Origin and propagation of extremely high-energy cosmic rays
Section snippets
Introduction and scope of this review
The cosmic rays (CR) of extremely high-energy (EHE) – those with energy ≳1020 eV [1], [2], [3], [6], [7], [8] – pose a serious challenge for conventional theories of origin of CR based on acceleration of charged particles in powerful astrophysical objects. The question of origin of these extremely high-energy cosmic rays (EHECR)1
The observed cosmic rays
In this section we give a brief overview of CR observations in general. Since this is a very rich topic with a tradition of almost 90 years, only the most important facts can be summarized. For more details the reader is referred to recent monographs on CR [35], [36] and to rapporteur papers presented at the biennial International Cosmic Ray Conference (ICRC) (see, e.g., [37], [38], [39]) for updates on the data situation. The relatively young field of γ-ray astrophysics which has now become an
Origin of bulk of the cosmic rays: general considerations
The question of origin of cosmic rays continues to be regarded as an “unsolved problem” even after almost 90 years of research since the announcement of their discovery in 1912. Although the general aspects of the question of CR origin are regarded as fairly well-understood now, major gaps and uncertainties remain, the level of uncertainty being in general a function that increases with energy of the cosmic rays.
The total CR energy density measured above the atmosphere is dominated by particles
Propagation and interactions of ultra-high-energy radiation
Since implications and predictions of the spectrum of UHECR depend on their composition which is uncertain, we will in this chapter review the propagation of all types of particles that could play the role of UHECR. We start with the hadronic component, continue with discussion on electromagnetic cascades initiated by UHE photons in extragalactic space, and then comment on more exotic options such as UHE neutrinos and new neutral particles predicted in certain supersymmetric models of particle
Origin of UHECR: acceleration mechanisms and sources
As mentioned in Section 3.3, the first-order Fermi acceleration in the form of DSAM when applied to shocks in supernova remnants can accelerate particles to energies perhaps up to ∼1017 eV (see, e.g., Ref. [16]), but probably not much beyond. Thus, SNRs are unlikely to be the sources of the UHECR above ∼1017 eV. Instead, for UHECR, one has to invoke shocks on larger scales, namely extragalactic shocks. Several papers have, therefore, focussed on extragalactic objects such as AGNs and
The basic idea
As discussed in the Section 5, the shock acceleration mechanism is a self-limiting process: for any given set of values of dimension of the acceleration region (fixed by, say, the radius R of the shock) and the magnetic field strength (B), simple criterion of Larmor containment of a particle of charge Ze within the acceleration region implies that there is a maximum energy Emax∼ZeBR up to which the particle can be accelerated before it escapes from the acceleration region, thus preventing
Constraints on the topological defect scenario
Scenarios of UHECR production that are related to new physics near the Grand Unification scale exhibit a striking difference to conventional acceleration models: whereas acceleration is tied, in one form or another, to astrophysical objects and magnetized shocks associated with them and took place at redshifts not greater than a few, energy release associated with Grand Unification scale physics takes place not only today, but already in the early Universe up to temperatures corresponding to
Summary and conclusions
It is clear from the discussions in the previous sections that the problem of origin of EHECR continues to remain as a major unsolved problem.
The EHECR present a unique puzzle: recall that for lower-energy cosmic rays (below about ) there is a strong belief that these are produced in supernova remnants (SNRs) in the Galaxy. However, because of the twists and turns the trajectories of these particles suffer in propagating through the Galactic magnetic field, it is not possible to point
Acknowledgements
We are most grateful to the late David Schramm whose insights, encouragements and constant support had been crucial to us in our efforts in this exciting area of research over the past several years. Indeed it was he who first urged us to undertake the project of writing this Report. We also wish to thank Felix Aharonian, Peter Biermann, Paolo Coppi, Veniamin Berezinsky, Chris Hill, Karsten Jedamzik, Sangjin Lee, Martin Lemoine, Angela Olinto, (the late) Narayan Rana, Qaisar Shafi, Floyd
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