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Origin and Propagation of Extremely High Energy Cosmic Rays

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Origin and Propagation of Extremely High Energy Cosmic Rays To appear in Physics Reports E-Print astro-ph/9811011 Origin and Propagation of Extremely High Energy Cosmic Rays Pijushpani Bhattacharjee1 Indian Institute of Astrophysics, Bangalore-560 034, India. G¨unter Sigl2 Astronomy & Astrophysics Center, Enrico Fermi Institute, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA Abstract Cosmic ray particles with energies in excess of 1020 eV have been detected. The sources as well as the physical mechanism(s) responsible for endowing cosmic ray particles with such enormous energies are unknown. This report gives a review of the physics and astrophysics associated with the questions of origin and propagation of these Extremely High Energy (EHE) cosmic rays in the Universe. After a brief review of the observed cosmic rays in general and their possible sources and acceleration mechanisms, a detailed discussion is given of possible “top-down” (non-acceleration) scenarios of origin of EHE cosmic rays through decay of sufficiently massive particles originating from processes in the early Universe. The massive particles can come from collapse and/or annihilation of cosmic topological defects (such as monopoles, cosmic strings, etc.) associated with Grand Unified Theories or they could be some long- lived metastable supermassive relic particles that were created in the early Universe and are decaying in the current epoch. The highest energy end of the cosmic ray spectrum can thus be used as a probe of new fundamental physics beyond Standard Model. We discuss the role of existing and proposed cosmic ray, gamma-ray and neutrino experiments in this context. We also discuss how observations with next generation experiments of images and spectra of EHE cosmic ray sources can be used to obtain new information on Galactic and extragalactic magnetic fields and possibly their origin. arXiv:astro-ph/9811011v2 23 Aug 1999 1e-mail: [email protected] 2e-mail: [email protected] Contents 1 Introduction and Scope of This Review 3 2 The Observed Cosmic Rays 5 2.1 Detection Methods at Different Energies . ............... 5 2.2 TheMeasuredEnergySpectrum . .......... 7 2.3 Events above 1020 eV........................................ 7 2.4 Composition ..................................... ....... 8 2.5 Anisotropy ...................................... ....... 9 2.6 Next-generation Experiments on Ultrahigh Energy Cosmic Ray, γ Ray, and Neutrino As- − trophysics ......................................... ..... 9 3 Origin of Bulk of the Cosmic Rays: General Considerations 10 3.1 Energetics...................................... ........ 11 3.2 Galactic versus Extragalactic Origin of the Bulk of the CR .................. 12 3.3 Acceleration Mechanisms and Possible Sources . ................. 13 4 Propagation and Interactions of Ultra-High Energy Radiation 16 4.1 Nucleons, Nuclei, and the Greisen-Zatsepin-Kuzmin Cutoff .................. 17 4.2 UHE Photons and Electromagnetic Cascades . .............. 18 4.3 Propagation and Interactions of Neutrinos and “Exotic” Particles . 21 4.3.1 Neutrinos ..................................... ..... 21 4.3.2 SupersymmetricParticles . .......... 26 4.3.3 OtherParticles ................................ ....... 27 4.4 Signatures of Galactic and Extragalactic Magnetic Fields in UHECR Spectra and Images . 27 4.4.1 Synchrotron Radiation and Electromagnetic Cascades ................. 27 4.4.2 Deflection and Delay of Charged Hadrons . ........... 28 4.5 Constraints on EHECR Source Locations . ............. 29 4.6 SourceSearchforEHECREvents. ........... 30 4.7 Detailed Calculations of Ultra-High Energy Cosmic Ray Propagation . 31 4.7.1 Average Fluxes and Transport Equations in One Dimension.............. 31 4.7.2 Angle-Time-Energy Images of Ultra-High-Energy CosmicRaySources . 33 4.8 Anomalous Kinematics, Quantum Gravity Effects, Lorentz symmetry violations . 38 5 Origin of UHECR: Acceleration Mechanisms and Sources 40 5.1 Maximum Achievable Energy within Diffusive Shock Acceleration Mechanism . 40 5.2 SourceCandidatesforUHECR . .......... 42 5.2.1 AGNs and Radio-Galaxies . ........ 42 5.2.2 Pulsars ....................................... 44 5.2.3 OtherCandidateSources . ........ 45 5.3 A Possible Link Between Gamma-Ray Bursts and Sources of E > 1020 eV Events? . 45 6 Non-acceleration Origin of Cosmic Rays above 1020 eV 47 6.1 TheBasicIdea .................................... ....... 48 6.2 From X Particles to Observable Particles: Hadron spectra in Quark Hadron Fragmentation 49 → 6.2.1 Local Parton-Hadron Duality . .......... 50 6.2.2 Nucleon, Photon and Neutrino Injection Spectra . ............... 54 1 6.2.3 X Particle Production/Decay Rate Required to Explain the Observed EHECR Flux: ABenchmarkCalculation . 55 6.3 Cosmic Topological Defects as Sources of X Particles: General Considerations . 57 6.4 X particle production from Cosmic Strings . ................ 59 6.4.1 EvolutionofCosmicStrings. .......... 60 6.4.2 Intercommuting of Long Strings . .......... 63 6.4.3 FinalStageofLoopShrinkage . ......... 63 6.4.4 CuspEvaporation ............................... ...... 63 6.4.5 Collapse or Repeated Self-intersections of Closed Loops ................ 65 6.4.6 Direct Emission of X Particles from Cosmic Strings . ................ 67 6.5 X Particles from Superconducting Cosmic Strings . .................. 68 6.6 X Particles from Decaying Vortons . ............. 71 6.7 XparticlesfromMonopoles . ........... 72 6.8 X Particles from Cosmic Necklaces . ............. 74 6.9 A General Parametrization of Production Rate of X Particles from Topological Defects . 75 6.10 TDs, EHECR, and the Baryon Asymmetry of the Universe . ............... 76 6.11 TeV-Scale Higgs X Particles from Topological Defects in Supersymmetric Theories . 77 6.12 TDs Themselves as EHECR Particles . ............ 77 6.12.1 Monopoles as EHECR Particles . .......... 77 6.12.2 Vortons as EHECR Particles . ......... 78 6.13 EHECR from Decays of Metastable Superheavy Relic Particles ................ 79 6.13.1 GeneralConsiderations . .......... 79 6.13.2 Anisotropy ................................... ...... 81 6.14 Cosmic Rays from Evaporation of Primordial Black Holes ................... 82 7 Constraints on the Topological Defect Scenario 83 7.1 Low-Energy Diffuse γ ray Background: Role of Extragalactic Magnetic Field and Cosmic − InfraredBackground ................................. ....... 83 7.2 Constraints from Primordial Nucleosynthesis . .................. 88 7.3 Constraints from Distortions of the Cosmic Microwave Background.............. 88 7.4 ConstraintsonNeutrinoFluxes . ............. 89 8 Summary and Conclusions 92 Acknowledgments 94 References 94 Figures 117 2 1 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)3 is, therefore, currently a subject of much intense debate and discussions; see Refs. [4, 5, 9], and Ref. [10] for a recent brief review. The current theories of origin of EHECR can be broadly categorized into two distinct “scenarios”: the “bottom-up” acceleration scenario, and the “top-down” decay scenario, with various different models within each scenario. As the names suggest, the two scenarios are in a sense exact opposite of each other. In the bottom-up scenario, charged particles are accelerated from lower energies to the requisite high energies in certain special astrophysical environments. Examples are acceleration in shocks associated with supernova remnants, active galactic nuclei (AGNs), powerful radio galaxies, and so on, or acceleration in the strong electric fields generated by rotating neutron stars with high surface magnetic fields, for example. In the top-down scenario, on the other hand, the energetic particles arise simply from decay of certain sufficiently massive particles originating from physical processes in the early Universe, and no acceleration mechanism is needed. The problems encountered in trying to explain the EHECR in terms of acceleration mechanisms have been well-documented in a number of studies; see, e.g., Refs. [11, 12, 13, 14]. Even if it is possible, in principle, to accelerate particles to EHECR energies of order 100 EeV in some astrophysical sources, it is generally extremely difficult in most cases to get the particles come out of the dense regions in and/or around the sources without losing much energy. Currently, the most favorable sources in this regard are perhaps a class of powerful radio galaxies (see, e.g., Refs. [15, 16, 17, 18, 19, 20] for recent reviews and references to literature), although the values of the relevant parameters required for acceleration to energies > 100 EeV are somewhat on the side of extreme [13]. However, even if the requirements of energetics are met,∼ the main problem with radio galaxies as sources of EHECR is that most of them seem to lie at large cosmological distances, 100 Mpc, from Earth. This is a major problem if EHECR particles are ≫ conventional particles such as nucleons or heavy nuclei. The reason is that nucleons above 70 EeV ≃ lose energy drastically during their propagation from the source to Earth due to Greisen-Zatsepin-Kuzmin (GZK) effect [21, 22], namely, photo-production of pions when the nucleons collide with photons of the cosmic microwave background (CMB), the mean-free path for which is few Mpc [23].
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