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Register a new userIndications of Proton-Dominated Cosmic Ray Composition above 1.6 EeV
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We report studies of ultra-high energy cosmic ray composition via analysis of depth of airshower maximum (Xmax), for airshower events collected by the High Resolution Flys Eye (HiRes) observatory. The HiRes data are consistent with a constant elongation rate d<Xmax>/d(log(E)) of 47.9 +- 6.0 (stat.) +- 3.2 (syst.)g/cm^2/decade for energies between 1.6 EeV and 63 EeV, and are consistent with a predominantly protonic composition of cosmic rays when interpreted via the QGSJET01 and QGSJET-II high-energy hadronic interaction models. These measurements constrain models in which the galactic-to-extragalactic transition is the cause of the energy spectrum ankle at 4 EeV.
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The lateral distribution of muons with ~1.0xsec(theta) GeV in extensive air showers within ~10^{17}-10^{19} eV energy region obtained during different observational periods from November 1987 to June 2013 has been analyzed. Experimental data have been compared to predictions of various hadron interaction models. The best agreement is observed with QGSJETII-04. Until 1996, the mass composition of cosmic rays with energy below 2x10^{18} eV was significantly lighter than in later periods.
We present a detailed study of the large-scale anisotropies of cosmic rays with energies above 4 EeV measured using the Pierre Auger Observatory. For the energy bins [4,8] EeV and $Egeq 8$ EeV, the most significant signal is a dipolar modulation in right ascension at energies above 8 EeV, as previously reported. In this paper we further scrutinize the highest-energy bin by splitting it into three energy ranges. We find that the amplitude of the dipole increases with energy above 4 EeV. The growth can be fitted with a power law with index $beta=0.79pm 0.19$. The directions of the dipoles are consistent with an extragalactic origin of these anisotropies at all the energies considered. Additionally we have estimated the quadrupolar components of the anisotropy: they are not statistically significant. We discuss the results in the context of the predictions from different models for the distribution of ultrahigh-energy sources and cosmic magnetic fields.
We report on a measurement of the cosmic ray composition by the Telescope Array Low-Energy Extension (TALE) air fluorescence detector (FD). By making use of the Cherenkov light signal in addition to air fluorescence light from cosmic ray (CR) induced extensive air showers, the TALE FD can measure the properties of the cosmic rays with energies as low as $sim 2$ PeV and exceeding 1 EeV. In this paper, we present results on the measurement of $X_{rm max}$ distributions of showers observed over this energy range. Data collected over a period of $sim 4$ years was analyzed for this study. The resulting $X_{rm max}$ distributions are compared to the Monte Carlo (MC) simulated data distributions for primary cosmic rays with varying composition and a 4-component fit is performed. The comparison and fit are performed for energy bins, of width 0.1 or 0.2 in $log_{10} (E/{rm eV})$, spanning the full range of the measured energies. We also examine the mean $X_{rm max}$ value as a function of energy for cosmic rays with energies greater than $10^{15.8}$ eV. Below $10^{17.3}$ eV, the slope of the mean $X_{rm max}$ as a function of energy (the elongation rate) for the data is significantly smaller than that of all elements in the models, indicating that the composition is becoming heavier with energy in this energy range. This is consistent with a rigidity-dependent cutoff of events from galactic sources. Finally, an increase in the $X_{rm max}$ elongation rate is observed at energies just above $10^{17}$ eV indicating another change in the cosmic rays composition.
Blazars are potential candidates of cosmic-ray acceleration up to ultrahigh energies ($Egtrsim10^{18}$ eV). For an efficient cosmic-ray injection from blazars, $pgamma$ collisions with the extragalactic background light (EBL) and cosmic microwave background (CMB) can produce neutrino spectrum peaks near PeV and EeV energies, respectively. We analyze the contribution of these neutrinos to the diffuse background measured by the IceCube neutrino observatory. The fraction of neutrino luminosity originating from individual redshift ranges is calculated using the distribution of BL Lacs and FSRQs provided in the textit{Fermi}-LAT 4LAC catalog. Furthermore, we use a luminosity dependent density evolution to find the neutrino flux from unresolved blazars. The results obtained in our model indicate that as much as $approx10%$ of the flux upper bound at a few PeV energies can arise from cosmic-ray interactions on EBL. The same interactions will also produce secondary electrons and photons, initiating electromagnetic cascades. The resultant photon spectrum is limited by the isotropic diffuse $gamma$-ray flux measured between 100 MeV and 820 GeV. The latter, together with the observed cosmic-ray flux at $E>10^{16.5}$ eV, can constrain the baryonic loading factor depending on the maximum cosmic-ray acceleration energy.
All information about primary cosmic rays above the knee has been obtained from results of EAS investigations. At that, two alternative approaches exist: cosmophysical and nuclear physical. In the frame of the first one, all changes in measured EAS characteristics are explained by the changes in energy spectrum and mass composition of primary cosmic rays. In this paper, the second approach is considered, in frame of which corresponding changes in EAS parameters are explained by changes of interaction model above the knee. Some experimental possibilities of proof of the correctness of the nuclear physical approach are considered.
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