Indications of Proton-Dominated Cosmic-Ray Composition above 1.6 EeV

Department of Physics, University of Utah, Salt Lake City, Utah, USA.
Physical Review Letters (Impact Factor: 7.51). 04/2010; 104(16):161101. DOI: 10.1103/PhysRevLett.104.161101
Source: arXiv


We report studies of ultrahigh-energy cosmic-ray composition via analysis of depth of air shower maximum (X(max)), for air shower events collected by the High-Resolution Fly's Eye (HiRes) observatory. The HiRes data are consistent with a constant elongation rate d<X(max)>/d[log(E)] of 47.9+/-6.0(stat)+/-3.2(syst) g/cm2/decade for energies between 1.6 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 4x10(18) eV.

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    • "In this model the ankle is a signature of pair production of UHE protons when interacting with the background photon fields. This later interpretation requires a predominantly protonic component up to the highest energies and is in tension with data from Auger [2], but not with HiRes [3] and TA [4]. "
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    ABSTRACT: Ultra-high energy cosmic rays can propagate diffusively in cosmic magnetic fields. When their propagation time is comparable to the age of the universe, a suppression in the flux relative to the case in the absence of magnetic fields will occur. In this work we find an approximate parametrization for this suppression at energies below $\sim$ Z EeV using several magnetic field distributions obtained from cosmological simulations of the magnetized cosmic web. We assume that the magnetic fields have a Kolmogorov power spectrum with the field strengths distributed according to these simulations. We show that, if magnetic fields are coupled to the matter distribution, low field strengths will fill most of the volume, making the suppression milder compared to the case of a constant magnetic field with strength equal to the mean value of this distribution. We also derive upper limits for this suppression to occur for some models of extragalactic magnetic fields, as a function of the coherence length of these fields.
    Full-text · Article · Jul 2014 · Journal of Cosmology and Astroparticle Physics
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    • "No UHE photons have been observed so far and upper limits to their flux have been placed [6] [7], partly disfavoring exotic models. Current experimental results on the chemical composition of UHE cosmic rays [8] [9] [10] [11] [12] suggest a mixed composition from light to heavier nuclei at the highest energies. As will be discussed in the next sections, the expected flux of photons depends, among other parameters, also on the chemical composition of CR at the source. "
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    ABSTRACT: Ultra-high energy (UHE) photons play an important role as an independent probe of the photo-pion production mechanism by UHE cosmic rays. Their observation, or non-observation, may constrain astrophysical scenarios for the origin of UHECRs and help to understand the nature of the flux suppression observed by several experiments at energies above $10^{19.5}$ eV. Whereas the interaction length of UHE photons above $10^{17}$ eV ranges from a few hundred kpc up to tenths of Mpc, photons can interact with the extragalactic background radiation initiating the development of electromagnetic cascades which affect the fluxes of photons observed at Earth. The interpretation of the current experimental results rely on the simulations of the UHE photon propagation. In this paper, we present the novel Monte Carlo code EleCa to simulate the $Ele$ctromagnetic $Ca$scading initiated by high-energy photons and electrons. We provide an estimation of the surviving probability for photons inducing electromagnetic cascades as a function of their distance from the observer and we calculate the distances within which we expect to observe UHE photons with energy between $10^{17}$ and $10^{19}$ eV. Furthermore, the flux of GZK photons at Earth is investigated in several astrophysical scenarios where we vary both injection spectrum and composition at the source and the intensity of the intervening extragalactic magnetic field. Although the photon propagation depends on several astrophysical factors, our numerical predictions combined with future experimental observations (or non-observations) of UHE photons -- in the energy range between $10^{17.5}$ eV and $10^{20}$ eV -- can help to constrain these scenarios.
    Preview · Article · Nov 2013 · Astroparticle Physics
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    • "The top, middle, and bottom panels correspond to the Augeruniform , Auger, and TA cases respectively, as in Fig 1. Pulsar and propagation parameters: wind acceleration coefficient η = 0.3, Galactic magnetic field coherence length l c = 20 pc, magnetic halo height H = 2 kpc. – 20 – Figure 5. Average logarithmic mass of cosmic ray derived from X max measurements from [75] with data from Tunka [76], Yakutsk [77] [78], CASA-BLANCA [79], HiRes/MIA [80], HiRes [81], KASCADE-Grande [72], Auger [15] and TA [73] for hadronic interaction model EPOS v1.99 [57] compare with simulation predictions (red lines) as in Fig. 4. Dashed lines indicate the energy range where pulsars contribute less than 80% to the total flux (see Fig. 4 "
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    ABSTRACT: The acceleration of ultrahigh energy nuclei in fast spinning newborn pulsars can explain the observed spectrum of ultrahigh energy cosmic rays and the trend towards heavier nuclei for energies above $10^{19}\,$eV as reported by the Auger Observatory. Pulsar acceleration implies a hard injection spectrum ($\sim E^{-1}$) due to pulsar spin down and a maximum energy $E_{\rm max} \sim Z \, 10^{19}$ eV due to the limit on the spin rate of neutron stars. We have previously shown that the escape through the young supernova remnant softens the spectrum, decreases slightly the maximum energy, and generates secondary nuclei. Here we show that the distribution of pulsar birth periods and the effect of propagation in the interstellar and intergalactic media modifies the combined spectrum of all pulsars. By assuming a normal distribution of pulsar birth periods centered at 300 ms, we show that the contribution of extragalactic pulsar births to the ultrahigh energy cosmic ray spectrum naturally gives rise to a contribution to very high energy cosmic rays (VHECRs, between $10^{16}$ and $10^{18}$ eV) by Galactic pulsar births. The required injected composition to fit the observed spectrum depends on the absolute energy scale, which is uncertain, differing between Auger Observatory and Telescope Array. The contribution of Galactic pulsar births can also bridge the gap between predictions for cosmic ray acceleration in supernova remnants and the observed spectrum just below the ankle, depending on the composition of the cosmic rays that escape the supernova remnant and the diffusion behavior of VHECRs in the Galaxy.
    Preview · Article · Feb 2013 · Journal of Cosmology and Astroparticle Physics
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