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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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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]

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


HiRes elongation rate


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)


Adiabatic losses

Universe expansion

Pair production

p  p e+ e-

UHE Proton energy losses



Universe size

1000 Mpc

log10[ latt (Mpc)]

100 Mpc

Photopion production

p   p 0

 n 

log10[ E (eV)]


Pair production

A  A e+ e-


Universe size


A   (A-1) N

 (A-2) 2N

UHE Nuclei energy losses


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


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


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


Proton Dip

Best fit values:

γ= 2.7

Jp = O(10erg s-1Mpc-3

Berezinsky et al. (2002-2005)


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



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/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)


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)


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


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)


Galactic and ExtraGalactic I

dip scenario

 =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)


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)

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



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.