Transcript Slide 1

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.
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]
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?
Hillas Plot
Maximum energy ~ Z(v/c)BL
(charge*speed*B_field*size)
EEeV <~ 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 v=c. 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.
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
Cosmic ray
spectrum
Knee
multiplied by
E2.5
Ankle
Toes?
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.
Eight years ago: An apparent GZK paradox
[AGASA]
The AGASA evidence against a GZK effect on the spectrum.
The Utah Fly’s Eye measured one event at 300 EeV.
Early HiRes analysis (1999) indicated a continuing power law.
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.
Full Auger
Auger South
HiRes
TA
Full Auger
Auger South
The Quest for Exposure
A concave downward transition
requires at least a double
coincidence:
A transition between power laws is
necessarily a concave upward feature.
( Ian Axford )
(1) One spectrum ends at the
same energy where the other
starts.
(2) Both have the same flux at
that energy.
No evident transition to extragalactic population before the ankle
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 e+e- production by collisions with microwave photons
(MeV gamma rays in the proton’s 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”).
[Berezinsky]
The shape of the spectrum from each
experiment is the same as expected
due to e+e- 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.
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.
1 kpc
Galaxy
Center
Larmor radius:
Rkpc ≈ EEeV / (Z BmG)
RMpc ≈ EEeV / (Z BnG)
For 60-EeV protons, deflection is
θ ~ 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 Galaxy’s
magnetic field because B~3 μG extending for about 1 kpc.
Veron-Cetty AGNs (red dots)
Supergalactic Plane (blue line)
Swift x-ray galactic black holes (blue circles)
The AGN correlation (Auger)
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 > 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
He
CNO
10
15
20
Fe
Depth of Shower Maximum
As a Function of Energy
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
(<2% of flux at 10 EeV)
Netrino Limit
(No candidate events)
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.)