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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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Transition from Galactic to
Extragalactic Cosmic Rays
INFN – Laboratori Nazionali del Gran Sasso
Aspen Workshop on Cosmic Rays Physics
Aspen 15-19 April 2007
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
Proton composition at E>1018 eV
not disfavored by experimental
HiRes elongation rate
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=1019.730.07 eV in perfect agreement with the theoretically predicted value for protons E1/2=1019.72 (Berezinsky & Grigorieva 1988)
p p e+ e-
UHE Proton energy losses
log10[ latt (Mpc)]
p p 0
log10[ E (eV)]
A A e+ e-
A (A-1) N
UHE Nuclei energy losses
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
Continuum Energy Losses
Protons lose energy but do not disappear.
Fluctuations in the pγ interaction start to
be important only at E>51019 eV.
Berezinsky, Grigorieva, Gazizov (2006)
Uniform distribution of sources
the UHECR sources are continuously
distributed with a density ns.
the UHECR sources are discretely
distributed with a spacing d.
γ> 2 injection power law
Jp=Lp nS source emissivity
Injection spectrum number of particles injected
at the source per unit time and energy
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
Best fit values:
Jp = O(10erg s-1Mpc-3
Berezinsky et al. (2002-2005)
Calibrating the energy through the Dip gives an energy shift E→ λE(fixed by
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
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…)
heavy nuclei fraction at E>1018 eV
larger than 15% (primordial He has nHe/nH0.08)
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)
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)
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
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
E1018 eV 2nd Knee
RA & Berezinsky (2005)
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)
mix comp scenario
Allard, Parizot, Olinto (2005-2007)
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
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
2. Where is the transition Galactic-ExtraGalactic CRs?
Precise determination of the mass composition in the energy
range 1018 - 1019 eV.