Astrophysical origins of ultrahigh energy cosmic rays

Abstract

In the first part of this review we discuss the basic observational features at the end of the cosmic ray energy spectrum. We also present there the main characteristics of each of the experiments involved in the detection of these particles. We then briefly discuss the status of the chemical composition and the distribution of arrival directions of cosmic rays. After that, we examine the energy losses during propagation, introducing the Greisen-Zaptsepin-Kuzmin (GZK) cutoff, and discuss the level of confidence with which each experiment have detected particles beyond the GZK energy limit. In the second part of the review, we discuss astrophysical environments able to accelerate particles up to such high energies, including active galactic nuclei, large scale galactic wind termination shocks, relativistic jets and hot-spots of Fanaroff-Riley radiogalaxies, pulsars, magnetars, quasar remnants, starbursts, colliding galaxies, and gamma ray burst fireballs. In the third part of the review we provide a brief summary of scenarios which try to explain the super-GZK events with the help of new physics beyond the standard model. In the last section, we give an overview on neutrino telescopes and existing limits on the energy spectrum and discuss some of the prospects for a new (multi-particle) astronomy. Finally, we outline how extraterrestrial neutrino fluxes can be used to probe new physics beyond the electroweak scale. Solicited Review Article Prepared for Reports on Progress in Physics 1 PREFACEReviewing cosmic ray physics is a risky business. Theoretical models continue to appear at an amazing rate, both in the astrophysical and more exotic domains. We warn the reader: we do not (nor we could) intend to make an homogeneous coverage of all the ideas of our fellow colleagues. Reading differently focused reviews on the issue (quoted here along the way) is, in our view, the best approach to such a wide topic of research.2 Contents

Evidence against Macroscopic Astrophysical Dyadospheres

“…For stellar-mass cores, this is 2Y3 orders of magnitude smaller than the energy of gamma-ray bursts Torres & Anchordoqui 2004), essentially because the efficiency is 3Y 4 orders of magnitude less than unity. Again, I should emphasize that all of these estimates and calculations give only very conservative upper limits on the efficiency and energy emitted, since they all assume that somehow one can hold the charge onto the surface of the collapsing core even when it is as large as the transition radius R t , which for 0 ¼ Q 0 /M > 10 À4 would be significantly greater than the size of a neutron star.…”

Cosmic ray composition and energy spectrum from 1–30 PeV using the 40-string configuration of IceTop and IceCube

“…However, a small sample of simulated proton and iron showers with snow attenuation was used to study this effect and to develop a method of accounting for it in event reconstruction. For each tank, the signal expectation is lowered by a factor which depends upon the measured snow depth above each tank, as well as the snow density, which was measured to be about 0.4 g/cm 3 . Thus, a correction has been made to the data at this reconstruction stage so that it corresponds to the standard simulation described above.…”

“…Currently, the most popular model predicts cosmic ray acceleration in shock fronts via the first order Fermi mechanism [1]. More specifically, at energies up to ∼ 10 17 eV, the source of this acceleration mechanism is often attributed to supernova remnants; a cut-off energy which depends upon nuclear charge (Z) of the particle accelerated at the source could be responsible for a mass-dependent knee; the ankle is then attributed to cosmic rays from extragalactic sources such as gamma-ray bursts or active galactic nuclei [2,3,4].…”

Ultrahigh energy nuclei propagation in a structured, magnetized universe

“…These models are based on particle acceleration at shocks in powerful extragalactic astrophysical objects, ranging from compact objects such as γ−ray bursts (GRBs) to large-scale radio lobes of active galactic nuclei (AGNs), see for example Ref. [12]. They have the obvious advantage to be exclusively based on known physics and astrophysical objects.…”