Cosmic Rays above the Knee Region

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Cosmic Rays above the Knee Region 3rd School on
Cosmic Rays and Astrophysics Paul Sommers Penn
State University
Lecture 1 Science issues and open questions.
Observational evidence.
Lecture 2 Air shower physics. Measurement
techniques.
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The Cosmic Ray Energy Spectrum Non-thermal,
approximate power law, up to about 3x1020
eV (possibly higher)
1 EeV 1018 eV 6 EeV 1 Joule
Simon Swordy
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Primary Questions Where do they originate? How
do they acquire macroscopic energy (Joules)?
Accelerated? Electromagnetic force? Fermi shock
model? B-field strength? Size of
accelerating region? Plasma speeds?
Escape time? Loss time (synchrotron radiation,
collisions)? Top-down production by decay or
annihilation of massive particles? Role of
dark matter or dark energy?
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Hillas Plot
Maximum energy Z(v/c)BL (chargespeedB_fiel
dsize) EEeV lt Z(v/c)BµGLkpc
Z(v/c)BnGLMpc Note Containment in a region
requires the size of the region to be greater
than the Larmor radius. This is the same as the
above inequality if vc. The energy upper limit
is not much stronger than requiring that B will
confine particles to a region of size L.
There are few types of astrophysical objects that
are candidates for accelerating particles to 100
EeV.
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Primary Types of Observations

• Anisotropy
• Large scale patterns (dipole, quadrupole,
  galactic plane,)
• Small scale (clustering of arrival directions,
  discrete sources)
• Energy Spectrum
• Power Law? Spectral index?
• Features (deviations from power law knee,
  second knee, ankle,
• suppression or cutoff)
• Particle types
• Nuclear mass distribution
• Photons
• Neutrinos

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Knee
Cosmic ray spectrum multiplied by E2.5
Ankle
Toes?
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The GZK effect (Greisen-Zatsepin-Kuzmin)
Cronin
To a proton above about 60 EeV, the CMB photons
appear to be a beam of gamma rays energetic
enough to produce a pion by collision. Protons
cannot travel more than 100 Mpc without dropping
below that energy threshold.
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Eight years ago An apparent GZK paradox
AGASA
The AGASA evidence against a GZK effect on the
spectrum. The Utah Flys Eye measured one event
at 300 EeV. Early HiRes analysis (1999) indicated
a continuing power law.
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The spectrum does steepen at the GZK threshold
energy. The AGN correlation confirms that
it is a propagation effect It is not due to
the sources running out of steam at that energy.
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Full Auger
Auger South
TA
HiRes
Full Auger
Auger South
The Quest for Exposure
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• A concave downward transition requires at least a
  double coincidence
• One spectrum ends at the same energy where the
  other starts.
• Both have the same flux at that energy.

A transition between power laws is necessarily a
concave upward feature.
( Ian Axford )
No evident transition to extragalactic population
before the ankle
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• Traditional view
• Low energy cosmic rays are known to be of
  galactic origin.
• The highest energy cosmic rays almost surely
  originate outside the Galaxy, because they do not
  exhibit galactic anisotropy (and it is hard to
  imagine how they accelerate to such high energies
  in our Galaxy).
• The ankle is the only concave upward feature in
  the spectrum. It must therefore be the
  transition energy between galactic and
  extragalactic cosmic rays.
• A contemporary alternative view (after
  Berezinsky)
• The dip of the ankle has exactly the shape
  expected for energy loss by protons due to ee-
  production by collisions with microwave photons
  (MeV gamma rays in the protons frame).
• The highest energy cosmic rays must be
  extragalactic protons, and the transition from
  galactic cosmic rays occurs at a lower energy
  (below the start of the dip, sometimes called the
  second knee).

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Berezinsky

• The shape of the spectrum from each experiment is
  the same as expected due to ee- losses. By
  small shifts in normalization, the spectra come
  into agreement with each other.

The energy of transition to extragalactic cosmic
rays has not been determined yet.
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• Above about 60 EeV, the distant universe
  disappears.
• Without that large isotropic background, sources
  within the GZK sphere (roughly 100 Mpc radius)
  may be individually detectable.
• Also
• Deflection by magnetic fields decreases with
  energy as 1/E.
• For trans-GZK protons, the magnetic deflections
  may be small.
• Look for clusters of arrival directions on an
  empty sky.

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Galaxy Center
1 kpc
Larmor radius Rkpc EEeV / (Z BmG)
RMpc EEeV / (Z BnG)
For 60-EeV protons, deflection is ? LkpcBµG (in
degrees) for a path length Lkpc through a regular
transverse field BµG.
Deflections of only a few degrees are expected
due to the Galaxys magnetic field because B3 µG
extending for about 1 kpc.
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Veron-Cetty AGNs (red dots) Supergalactic Plane
(blue line) Swift x-ray galactic black holes
(blue circles)
The AGN correlation (Auger)
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What are the particle types? They must be protons
for the Berezinsky model. Are they? Can we
deduce from the AGN correlation that they are
protons at 60 EeV and above? If we know they are
protons, then we can do interesting studies of
proton interactions (at CM energies gt 300 TeV,
much higher than 14 TeV of the LHC). Above the
GZK threshold, the composition can be protons
and/or heavy nuclei, but not intermediate masses.
They photodisintegrate too quickly.
At 60 EeV, the CMB photon energy needed for
photodisintegrating a nucleus depends on the
Lorentz gamma factor, hence on the nuclear
mass. The minimum energies are shown for He, CNO,
and Fe
frequency (1/cm) 5 10
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He CNO
Fe
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Depth of Shower Maximum As a Function of Energy
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Measuring the nuclear mass distribution is
difficult. Photons and neutrinos can be
identified with confidence, however. Auger has
not seen them and has derived upper
limits. Further details in my next lecture.
Photon Limits (lt2 of flux at 10 EeV)
Netrino Limit (No candidate events)
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Summary

• The sources of cosmic rays have not been
  identified at any energy.
• Low energy cosmic rays originate in the Galaxy,
  the highest energy cosmic rays are extragalactic.
  There are conflicting ideas about the energy
  where the transition occurs.
• By coincidence, the GZK effect removes the
  isotropic background at about the same energy
  where magnetic rigidity of protons becomes great
  enough to allow tight clusters of arrival
  directions.
• Charged particle astronomy is plausible
  above the GZK threshold energy.
• No help (yet) from neutral particles (gamma rays
  or neutrinos).
• New limits on the fraction of primary photons
  constrain models of top-down production of UHE
  cosmic rays.
• No neutrinos have been detected at UHE energies.
• Low-mass and intermediate-mass nuclei
  photodisintegrate rapidly by collisions with CMB
  photons at 60 EeV, so cosmic rays above 60 EeV
  must be protons and/or heavy nuclei. (The AGN
  correlation suggests protons, but measured air
  shower properties might favor heavies or else a
  revision of hadronic interaction models.)