency of relativistic jets to accelerate particles are continuously a matter of debate.…”