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Register a new userUltra high energy cosmic rays from extragalactic astrophysical sources: energy losses and spectra
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The energy losses and spectra of Ultra High Energy Cosmic Rays (UHECR) are calculated for protons as primary particles. The attention is given to the energy losses due to electron-positron production in collisions with the microwave 2.73 K photons. The energy spectra are calculated for several models, which differ by production spectra and by source distribution, namely: (i) Uniform distribution of the sources with steep generation spectra with indices 2.4 - 2.7, with cosmological evolution and without it. In this case it is possible to fit the shape of the observed spectrum up to 8.10^{19} eV, but the required CR emissivity is too high and the GZK cutoff is present. (ii) Uniform distribution of the sources with flat generation spectrum dE/E^2 which is relevant to GRBs. The calculated spectrum is in disagreement with the observed one. The agreement at Elesssim 8.10^{19} eV can be reached using a complex generation spectrum, but the required CR emissivity is three orders of magnitude higher than that of GRBs, and the predicted spectrum has the GZK cutoff. (iii) The case of local enhancement within region of size 10 - 30 Mpc with overdensity given by factor 3- 30. The overdensity larger than 10 is needed to eliminate the GZK cutoff.
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More than 100 years after the discovery of cosmic rays and various experimental efforts, the origin of ultra-high energy cosmic rays (E > 100 PeV) remains unclear. The understanding of production and propagation effects of these highest energetic particles in the universe is one of the most intense research fields of high-energy astrophysics. With the advent of advanced simulation engines developed during the last couple of years, and the increase of experimental data, we are now in a unique position to model source and propagation parameters in an unprecedented precision and compare it to measured data from large scale observatories. In this paper we revisit the most important propagation effects of cosmic rays through photon backgrounds and magnetic fields and introduce recent developments of propagation codes. Finally, by comparing the results to experimental data, possible implications on astrophysical parameters are given.
We present a strong hint of a connection between high energy $gamma$-ray emitting blazars, very high energy neutrinos, and ultra high energy cosmic rays. We first identify potential hadronic sources by filtering $gamma$-ray emitters %from existing catalogs that are in spatial coincidence with the high energy neutrinos detected by IceCube. The neutrino filtered $gamma$-ray emitters are then correlated with the ultra high energy cosmic rays from the Pierre Auger Observatory and the Telescope Array by scanning in $gamma$-ray flux ($F_{gamma}$) and angular separation ($theta$) between sources and cosmic rays. A maximal excess of 80 cosmic rays (42.5 expected) is found at $thetaleq10^{circ}$ from the neutrino filtered $gamma$-ray emitters selected from the second hard {it Fermi}-LAT catalogue (2FHL) and for $F_gammaleft(>50:mathrm{GeV}right)geq1.8times10^{-11}:mathrm{ph},mathrm{cm}^{-2},mathrm{s}^{-1}$. The probability for this to happen is $2.4 times 10^{-5}$, which translates to $sim 2.4 times 10^{-3}$ after compensation for all the considered trials. No excess of cosmic rays is instead observed for the complement sample of $gamma$-ray emitters (i.e. not in spatial connection with IceCube neutrinos). A likelihood ratio test comparing the connection between the neutrino filtered and the complement source samples with the cosmic rays favours a connection between neutrino filtered emitters and cosmic rays with a probability of $sim1.8times10^{-3}$ ($2.9sigma)$ after compensation for all the considered trials. The neutrino filtered $gamma$-ray sources that make up the cosmic rays excess are blazars of the high synchrotron peak type. More statistics is needed to further investigate these sources as candidate cosmic ray and neutrino emitters.
The origin of the ultra high energy cosmic rays (UHECR) with energies above E > 1017eV, is still unknown. The discovery of their sources will reveal the engines of the most energetic astrophysical accelerators in the universe. This is a written version of a series of lectures devoted to UHECR at the 2013 CERN-Latin-American School of High-Energy Physics. We present an introduction to acceleration mechanisms of charged particles to the highest energies in astrophysical objects, their propagation from the sources to Earth, and the experimental techniques for their detection. We also discuss some of the relevant observational results from Telescope Array and Pierre Auger Observatory. These experiments deal with particle interactions at energies orders of magnitude higher than achieved in terrestrial accelerators.
We explore the joint implications of ultrahigh energy cosmic ray (UHECR) source environments -- constrained by the spectrum and composition of UHECRs -- and the observed high energy astrophysical neutrino spectrum. Acceleration mechanisms producing power-law CR spectra $propto E^{-2}$ are compatible with UHECR data, if CRs at high rigidities are in the quasi-ballistic diffusion regime as they escape their source environment. Both gas- and photon-dominated source environments are able to account for UHECR observations, however photon-dominated sources do so with a higher degree of accuracy. However, gas-dominated sources are in tension with current neutrino constraints. Accurate measurement of the neutrino flux at $sim 10$ PeV will provide crucial information on the viability of gas-dominated sources, as well as whether diffusive shock acceleration is consistent with UHECR observations. We also show that UHECR sources are able to give a good fit to the high energy portion of the astrophysical neutrino spectrum, above $sim$ PeV. This common origin of UHECRs and high energy astrophysical neutrinos is natural if air shower data is interpreted with the textsc{Sibyll2.3c} hadronic interaction model, which gives the best-fit to UHECRs and astrophysical neutrinos in the same part of parameter space, but not for EPOS-LHC.
The origin of ultra high energy cosmic rays promises to lead us to a deeper understanding of the structure of matter. This is possible through the study of particle collisions at center-of-mass energies in interactions far larger than anything possible with the Large Hadron Collider, albeit at the substantial cost of no control over the sources and interaction sites. For the extreme energies we have to identify and understand the sources first, before trying to use them as physics laboratories. Here we describe the current stage of this exploration. The most promising contenders as sources are radio galaxies and gamma ray bursts. The sky distribution of observed events yields a hint favoring radio galaxies. Key in this quest are the intergalactic and galactic magnetic fields, whose strength and structure are not yet fully understood. Current data and statistics do not yet allow a final judgment. We outline how we may progress in the near future.
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