On the Origin of Ultra High Energy Cosmic Rays

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On the Origin of Ultra High Energy Cosmic Rays

• Chia-Chun Lu
• Institute of Physics, National Chiao-Tung
  University, Taiwan
• Guey-Lin Lin
• Institute of Physics, National Chiao-Tung
  University, Taiwan
• Leung Research Center for Cosmology and Particle
  Astrophysics

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Outline

• Brief facts about cosmic rays.
• Detection technique of UHECR.
• Research purpose and three main questions.
• Observation results.
• Conclusion and the future work.

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A Timeline of Cosmic Rays History

• 1912 Hess discovered cosmic rays
• 1927 Cosmic rays seen in the cloud chamber
• 1937 Discovery of muon
• 1938 Auger discovered extensive air showers
• 1962 First 1020 eV cosmic ray detected
• 1962Now A lot of observatories and efforts on
  the study of cosmic rays

S
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Left Victor Hess before his balloon
flight. Right Hess's balloon.
In 1912 he took a balloon to an altitude of 4.8
km, and used a a gold leaf electroscope to
measure the radiation. He found that the
radiation intensity increased with the altitude.

B
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Cosmic Ray Spectrum
E-2.7
E-3.0
LHC
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Detection Technique of UHECR
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Air Showers
http//www.mpi-hd.mpg.de/hfm/CosmicRay/Showers.htm
l
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Fluorescence Surface Detectors
Fluorescence detectors can better determine the
energy, direction and composition of CR, but
they can only operate in moonless night. Thus the
exposure time is limited.
Watson, ICRC 2007
http//www.auger.org/
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Composition Measurement
Q
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Questions

• Is there an END of the cosmic ray spectrum ?
• What is the composition ?
• (Protons or other heavy nuclei)
• What is the origin of the ultra-high energy
  cosmic rays?
• (Top-down model bottom-up model)

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The end of the spectrum?--GZK Cutoff
Greisen-Zatsepin-Kuzmin cutoff. K. Greisen, Phys.
Rev. Lett. 16, 748 - 750 (1966) G. T. Zatepin
and V. A. Kuzmin, Sov. Phys. JETP. Lett. 4
(1966) 78.
latte 150 Mpc at the resonance Eth 4?1019 eV
Q
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Top-down Bottom-up Models

• Top-down model
• Decay of heavy relic particles produced in the
  early Universe.
• gt Ruled out because of over-produced neutrino
  flux
• which is not seen in observation.
• Bottom-up model
• Acceleration in astronomical objects.
• Candidates in Hillas plot

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Candidates of Sources for Bottom-up Models
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Observation Results of UHECR

• Can we answer the three questions?
• (the end of the spectrum, the composition, the
  origin)

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Yakutsk,Russia
HiRes,Utah
AGASA,Japan
The figure taken from http//www.auger.org/
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Q1. HiRes V.S. AGASA

• HiRes data confirmed the GZK cutoff, while AGASA
  conflicted with it.

R.U. Abbasi et al. HiRes Collaboration, Phys.
Rev. Lett. (2008) arXivastro-ph/0703099.
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Auger's Spectrum
The red lines represent different models of
source evolution derived from star formation
history.
A. Watson, ICRC 2007
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Q2. Composition Measurement
L. A. Anchordoqui, H. Goldberg, D. Hooper, S.
Sarkar and A. M. Taylor, Phys. Rev. D 76 (2007)
123008 arXiv0709.0734 astro-ph.
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Q3. Correlated Sources found in Auger's New Result
November, 2007
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Event Energy gt 57 EeV Dmax lt 71Mpc ?3.2o
(opening angle of the circle ) gt protons
dominated
This result is based upon matching the data with
V-C catalog which is valid only up to 100Mpc. Is
this a consistent approach?
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GZK Horizon
Accumulative event probabilities of UHECR
N(l, Eth) dl is the number of cosmic ray events
which are originated from sources at distances
between l and l dl from the Earth and arrive at
the detector with energies above Eth. D is the
maximal distance of sources.
P(D,Eth)0.9 D is the GZK horizon for
corresponding Eth. It means that 90 of the
events with energies above Eth come from the
sources at distances smaller then D.
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Inconsistency of GZK Horizon and Valid distance
of V-C catalog

• D220 Mpc for Eth57 EeV, while the valid
  distance of V-C catalog is 100Mpc. (Assuming
  uniform distribution of sources.)
• Could local over-density of sources resolve this
  inconsistency?

GZK Horizon for different degrees of over-density
and Eth
Lu Lin, arXiv0804.3122
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Fittings with Source Over-density to the Auger
Spectrum
The model n(llt30Mpc)/n010 with ?2/d.o.f.2 is
disfavored by the spectrum.
Lu Lin, arXiv0804.3122
FIG. 2 Fittings to the Auger measured UHECR
spectrum where the red, green, blue and black
curves denote the model with the local
over-density n(l lt 30Mpc)/n0 1, 2, 4, and 10
respectively. Solid curves correspond to ? 2.6
while dash curves correspond to ? 2.5. We take
the source evolution parameter m 3 throughout
the calculations.
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Energy Calibration
Keeping the HiRes energy scale unchanged, the
energy-adjustment factor ? is found to be 1.2,
0.75, 0.83 and 0.625 respectively for Auger,
AGASA, Akeno and Yakutsk. And a different shower
energy reconstruction method infers a 30 higher
UHECR energy than that determined by Augers
?uorescence detector-based shower reconstruction.
V. Berezinsky, arXiv0801.3028 astro-ph
R. Engel for the Pierre Auger Collaboration,
arXiv0706.1921 astro-ph.
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Fittings to the Shifted Spectrum
The energy calibration is very crucial !
Lu Lin, arXiv0804.3122
The ?2 values here are smaller than the
corresponding ones associated with unshifted
spectrum and the best fit fall in the case of
local over-density n/n02-4.
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Conclusion and the Future
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Answers to Questions

• Is there an END of the cosmic ray spectrum ?
• The GZK cutoff is confirmed by HiRes and
  Auger.
• What is the composition of ultra-high energy
  cosmic rays?
• NOT RESOLVED YET ! The composition
  measurement of HiRes and Auger dont agree with
  each other.
• 3. What is the origin of the ultra-high energy
  cosmic rays?
• NOT RESOLVED YET ! We think Augers
  correlation analysis is not consistent. And this
  correlation was not found in HiRes data.

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More Events!

• More events for accurate spectrum measurement. ?
  The degree of over-density can be better
  determined.
• More fluorescence events for composition
  measurement.
• More events for source correlation search.
• More events for cosmic ray astronomy !

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Prepared and waiting, they will come!
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Supplement
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http//www.mpi-hd.mpg.de/hfm/CosmicRay/Showers.htm
l
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http//www.aip.org/png/2006/248.htm
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The Pierre Auger Observatory

• Proposed in 1992 by Jim Cronin and Alan Watson
• 50 million construction budget
• With energy band in 1018 1020 eV
• Two independent detection methods
• Fluorescence detectors
• Ground-based tanks

All information taken from http//www.auger.org/
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The figure taken from http//www.auger.org/
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Surface Detectors
The hybrid nature of the Pierre Auger Observatory
provides for two independent ways to see cosmic
rays. These two ways are an array of surface
detectors, and a collection of air fluorescence
telescopes. A conceptual view of how these
systems complement each other is shown in the
following diagram.
The figure taken from http//www.auger.org/
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Composition Measurement
R.U. Abbasi et al. The High Resolution Fly's
Eye Collaboration, Astrophys. J. 622, 910 (2005)
arXivastro-ph/0407622.
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Local Over-density of Sources

• Motivated by the existence of Local Supercluster.
• Invoked to account for AGASA data.

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http//www.auger.org/
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Pierre Auger, who had positioned particle
detectors high in the Alps, noticed that two
detectors located many meters apart both signaled
the arrival of particles at exactly the same
time. Auger concluded that he had observed
showers with energies of 1015 eV -- ten million
times higher than any known before.
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The horizontal dash line in each panel denotes P
(D, Eth) 0.9. The intersection of this line
with each color curve gives the GZK horizon
corresponding to a speci?c local over-density
characterized by the ratio n(l lt 30Mpc)/n0.
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http//www.aip.org/png/2006/248.htm