Ultra-High Energy Cosmic Rays in a Structured and Magnetized Cosmic Environment

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Ultra-High Energy Cosmic Rays in a Structured and
Magnetized Cosmic Environment

• General facts and the experimental situation
• Acceleration (bottom-up scenario)
• Cosmic magnetic fields and their role in cosmic
  ray physics

Günter Sigl GReCO, Institut dAstrophysique de
Paris, CNRS http//www.iap.fr/users/sigl/homepage.
html
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The cosmic ray spectrum stretches over some 12
orders of magnitude in energy and some 30 orders
of magnitude in differential flux
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The structure of the spectrum and scenarios of
its origin
knee
ankle
toe ?
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Atmospheric Showers and their Detection
Flys Eye technique measures fluorescence
emission The shower maximum is given by Xmax
X0 X1 log Ep where X0 depends on primary
type for given energy Ep
Ground array measures lateral distribution Primary
energy proportional to density 600m from shower
core
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Current data at the highest energies
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A Tension between the Newest Fluorescence Data
(HiRes) and Ground Array Results? Or Is there a
Cut-Off after all?
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HiRes collaboration, astro-ph/0208301
Lowering the AGASA energy scale by about 20
brings it in accordance with HiRes up to the GZK
cut-off, but not beyond.
May need need an experiment combining ground
array with fluorescence such as the Auger project
to resolve this issue.
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But HiRes has also seen a gt200 EeV event in
stereo mode with only 20 exposure of the
mono-mode
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Next-Generation Ultra-High Energy Cosmic Ray
Experiments
compare to AGASA acceptance 230 km2sr
Experiments starting date acceptance in km2sr angular resolution energy resolution
High Res Flys Eye since 1999 350-1000 few degrees 40 mono 10 stereo
Telescope Array maybe with Auger North 1700-5000 1o ? 20 ?
Auger ground full size in about 2004 gt7000 lt 2o 15
Auger hybrid 2004 gt700 0.25o 8
EUSO/OWL space-based gt2010 105 ? 1o ? lt30 ?
radio detection ??? gt1000 ? few degrees ? ???
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The southern Auger site is under construction.
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The Ultra-High Energy Cosmic Ray Mystery consists
of (at least) Three Interrelated Challenges
1.) electromagnetically or strongly interacting
particles above 1020 eV loose energy within
less than about 50 Mpc.
2.) in most conventional scenarios exceptionally
powerful acceleration sources within that
distance are needed.
3.) The observed distribution seems to be very
isotropic (except for a possible interesting
small scale clustering)
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The Greisen-Zatsepin-Kuzmin (GZK) effect
Nucleons can produce pions on the cosmic
microwave background
?
nucleon

• sources must be in cosmological backyard
• Only Lorentz symmetry breaking at ?gt1011
• could avoid this conclusion.

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First Order Fermi Shock Acceleration
This is the most widely accepted scenario of
cosmic ray acceleration
u1
u2
The fractional energy gain per shock crossing
depends on the velocity jump at the
shock. Together with loss processes this leads to
a spectrum E-q with q gt 2 typically. When the
gyroradius becomes comparable to the shock
size, the spectrum cuts off.
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A possible acceleration site associated with
shocks in hot spots of active galaxies
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A possible acceleration site associated with
shocks formed by colliding galaxies
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Or Can Plasma Waves in Relativistic Shocks
Occuring in ?-Ray Bursts accelerate up to 1024
eV?
Chen, Tajima, Takahashi, astro-ph/0205287
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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.
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HiRes sees no significant anisotropy above 1018 eV
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Cosmic Magnetic Fields and their Role in Cosmic
Ray Physics
1.) Cosmic rays up to 1018 eV are confined in
the Galaxy Energy densities in cosmic rays,
in the galactic magnetic field, in the
turbulent flow, and gravitational energy are of
comparable magnitude. The galactic cosmic
ray luminosity LCR required to maintain its
observed density uCR in the galactic volume
Vgal for a confinement time tCR107 y, LCR
uCR Vgal / tCR, is 10 of the kinetic energy
rate of galactic supernovae.
2.) Cosmic rays above 1019 eV are probably
extragalactic and may be deflected mostly by
extragalactic fields BXG rather than by galactic
fields. However, very little is known about
about BXG It could be as small as 10-20 G
(primordial seeds, Biermann battery) or up to
fractions of micro Gauss if concentrated in
the local Supercluster (equipartition with
plasma).
3.) Magnetic fields are main players in cosmic
ray acceleration.
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Example Magnetic field of 10-10 Gauss, coherence
scale 1 Mpc burst source at 50 Mpc distance
time delay
cuts through the energy-time distribution
differential spectrum
Lemoine, Sigl
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Transition rectilinear-diffusive regime
Neglect energy losses for simplicity.
Time delay over distance d in a field Brms of
coherence length ?c for small deflection
This becomes comparable to distance d at energy
Ec
In the rectilinear regime for total differential
power Q(E) injected inside d, the differential
flux reads
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In the diffusive regime characterized by a
diffusion constant D(E), particles are confined
during a time scale
which leads to the flux
For a given power spectrum B(k) of the magnetic
field an often used (very approximate) estimate
of the diffusion coefficient is
where Brms2?08dkk2ltB2(k)gt, and the gyroradius is
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IF EltltEc and IF energy losses can be approximated
as continuous, dE/dt-b(E) (this is not the case
for pion production), the local cosmic
ray density n(E,r) obeys the diffusion equation
Where now q(E,r) is the differential injection
rate per volume, Q(E)?d3rq(E,r). Analytical
solutions exist (Syrovatskii), but the
necessary assumptions are in general too
restrictive for ultra-high energy cosmic rays.
Monte Carlo codes are therefore in general
indispensable.
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Strong fields in our Supergalactic Neighbourhood ?
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Principle of deflection code
sphere around observer
source
A particle is registered every time a trajectory
crosses the sphere around the observer. This
version to be applied for individual source/magnet
ic field realizations and inhomogeneous
structures.
sphere around source
A particle is registered every time a
trajectory crosses the sphere around the source.
This version to be applied for homogeneous structu
res and if only interested in average distribution
s.
source
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Effects of a single source Numerical simulations
A source at 3.4 Mpc distance injecting protons
with spectrum E-2.4 up to 1022 eV A uniform
Kolmogorov magnetic field of strength 0.3 micro
Gauss and largest turbulent eddy size of 1 Mpc.
105 trajectories, 251 images between 20 and 300
EeV, 2.5o angular resolution
Isola, Lemoine, Sigl
Conclusions 1.) Isotropy is inconsistent
with only one source. 2.) Strong fields
produce interesting lensing (clustering) effects.
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Same scenario, averaged over many magnetic field
realisations
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That the flux produced by CenA is too anisotropic
can also be seen from the realization averaged
spectra visible by detectors in different
locations
southern hemisphere
AGASA, northern hemisphere
solid angle averaged
Isola, Lemoine, Sigl, Phys.Rev.D 65 (2002) 023004
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Summary of spectral effects
in rectilinear regime
in diffusive regime
in rectilinear regime
in diffusive regime
Continuous source distribution following the
Gaussian profile. B3x10-7 G, d10 Mpc
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More detailed scenarios of large scale magnetic
fields use large scale structure simulations with
magnetic fields followed passively and normalized
to a few micro Gauss in galaxy clusters.
We use a (75 Mpc)3 box, repeated by periodic
boundary conditions, to take into account sources
at cosmological distances.
We then consider different observer and source
positions for structured and unstructured
distributions with and without magnetization.
We analyze these scenarios and compare them with
data based on large scale multi-poles,
auto-correlations, and clustering.
Sigl, Miniati, Ensslin, astro-ph/0309695
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Observer immersed in fields of order 0.1
micro Gauss
Observer immersed in fields of order 10-11 Gauss
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Observer immersed in fields of order 10-11 Gauss
Cut thru local magnetic field strength
Filling factors of magnetic fields from the large
scale structure simulation.
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Result Magnetized, structured sources are
marginally favored if the observer is immersed in
negligible fields.
Strong field observer ruled out by
isotropy around 1019 eV.
Weak field observer allowed.
However, even if fields around observer are
negligible, deflection in magnetized structures
surrounding the sources lead to off-sets of
arrival direction from source direction up to gt10
degrees up to 1020 eV in our simulations. This is
contrast to Dolag et al., astro-ph/0310902. gt
Particle astronomy not necessarily possible !
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Unmagnetized, Unstructured Sources
Source density2.4x10-4 Mpc-3
Source density2.4x10-6 Mpc-3
Autocorrelation function sensitive to source
density in this case
Comparison with AGASA data gt The required source
density is 10-5 Mpc-3. Similar numbers were
found in several independent studies, e.g.
Yoshiguchi et al. ApJ. 586 (2003) 1211, Blasi and
de Marco, astro-ph/0307067
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Magnetized, Structured Sources
Comparing predicted autocorrelations for source
density 2.4x10-4 Mpc-3 (upper set) and 2.4x10-5
Mpc-3 (lower set) for an Auger-type exposure.
Deflection in magnetic fields makes
autocorrelation and power spectrum much less
dependent on source density and distribution !
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The spectrum in the magnetized source scenario
shows a pronounced GZK cut-off.
Deflection can be substantial even up to 1020 eV.
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Comparing predicted autocorrelations for source
density 2.4x10-5 Mpc-3 with (lower set) and
without (upper set) magnetization for an
Auger-type exposure.
In the future, a suppressed auto-correlation
function will be a signature of magnetized
sources.
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Generalization to heavy nuclei
All secondary nuclei are followed and registered
upon crossing a sphere around the source.
B10-12 G, Egt1019 eV
Example If source injects heavy
nuclei, diffusion can enhance the heavy component
relative to the weak-field case.
B2x10-8 G, Egt1019 eV
Here we assume E-2 iron injection up to 1022 eV.
Bertone, Isola, Lemoine, Sigl, astro-ph/0209192
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B2x10-8 G, d7.1 Mpc
Composition as function of energy.
However, the injection spectrum necessary to
reproduce observed spectrum is E-1.6 and thus
rather hard.
B2x10-8 G, d3.2 Mpc
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2nd example Helium primaries do not survive
beyond 20 Mpc at the highest
energies
B10-12 G, Egt1020 eV
Bertone, Isola, Lemoine, Sigl, astro-ph/0209192
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Conclusions
1.) The origin of very high energy cosmic rays is
one of the fundamental unsolved questions of
astroparticle physics. This is especially
true at the highest energies, but even the origin
of Galactic cosmic rays is not resolved
beyond doubt.
2.) Acceleration and sky distribution of cosmic
rays are strongly linked to the in part
poorly known strength and distribution of cosmic
magnetic fields.
3.) Already current cosmic ray data (isotropy)
favor an observer immersed in fields lt
10-11 G. Future data (auto-correlation) will test
source magnetization.
4.) The coming 3-5 years promise an about
100-fold increase of ultra-high energy
cosmic ray data due to experiments that are under
either construction or in the proposal stage.