1. Composition of Cosmic Rays at Ultra High Energies
Leonid Ksenofontov,
E.G.Berezhko, S.P.Knurenko,
Yu.G.Shafer Institute of Cosmophysical Research and Aeronomy, Yakutsk, Russia
Abstract ID: 154

2. Outline
We present measurements of the CR composition above 1015eV, performed with the Yakutsk extensive air shower array.
Almost events above 1017eV observed by Cherenkov detectors are selected for the analysis of the depth of maximum of the longitudinal development of air showers induced by CRs.
The interpretation of these results in terms of CR mass composition is given.

3. Cosmic Ray Flux CR sources: knee 1 knee 2 GZK cutoff Galactic SNRs
SNRs (Berezhko & Völk (2007)
knee 2
SNRs(?)
Reacceleration(?)
Extragalactic (?)
GZK cutoff

4. Energy spectrum of CRs Dip Dip scenario p + γ → p + e+ + e- GZK cutoff
p + γ → N + π
Berezhko & Völk (2007)
CR spectrum,
produced in SNRs
CR spectrum from extragalactic sources JEG~ε -2.7
(Berezinsky et al.2006)

5. Energy spectrum of CRs Ankle scenario CR from SNRs: SN Ia + SN IIb
CR spectrum JEG~ε -2
from extragalactic sources
(Berezinsky et al.2006)

6. Experiment
Yakutsk Extensive Air Shower Array is a ground based experiment for registration of Extensive Air Showers of secondary particles, produced by high energy Cosmic Ray particle in the Earth atmosphere
Located near Yakutsk, Russia (61.661oN, oE),100 m above the sea level.
Operates since 1973.
At present it consists of
60 ground-based and 6 underground scintillation detector stations to measure charged particles (electrons and muons) and 50 open photomultiplier tubes to observe the atmospheric Cherenkov light.
The total area covered by the detectors is 12 km2.

7. Experiment
The measurements of the light-integrating Cherenkov detectors can be analyzed separately from the measurements of the other detectors of the Yakutsk EASA.
These Cherenkov detectors constitute an array of photomultiplier tubes with light collection cones looking upwards into the night sky which measures the lateral distribution of the Cherenkov light at ground level.
The energy of the charged particle initiating the shower is determined in a model-independent way using the measurements of the Cherenkov detectors together with the data of the charged-particle detectors (Knurenko et al. 2006, Ivanov et al. 2007).
The depth of shower maximum Xmax is derived from observations of the lateral distribution of the Cherenkov light.
(Hörandel, 2006)
Compared with earlier considerations we present here the most complete set of events detected with the Cherenkov detectors of Yakutsk EASA.

8. Nonlinear kinetic (time-dependent) model of CR acceleration in SNRs in spherical symmetry
Kang & Jones 2006,
Ptuskin & Zirakashvili 2008
Gas dynamic equations
CR transport equation
Suprathermal particle injection
Gas heating due to wave dissipation
Time-dependent (amplified)
magnetic field
Applied to an individual SNR theory gives time evolution for t>0 : nuclear particle Np(p,r, t ), and electron Ne(p,r, t ) momentum and spatial
distributions, which in turn can be used for determination of the expected
nonthermal emissions Fγ(εγ)

9. Magnetic field amplification
ρISM
Beff
Results of modeling (Lucek & Bell, 2000)
& theoretical considerations (Bell 2004;
Pelletier et al. 2006; Zirakashvili et al. 2007)+
Spectral properties of SNR synchrotron
emission +
Fine structure of nonthermal X-ray emission VS
BISM L
shock
SNR magnetic field is considerably
amplified
Beff2/8π ≈ 10-2ρISMVS2 (Völk et al, 2005)
Beff >> BISM

10. SN Ia + SN IIb w = 10 km/s SN IIb : ESN = 3×1051 erg RSG wind: Data:
CAPRICE (Boezio et al 2003)
BESS (Haino et al. 2004)
ATIC-2 (Panov et al. 2006)
CREAM (Yoon et al. 2011)
JACEE (Asakimori et al. 1998)
KASCADE (Antoni et al. 2005)

11. Energy spectrum of CRs Ankle scenario CR spectrum JEG~ε -2
ATIC-2 (Panov et al. 2006)
JACEE (Asakimori et al. 1998)
KASCADE (Antoni et al. 2005)
Auger (Abraham et al. 2008)
HiRes (Bergman et al. 2007)
Yakutsk:
at e<3x1017eV (Ivanov et al 2009)
at e>3x1017eV (Egorovaet al 2004)
CR spectrum JEG~ε -2
from extragalactic sources
(Berezinsky et al.2006)
CR from SNRs:
SN Ia + SN IIb

12. Mean logarithm of CR atomic number
Galactic SNRs:
SN Ia + SN IIb
ATIC-2 (Panov et al. 2009)
JACEE,KASCADE (Horandel 2006)
Auger (Abraham et al. 2010)
HiRes (Abbashi et al. 2005, 2010)
Yakutsk: this work
Galactic SNRs (SN Ia) +
Extragalactic CRs J ~ ε-2.7

13. Summary
Composition of ultra high energy CRs measured by Yakutsk EASA is characterized by the peak value of <ln A>≈ 2.5 achieved at ε~1017eV and its substantial decrease within the energy interval eV to <ln A> ≈ 1 at ε~1018eV.
Such a behavior is consistent with the HiRes data that has to be considered as indication for the transition from galactic CR component, which is produced in galactic SNRs, to extragalactic component at ε~ eV.
Ankle scenario which predict this transition at ε~3x eV is clearly inconsistent with Yakutsk and HiRes data.
At higher energies ε~ eV Yakutsk data are also consistent with the dip scenario, which predicts relatively small variation of the mean CR atomic number near the value <ln A>=1.
However the existing data are quite contradictory at these energies: <ln A> ≈ 0 according to HiRes data and <ln A> ≈ 2 according to AUGER data. This very uncertain experimental situation does not allow to make any strict conclusion about CR composition at ε~1019eV.