An Estimate of the Spectral Intensity Expected from the Molecular Bremsstrahlung Radiation in Extensive Air Showers

A detection technique of ultra-high energy cosmic rays, complementary to the fluorescence technique, would be the use of the molecular Bremsstrahlung radiation emitted by low-energy electrons left after the passage of the showers in the atmosphere. The emission mechanism is expected from quasi-elastic collisions of electrons produced in the shower by the ionisation of the molecules in the atmosphere. In this article, a detailed calculation of the spectral intensity of photons at ground level originating from the transitions between unquantised energy states of free ionisation electrons is presented. In the absence of absorption of the emitted photons in the plasma, the obtained spectral intensity is shown to be 5 10^{-26} W m^{-2}Hz^{-1} at 10 km from the shower core for a vertical shower induced by a proton of 10^{17.5} eV.Comment: 16 pages, 6 figures, accepted in Astroparticle Physics. Compared to v1 version: 1. Inclusion of ro-vibrational processes. 2. Use of more accurate ionization potential values and energy distribution of the secondary electron

Molecular Bremsstrahlung Radiation at GHz Frequencies in Air

A detection technique for ultra-high energy cosmic rays, complementary to the fluorescence technique, would be the use of the molecular Bremsstrahlung radiation emitted by low-energy ionization electrons left after the passage of the showers in the atmosphere. In this article, a detailed estimate of the spectral intensity of photons at ground level originating from this radiation is presented. The spectral intensity expected from the passage of the high-energy electrons of the cascade is also estimated. The absorption of the photons in the plasma of electrons/neutral molecules is shown to be negligible. The obtained spectral intensity is shown to be $2\times10^{-21}$W cm$^{-2}$ GHz$^{-1}$ at 10 km from the shower core for a vertical shower induced by a proton of $10^{17.5}$ eV. In addition, a recent measurement of Bremsstrahlung radiation in air at gigahertz frequencies from a beam of electrons produced at 95 keV by an electron gun is also discussed and reasonably reproduced by the model.Comment: 20 pages, 9 figures, figures (2,4,7) improved in v2, accepted by Phys. Rev.

UHE tau neutrino flux regeneration while skimming the Earth

The detection of Earth-skimming tau neutrinos has turned into a very promising strategy for the observation of ultra-high energy cosmic neutrinos. The sensitivity of this channel crucially depends on the parameters of the propagation of the tau neutrinos through the terrestrial crust, which governs the flux of emerging tau leptons that can be detected. One of the characteristics of this propagation is the possibility of regeneration through multiple $\nu_\tau \leftrightarrow \tau$ conversions, which are often neglected in the standard picture. In this paper, we solve the transport equations governing the $\nu_\tau$ propagation and compare the flux of emerging tau leptons obtained allowing regeneration or not. We discuss the validity of the approximation of neglecting the $\nu_\tau$ regeneration using different scenarios for the neutrino-nucleon cross-sections and the tau energy losses.Comment: 8 pages, 8 figure

Angular Power Spectrum Estimation of Cosmic Ray Anisotropies with Full or Partial Sky Coverage

We study the angular power spectrum estimate in order to search for large scale anisotropies in the arrival directions distribution of the highest-energy cosmic rays. We show that this estimate can be performed even in the case of partial sky coverage and validated over the full sky under the assumption that the observed fluctuations are statistically spatial stationary. If this hypothesis - which can be tested directly on the data - is not satisfied, it would prove, of course, that the cosmic ray sky is non isotropic but also that the power spectrum is not an appropriate tool to represent its anisotropies, whatever the sky coverage available. We apply the method to simulations of the Pierre Auger Observatory, reconstructing an input power spectrum with the Southern site only and with both Northern and Southern ones. Finally, we show the improvement that a full-sky observatory brings to test an isotropic distribution, and we discuss the sensitivity of the Pierre Auger Observatory to large scale anisotropies.Comment: 16 pages, 6 figures, version accepted for publication by JCA

Tau energy losses at ultra-high energy: continuous versus stochastic treatment

We study the energy losses of the tau lepton in matter through electromagnetic processes at ultra-high energy (UHE). We use both a stochastic and a continuous framework to treat these interactions and compare the flux of tau leptons propagated after some amount of matter. We discuss the accuracy of the approximation of continuous energy losses by studying the propagation in standard rock of taus with both mono-energetic and power law injection spectra.Comment: 7 pages, 8 figure

The Use of the Signal at an Optimal Distance from the Shower Core as a Surrogate for Shower Size

When analysing data from air-shower arrays, it has become common practice to use the signal at a considerable distance from the shower axis (ropt) as a surrogate for the size of the shower. This signal, S(ropt), can then be related to the primary energy in a variety of ways. After a brief review of the reasons behind the introduction of ropt laid out in a seminal paper by Hillas in 1969, it will be shown that ropt, is a more effective tool when detectors are laid out on a triangular grid than when detectors are deployed on a square grid. This result may have implications for explaining the differences between the flux observed by the Auger and Telescope collaborations above 10\,EeV and should be kept in mind when designing new shower arrays

The Use of the Signal at an Optimal Distance from the Shower Core as a Surrogate for Shower Size

When analysing data from air-shower arrays, it has become common practice to use the signal at a considerable distance from the shower axis (_opt) as a surrogate for the size of the shower. This signal, (_opt), can then be related to the primary energy in a variety of ways. After a brief review of the reasons behind the introduction of opt laid out in a seminal paper by Hillas in 1969, it will be shown that _opt, is a more effective tool when detectors are laid out on a triangular grid than when detectors are deployed on a square grid. This result may have implications for explaining the differences between the flux observed by the Auger and Telescope collaborations above 10 EeV and should be kept in mind when designing new shower arrays

Astrophysical interpretation of the medium scale clustering in the ultra-high energy sky

We compare the clustering properties of the combined dataset of ultra-high energy cosmic rays events, reported by the AGASA, HiRes, Yakutsk and Sugar collaborations, with a catalogue of galaxies of the local universe (redshift z<~0.06). We find that the data reproduce particularly well the clustering properties of the nearby universe within z <~0.02. There is no statistically significant cross-correlation between data and structures, although intriguingly the nominal cross-correlation chance probability drops from ~50% to ~10% using the catalogue with a smaller horizon. Also, we discuss the impact on the robustness of the results of deflections in some galactic magnetic field models used in the literature. These results suggest a relevant role of magnetic fields (possibly extragalactic ones, too) and/or possibly some heavy nuclei fraction in the UHECRs. The importance of a confirmation of these hints by Auger data is emphasized.Comment: 10 pages, 7 figures; one reference adde

Tau Neutrinos in the Auger Observatory : A New Window to UHECR Sources

The cosmic ray spectrum has been shown to extend well beyond 10^{20}eV. With nearly 20 events observed in the last 40 years, it is now established that particles are accelerated or produced in the universe with energies near 10^{21}eV. In all production models neutrinos and photons are part of the cosmic ray flux. In acceleration models (bottom-up models), they are produced as secondaries of the possible interactions of the accelerated charged particle; in direct production models (top-down models) they are a dominant fraction of the decay chain. In addition, hadrons above the GZK threshold energy will also produce, along their path in the Universe, neutrinos and photons as secondaries of the pion photo-production processes. Therefore, photons and neutrinos are very distinctive signatures of the nature and distribution of the potential sources of ultra high energy cosmic rays. In the following we describe the tau neutrino detection and identification capabilities of the Auger observatory. We show that in the range 3x10^{17}-3x10^{20}eV the Auger effective apperture reaches a few tenths of km^2.sr, making the observatory sensitive to fluxes as low as a few tau neutrinos per km^2.sr.year. In the hypothesis of nu_mu nu_tau oscillations with full mixing, this sensitivity allows to probe the GZK cutoff as well as to provide model independent constraints on the mechanisms of production of ultra high energy cosmic rays.Comment: 10 pages, 11 figures, accepted by Astroparticle physic

The signature of local cosmic structures on the ultra-high energy cosmic ray anisotropies

Current experiments collecting high statistics in ultra-high energy cosmic rays (UHECRs) are opening a new window on the universe facing the possibility to perform UHECR astronomy. Here we discuss a large scale structure (LSS) model for the UHECR origin for which we evaluate the expected large scale anisotropy in the UHECR arrival distribution. Employing the IRAS PSCz catalogue as tracer of the LSS, we derive the minimum statistics needed to reject or assess the correlation of the UHECRs with the baryonic distribution in the universe, in particular providing a forecast for the Auger experiment.Comment: 8 pages, 6 figures, contribution to the CRIS06 proceedings (Catania, Italy, May 29 - June 2, 2006