Knee of spectrum

1
Knee of spectrum

• Differential spectral index changes at 3 x
  1015eV
• a 2.7 ? a 3.0
• Continues to 3 x 1018 eV
• Expect exp-E / Z Emax cutoff for each Z
• Fine-tuning problem
• to match smoothly a new source with a steeper
  spectrum (Axford)
• How serious is this?

2
Different systematics
Variation of cerenkov intensity
Number of penetrating muons per shower electron.
Similar results from fits
3
The knee is probably a deconfinement effect as
suggested by rigidity dependence measured by
KASCADE
4
Ultra-high-energy Cosmic rays
The maximum energy a source can produce depends
on the

• Size of the object
• The local magnetic field
• To a lesser extent on the photon density

5
Only a few objects can produce energies of 1020 eV
Of course heavy nuclei can be accelerated more
easily than protons.
6
Hot spots in active galaxies are candidates
7
AGN jets cosmic accelerators of UHE cosmic rays?
8
Transition to extragalactic origin?

• Ankle
• new population of particles?
• Suggestive evidence
• hardening of spectrum
• change of composition
• Measurements
• Energy
• Depth of maximum (Xmax)
• Nm / Ne

9
Xmax vs Energy

• Lines indicate trend of data
• Light to heavy above the knee (1016 ? 1017 eV)
• Heavy to light at the ankle (1018 ? 1019 eV)
• AGASA looks at m/e ratio in shower front and sees
  no evidence for change of composition at the ankle

10
HiRes collaboration, astro-ph/0208301
At the highest energies there is some
disagreement between the experiments
HIRES (fluorescence) gives lower flux at 1019 eV
than AGASA (ground array)
Could be due to energy calibration. Shape and
flux agree when we take Eagasa -gt 0.8 Eagasa
New experiment needed to be sure of flux at
Egt1020 eV
11
The structure of the spectrum and scenarios of
its origin
knee
ankle
toe ?
12
The Greisen-Zatsepin-Kuzmin (GZK) effect
It is important to know if spectrum stops at 1020
eV or not.
If it stops then source is extra-galactic
(candidates), if not it is local (no candidates,
except dark matter).
nucleon
?
13
A Tension between the Newest Fluorescence Data
(HiRes) and Ground Array Results? Or Is there a
Cut-Off after all?
14
But HiRes has also seen a gt200 EeV event in
stereo mode with only 20 exposure of the
mono-mode
15
Arrival Directions of Cosmic Rays above 4x1019 eV
Akeno 20 km2, 17/02/1990 31/07/2001, zenith
angle lt 45o
Red squares events above 1020 eV, green circles
events of (4 10)x1019 eV
Shaded circles clustering within 2.5o.
Chance probability of clustering from isotropic
distribution is lt 1.
16
Arrival Directions of Cosmic Rays above 1019 eV
Arrival directions are uniform, but cluster a
little for the highest energies
17
Next-Generation Ultra-High Energy Cosmic Ray
Experiments
compare to AGASA acceptance 230 km2sr
18
The structure of the spectrum and scenarios of
its origin
knee
ankle
toe ?
19
Conclusions?

• The new HIRES fluorescence measurements in stereo
  show a GZK cut-off. This is proof of
  extra-galactic source.
• AGASA does not show clear cut-off so source very
  near.
• AGASA has an indication of clustering, indicating
  extra-galactic source.
• Neither show enhancement near the galactic center
  for energies Egt1018.
• Energy recalibration gives an improvement, but
  there is no direct evidence to support this.

A little confusing but my bias is toward a cut-off