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Signatures of Protons in UHECR Transition from Galactic to Extragalactic Cosmic Rays

Signatures of Protons in UHECR Transition from Galactic to Extragalactic Cosmic Rays. Roberto Aloisio. INFN – Laboratori Nazionali del Gran Sasso. Aspen Workshop on Cosmic Rays Physics. Aspen 15-19 April 2007. Chemical Composition. Fly's Eye [Dawson et al. 98].

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Signatures of Protons in UHECR Transition from Galactic to Extragalactic Cosmic Rays

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  1. Signatures of Protons in UHECR Transition from Galactic to Extragalactic Cosmic Rays Roberto Aloisio INFN – Laboratori Nazionali del Gran Sasso Aspen Workshop on Cosmic Rays Physics Aspen 15-19 April 2007

  2. Chemical Composition Fly's Eye[Dawson et al. 98] Transition from heavy (at 1017.5 eV) to light composition (at ~1019 eV) Haverah Park[Ave et al. 2001] No more than 54% can be Iron above 1019 eV No more than 50% can be photons above 4 1019 eV Similar limits from AGASA No conclusive observations at energies E>1018 eV Hires, HiresMIA, Yakutsk proton compositionFly’s Eye, Haverah Park, Akeno mixed composition Proton composition at E>1018 eV not disfavored by experimental observations HiRes elongation rate

  3. The End of the CR Spectrum? HiRes collaboration (2007) Strong evidences of an astrophysical proton dominated flux at the highest energies The last HiReS analysis confirms the expected Greisen Zatzepin Kuzmin suppression in the flux with E1/2=1019.730.07 eV in perfect agreement with the theoretically predicted value for protons E1/2=1019.72 (Berezinsky & Grigorieva 1988)

  4. Adiabatic losses Universe expansion Pair production p  p e+ e- UHE Proton energy losses protons CMB Universe size 1000 Mpc log10[ latt (Mpc)] 100 Mpc Photopion production p   p 0  n  log10[ E (eV)]

  5. Pair production A  A e+ e- Iron Universe size Photodisintegration A   (A-1) N  (A-2) 2N UHE Nuclei energy losses helium Universe size Pair production energy losses produce an early onset in the photo-disintegration flux depletion Depletion of the flux Iron E  1020 eV Helium E 1019 eV

  6. Protons propagation in Intergalactic Space Continuum Energy Losses Protons lose energy but do not disappear. Fluctuations in the pγ interaction start to be important only at E>51019 eV. Berezinsky, Grigorieva, Gazizov (2006) Uniform distribution of sources the UHECR sources are continuously distributed with a density ns. Discrete sources the UHECR sources are discretely distributed with a spacing d. model parameters γ> 2 injection power law Jp=Lp nS source emissivity Injection spectrum number of particles injected at the source per unit time and energy

  7. Modification Factor Jpunm(E) only redshift energy losses Jp(E) total energy losses DIP (p + CMB  p + e+ + e- ) GZK cut-off (p + CMB  N + ) Tiny dependence on the injection spectrum

  8. Proton Dip Best fit values: γ= 2.7 Jp = O(10erg s-1Mpc-3 Berezinsky et al. (2002-2005)

  9. Energy calibration by the Dip Calibrating the energy through the Dip gives an energy shift E→ λE(fixed by minimum χ2) λAGASA = 0.90 λHiRes = 1.21 λAuger = 1.26 NOTE: λ<1 for on-ground detectors and λ>1 for fluorescence light detectors (Auger energy calibration by the FD) Different experiments show different systematic in energy determination

  10. Robustness Protons in the Dip come from large distances, up to 103 Mpc. The Dip does not depend on: inhomogeneity, discreteness of sources source cosmological evolution maximum energy at the source intergalactic magnetic fields(see later…)

  11. Caveats heavy nuclei fraction at E>1018 eV larger than 15% (primordial He has nHe/nH0.08) Berezinsky et al. (2004) Allard et al. (2005) RA et al. (2006) the injection spectrum has < 2.4 The interpretation of the observed Spectrum in terms ofprotons pair-production losses FAILS if: RA, Berezinsky, Grigorieva (2007)

  12. Diffusive shock acceleration tipically shows  2.1  2.3 Maximum energy distribution The maximum acceleration energy is fixed by the geometry of the source and its magnetic field If the sources are distributed over Emax: (β ≈ 1.5) the overall UHECR generation rate has a steepening at some energy Ec (minimal Emax O(1018 eV)) E < Ec E > Ec Kachelriess and Semikoz (2005) RA, Berezinsky, Blasi, Grigorieva, Gazizov (2006)

  13. The IMF effect on the UHE proton spectrum Magnetic Horizon – Low Energy Steepening The diffusive flux presents a steeping due to proton energy losses and at lower energies an exponential suppression due to the magnetic horizon. The beginning of the steepening is independent of the IMF, it depends only on the proton energy losses and coincides with the observed 2nd Knee. The low energy cut-off is due to a suppression in the maximal contributing distance its position depends on the IMF. The low energy behavior (E<1018 eV) depends on the diffusive regime. B0=1 nG, lc=1 Mpc =2.7 no IMF The DIP survives also with IMF Combination of the UHECR low energy tail with the HE tail of galactic CR (transition Galactic-ExtraGalactic see later) Steepening in the flux at E1018 eV 2nd Knee RA & Berezinsky (2005) Lemoine (2005)

  14. Galactic and ExtraGalactic I dip scenario  =2.7 =2.7 without IMF with IMF The Galactic CR spectrum ends in the energy range 1017 eV, 1018 eV. 2nd Knee appears naturally in the extragalactic proton spectrum as the steepening energy corresponding to the transition from adiabatic energy losses to pair production energy losses. This energy is universal for all propagation modes (rectilinear or diffusive): E2K  1018 eV. RA & Berezinsky (2005)

  15. Galactic and ExtraGalactic II mix comp scenario Allard, Parizot, Olinto (2005-2007) • The transition is placed at Etr 31018 eV • The composition is dominated by galactic nuclei at E<Etr , • by extra-galactic nuclei at E>Etr and by extra-galactic protons • at the highest energies • High (Emax>1018 eV) maximum energy of galactic CR • Difficult to detect (nuclei before and after the transition)

  16. Galactic and ExtraGalactic III ankle scenario Traditionally (since 70s) the transition Galactic-ExtraGalactic CR was placed at the ankle ( 1019 eV). In this context ExtraGalactic protons start to dominate the spectrum only at the ankle energy with a more conservative injection spectrum 2.1  2.3. Problems in the Galactic component Galactic acceleration: Maximum acceleration energy required is very high Emax 1019 eV Composition: How the gap between Iron knee EFe 1017eV and the ankle (1019 eV) is filled

  17. Conclusions Galactic CR (nuclei) at E ≥ 1018 eV (ankle and mixed composition scenario) challenge for the acceleration of CR in the Galaxy (high Emax) ExtraGalactic CR (protons) at E ≥ 1018 eV (dip scenario) discovery of proton interaction with CMB confirmation of conservative models for Galactic CR challenge for the acceleration of UHECR (steep injection γ> 2.4) 1. Observation of the dip Spectrum in the range 1018 - 1019 eV could represent a signature of the proton interaction with CMB (as the GZK feature). 2. Where is the transition Galactic-ExtraGalactic CRs? Precise determination of the mass composition in the energy range 1018 - 1019 eV.

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