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Register a new userCascade photons as test of protons in UHECR
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An isotropic component of high energy $gamma$-ray spectrum measured by Fermi LAT constrains the proton component of UHECR. The strongest restriction comes from the highest, $(580-820)$ GeV, energy bin. One more constraint on the proton component is provided by the IceCube upper bound on ultrahigh energy cosmogenic neutrino flux. We study the influence of these restrictions on the source properties, such as evolution and distribution of sources, their energy spectrum and admixture of nuclei. We also study the sensitivity of restrictions to various Fermi LAT galactic foreground models (model B being less restrictive), to the choice of extragalactic background light model and to overall normalization of the energy spectrum. We claim that the $gamma$-ray-cascade constraints are stronger than the neutrino ones, and that however many proton models are viable. The basic parameters of such models are relatively large $gamma_g$ and not very large $z_{max}$. The allowance for H$e^4$ admixture also relaxes the restrictions. However we foresee that future CTA measurements of $gamma$-ray spectrum at $E_gamma simeq (600 - 800)$ GeV, as well as resolving of more individual $gamma$-ray sources, may rule out the proton-dominated cosmic ray models.
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We demonstrate that the energy spectra of Ultra High Energy Cosmic Rays (UHECR) as observed by AGASA, Flys Eye, HiRes and Yakutsk detectors, have the imprints of UHE proton interaction with the CMB radiation as the dip centered at $Esim 1times 10^{19}$ eV, beginning of the GZK cutoff, and very good agreement with calculated spectrum shape. This conclusion about proton composition agrees with recent HiRes data on elongation rate that support the proton composition at $Egeq 1times 10^{18}$ eV. The visible bump in the spectrum at $E sim 4times 10^{19}$ eV is not caused by pile-up protons, but is an artifact of multiplying the spectrum by $E^3$. We argue that these data, combined with small-angle clustering and correlation with AGN (BL Lacs), point to the AGN model of UHECR origin at energies $E leq 1times 10^{20}$ eV. The events at higher energies and the excess of the events at $E geq 1times 10^{20}$ eV, which is observed by AGASA (but absent in the HiRes data) must be explained by another component of UHECR, e.g. by UHECR from superheavy dark matter.
The signatures of Ultra High Energy (E >1 EeV) proton propagation through CMB radiation are pair-production dip and GZK cutoff. The visible characteristics of these two spectral features are ankle, which is intrinsic part of the dip, beginning of GZK cutoff in the differential spectrum and E_{1/2} in integral spectrum. Measured by HiRes and Telescope Array (TA) these characteristics agree with theoretical predictions. However, directly measured mass composition remains a puzzle. While HiRes and TA detectors observe the proton dominated mass composition, the data of Auger detector strongly evidence for nuclei mass composition becoming progressively heavier at energy higher than 4 EeV and reaching Iron at energy about 35 EeV. The models based on the Auger and HiRes/TA data are considered independently and classified using the transition from galactic to extragalactic cosmic rays. The ankle cannot provide this transition. since data of all three detector at energy (1 - 3) EeV agree with pure proton composition (or at least not heavier than Helium). If produced in Galaxy these particles result in too high anisotropy. This argument excludes or strongly disfavours all ankle models with ankle energy E_a > 3 EeV. The calculation of elongation curves, X_{max}(E), for different ankle models strengthens further this conclusion. Status of other models, the dip, mixed composition and Auger based models are discussed.
In the light of the recently predicted isotopic composition of the kpc-scale jet in Centaurus A, we re-investigate whether this source could be responsible for some of the ultra-high energy cosmic rays detected by the Pierre Auger Observatory. We find that a nearby source like Centaurus A is well motivated by the composition and spectral shape, and that such sources should start to dominate the flux above ~ 4 EeV. The best-fitting isotopes from our modelling, with the maximum 56Fe energy fixed at 250 EeV, are of intermediate mass, 12C to 16O, while the best-fitting particle index is 2.3.
Photon Astronomy ruled the last four centuries while wider photon band ruled last radio-X-Gamma century of discovery. Present decade may see the rise and competition of UHECR and UHE Neutrino Astronomy. Tau Neutrino may win and be the first flavor revealed. It could soon rise at horizons in AUGER at EeV energies, if nucleons are the main UHECR currier. If on the contrary UHECR are Lightest nuclei (He, Li. B) UHE tau neutrino maybe suppressed at EeV and enhanced at tens -hundred PeV. Detectable in AMIGA and HEAT denser sub-array in AUGER. Within a few years.
UHECR may be either nucleons or nuclei; in the latter case the Lightest Nuclei, as He, Li, Be, explains at best the absence of Virgo signals and the crowding of events around Cen-A bent by galactic magnetic fields. This model fit the observed nuclear mass composition discovered in AUGER. However UHECR nucleons above GZK produce EeV neutrinos while Heavy Nuclei, as Fe UHECR do not produce much. UHECR He nuclei at few tens EeV suffer nuclear fragmentation (producing low energetic neutrino at tens PeVs) but it suffer anyway photo-pion GZK suppression (leading to EeV neutrinos) once above one-few 10^{20} eV. Both these cosmogenic UHE secondary neutrinos signals may influence usual predicted GZK Tau Neutrino Astronomy in significant and detectable way; the role of resonant antineutrino electron-electron leading to Tau air-shower may also rise.
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