937,742 research outputs found
Characteristics of geomagnetic cascading of ultra-high energy photons at the southern and northern sites of the Pierre Auger Observatory
Cosmic-ray photons above 10^19 eV can convert in the geomagnetic field and
initiate a preshower, i.e. a particle cascade before entering the atmosphere.
We compare the preshower characteristics at the southern and northern sites of
the Pierre Auger Observatory. In addition to a shift of the preshower patterns
on the sky due to the different pointing of the local magnetic field vectors,
the fact that the northern Auger site is closer to the geomagnetic pole results
in a different energy dependence of the preshower effect: photon conversion can
start at smaller energies, but large conversion probabilitites (>90%) are
reached for the whole sky at higher energies compared to the southern Auger
site. We show how the complementary preshower features at the two sites can be
used to search for ultra-high energy photons among cosmic rays. In particular,
the different preshower characteristics at the northern Auger site may provide
an elegant and unambiguous confirmation if a photon signal is detected at the
southern site.Comment: 25 pages, 14 figures, minor changes, conclusions unchanged, Appendix
A replaced, accepted by Astroparticle Physic
On the nature of cosmic rays above the Greisen--Zatsepin--Kuz'min cut off
A re-examination of the atmospheric cascade profile of the highest energy
cosmic ray is presented. The study includes air-shower simulations considering
different cross sections, particle multiplicity and variation of the
hadronic-event-generator to model interactions above 200 GeV. The analysis
provides evidence that a medium mass nucleus primary reproduces the shower
profile quite well. This result does not support the idea, increasingly popular
at present, that the highest energy particles are protons, derived from the
decay of supermassive relic particles. On the other hand, we show that debris
of relativistic super-heavy nuclei, which can survive a 100 Mpc journey through
the primeval radiation are likely to generate such a kind of cascade.Comment: Revised version, improvements per referee's suggestions. To be
published in Phys. Lett.
Restricting UHECRs and cosmogenic neutrinos with Fermi-LAT
Ultrahigh energy cosmic ray (UHECR) protons interacting with the cosmic
microwave background (CMB) produce UHE electrons and gamma-rays that in turn
initiate electromagnetic cascades on CMB and infrared photons. As a result, a
background of diffuse isotropic gamma radiation is accumulated in the energy
range E\lsim 100 GeV. The Fermi-LAT collaboration has recently reported a
measurement of the extragalactic diffuse background finding it less intense and
softer than previously measured by EGRET. We show that this new result
constrains UHECR models and the flux of cosmogenic neutrinos. In particular, it
excludes models with cosmogenic neutrino fluxes detectable by existing neutrino
experiments, while next-generation detectors as e.g. JEM-EUSO can observe
neutrinos only for extreme parameters.Comment: 7 pages, 6 eps figures; v2: minor changes, v3: final version, added
discussion of EGMF influenc
Propagation of Ultra-High-Energy Cosmic Ray Nuclei in Cosmic Magnetic Fields and Implications for Anisotropy Measurements
(Abridged) Recent results from the Pierre Auger Observatory (PAO) indicate
that the composition of ultra-high-energy cosmic rays (UHECRs) with energies
above eV may be dominated by heavy nuclei. An important question is
whether the distribution of arrival directions for such UHECR nuclei can
exhibit observable anisotropy or positional correlations with their
astrophysical source objects despite the expected strong deflections by
intervening magnetic fields. For this purpose, we have simulated the
propagation of UHECR nuclei including models for both the extragalactic
magnetic field and the Galactic magnetic field. Assuming that only iron nuclei
are injected steadily from sources with equal luminosity and spatially
distributed according to the observed large scale structure in the local
Universe, at the number of events published by the PAO so far, the arrival
distribution of UHECRs would be consistent with no auto-correlation at 95%
confidence if the mean number density of UHECR sources
Mpc, and consistent with no cross-correlation with sources within 95%
errors for Mpc. On the other hand, with 1000 events
above eV in the whole sky, next generation experiments can
reveal auto-correlation with more than 99% probability even for Mpc, and cross-correlation with sources with more than 99%
probability for Mpc. In addition, we find that the
contribution of Centaurus A is required to reproduce the currently observed
UHECR excess in the Centaurus region. Secondary protons generated by
photodisintegration of primary heavy nuclei during propagation play a crucial
role in all cases, and the resulting anisotropy at small angular scales should
provide a strong hint of the source location if the maximum energies of the
heavy nuclei are sufficiently high.Comment: 17 pages, 15 figure
p, He, and C to Fe cosmic-ray primary fluxes in diffusion models: Source and transport signatures on fluxes and ratios
The propagated fluxes of proton, helium, and heavier primary cosmic-ray
species (up to Fe) are a means to indirectly access the source spectrum of
cosmic rays. We check the compatibility of the primary fluxes with the
transport parameters derived from the B/C analysis, but also if they bring
further constraints. Proton data are well described in the simplest model
defined by a power-law source spectrum and plain diffusion. They can also be
accommodated by models with, e.g., convection and/or reacceleration. There is
no need for breaks in the source spectral indices below TeV/n. Fits on
the primary fluxes alone do not provide physical constraints on the transport
parameters. If we let free the source spectrum and fix the diffusion coefficient such as to reproduce the B/C ratio, the MCMC analysis constrains
the source spectral index to be in the range for all primary
species up to Fe, regardless of the value of the diffusion slope . The
low-energy shape of the source spectrum is degenerate with the
low-energy shape of the diffusion coefficient: we find
for p and He data, but for C
to Fe primary species. This is consistent with the toy-model calculation in
which the shape of the p/He and C/O to Fe/O data is reproduced if
(no need for different slopes ). When
plotted as a function of the kinetic energy per nucleon, the low-energy p/He
ratio is shaped mostly by the modulation effect, whereas primary/O ratios are
mostly shaped by their destruction rate.Comment: 18 pages, 14 figures: accepted in A&A (1 table added
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