276 research outputs found
The flux suppression at the highest energies
Almost half a century ago, Greisen, Zatsepin and Kuz'min (GZK) predicted a
"cosmologically meaningful termination" of the spectrum of cosmic rays at
energies around eV due to their interaction with the cosmic microwave
background, as they propagate from distant extragalactic sources. A suppression
of the flux above eV is now confirmed. We argue that current
data are insufficient to conclude whether the observed feature is due to energy
loss during propagation, or else to the fact that the astrophysical
accelerators reach their limit, or indeed to a combination of both source
properties and propagation effects. We discuss the dependence of the spectral
steepening upon the cosmic-ray composition, source properties, and intervening
magnetic fields, and speculate on the additional information that may be
necessary to reach unambiguous conclusions about the origin of the flux
suppression and of the mechanisms behind the acceleration of cosmic rays up to
the highest observed energies.Comment: Invited review prepared for Comptes Rendus Physique (2014), in pres
Constraining non standard recombination: A worked example
We fit the BOOMERANG, MAXIMA and COBE/DMR measurements of the cosmic
microwave background anisotropy in spatially flat cosmological models where
departures from standard recombination of the primeval plasma are parametrized
through a change in the fine structure constant compared to its
present value. In addition to we vary the baryon and dark matter
densities, the spectral index of scalar fluctuations, and the Hubble constant.
Within the class of models considered, the lack of a prominent second acoustic
peak in the measured spectrum can be accomodated either by a relatively large
baryon density, by a tilt towards the red in the spectrum of density
fluctuations, or by a delay in the time at which neutral hydrogen formed. The
ratio between the second and first peak decreases by around 25% either if the
baryon density is increased or the spectral index decreased
by a comparable amount, or if neutral hydrogen formed at a redshift about
15% smaller than its standard value. We find that the present data is best
fitted by a delay in recombination, with a lower baryon density than the best
fit if recombination is standard. Our best fit model has ,
, , and . Compatible with
present data at 95% confidence level ,
, and .Comment: 9 pages, 6 figs. Version accepted for publication in Phys.Rev.
Anisotropies of ultra-high energy cosmic ray nuclei diffusing from extragalactic sources
We obtain the dipolar anisotropies in the arrival directions of ultra-high
energy cosmic ray nuclei diffusing from nearby extragalactic sources. We
consider mixed-composition scenarios in which different cosmic ray nuclei are
accelerated up to the same maximum rigidity, so that , with
the atomic number and the maximum proton energy. We adopt
EeV so as to account for an increasingly heavier
composition above the ankle. We obtain the anisotropies through Monte Carlo
simulations that implement the cosmic ray diffusion in extragalactic turbulent
fields as well as the effects of photo-disintegrations and other energy losses.
Dipolar anisotropies at the level of 5 to 10\% at energies ~EeV are
predicted for plausible values of the source density and magnetic fields
The most energetic particles in the universe
Several issues related to the lensing of ultra-high energy cosmic rays by the
Galactic magnetic field are discussed.Comment: Plenary talk given by E. Roulet at COSMO99, Trieste, Oct. 199
Angular distribution of cosmic rays from an individual source in a turbulent magnetic field
We obtain the angular distribution of the cosmic rays reaching an observer
from an individual source and after propagation through a turbulent magnetic
field, for different ratios between the source distance and the diffusion
length. We study both the high-energy quasi-rectilinear regime as well as the
transition towards the diffusive regime at lower energies where the deflections
become large. We consider the impact of energy losses, showing that they tend
to enhance the anisotropy of the source at a given energy. We also discuss
lensing effects, in particular those that could result from the regular
galactic magnetic field component, and show that the effect of the turbulent
extragalactic magnetic fields can smooth out the divergent magnification peaks
that would result for point-like sources in the limit of no turbulent
deflections.Comment: matches published versio
The shape of the extragalactic cosmic ray spectrum from Galaxy Clusters
We study the diffusive escape of cosmic rays from a central source inside a
galaxy cluster to obtain the suppression in the outgoing flux appearing when
the confinement times get comparable or larger than the age of the sources. We
also discuss the attenuation of the flux due to the interactions of the cosmic
rays with the cluster medium, which can be sizeable for heavy nuclei. The
overall suppression in the total cosmic ray flux expected on Earth is important
to understand the shape of the extragalactic contribution to the cosmic ray
spectrum for EeV. This suppression can also be relevant to interpret
the results of fits to composition-sensitive observables measured at ultra-high
energies
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