Detection of high energy showers by the Astroneu extensive air shower array

The Astroneu Extensive Air Shower (EAS) array comprises autonomous detection stations; each station consisting of 3 large scintillator detectors and one or more Radio Frequency (RF) antennas. The scintillator detectors of a station are able to detect showers with a low energy threshold of $20-30$ TeV, at a rate of $10-20$ showers per hour depending on the station geometrical layout. The RF antennas are used to detect very high energy EAS (E > 10¹⁷eV) through their radio wave emission. This work focuses in reconstructing and studying showers that have been detected synchronously by the scintillator detectors of two distant Astroneu stations. The performance of the array to detect and reconstruct the direction of such high energy (E > 5 · 10¹⁵eV) showers is evaluated by comparing the experimental measurements to the predictions of a detail Monte Carlo (MC) simulation.     

Upgrading and testing the 3D reconstruction of gamma-ray air showers as observed with an array of Imaging Atmospheric Cherenkov Telescopes

Stereoscopic arrays of Imaging Atmospheric Cherenkov Telescopes allow to reconstruct gamma-ray-induced showers in three dimensions, which offers several advantages: direct access to the shower parameters in space and straightforward calorimetric measurement of the incident energy. In addition, correlations between the different images of the same shower are taken into account. An analysis method based on a simple 3D-model of electromagnetic showers was recently implemented in the framework of the H.E.S.S. experiment. In the present article, the method is completed by an additional quality criterion, which reduces the background contamination by a factor of about 2 in the case of extended sources, while keeping gamma-ray efficiency at a high level. On the other hand, the dramatic flares of the blazar PKS 2155-304 in July 2006, which provided H.E.S.S. data with an almost pure gamma-ray sample, offered the unique opportunity of a precision test of the 3D-reconstruction method as well as of the H.E.S.S. simulations used in its calibration. An agreement at a few percent level is found between data and simulations for the distributions of all 3D shower parameters.     

Upgrading and testing the 3D reconstruction of gamma-ray air showers as observed with an array of Imaging Atmospheric Cherenkov Telescopes

Stereoscopic arrays of Imaging Atmospheric Cherenkov Telescopes allow to reconstruct gamma-ray-induced showers in three dimensions, which offers several advantages: direct access to the shower parameters in space and straightforward calorimetric measurement of the incident energy. In addition, correlations between the different images of the same shower are taken into account. An analysis method based on a simple 3D-model of electromagnetic showers was recently implemented in the framework of the H.E.S.S. experiment. In the present article, the method is completed by an additional quality criterion, which reduces the background contamination by a factor of about 2 in the case of extended sources, while keeping gamma-ray efficiency at a high level. On the other hand, the dramatic flares of the blazar PKS 2155-304 in July 2006, which provided H.E.S.S. data with an almost pure gamma-ray sample, offered the unique opportunity of a precision test of the 3D-reconstruction method as well as of the H.E.S.S. simulations used in its calibration. An agreement at a few percent level is found between data and simulations for the distributions of all 3D shower parameters.     

On the extension of the sensitive area of an extensive air shower surface array

A large distance between true and reconstructed core locations of an extensive air shower (EAS) may result in great systematic mis-estimation of EAS parameters. The reconstruction of those EASs whose core locations are outside the boundary of a surface array (outside EAS (OEAS)) results in a large distance of the reconstructed core location from the true one, especially when the true core is far outside the array. Although it may not be mentioned, the rejection of OEASs is a necessary and important step in the reconstruction procedure of an EAS. In this paper, an existing technique is optimized for the rejection of OEASs. The simultaneous use of this technique and a recently developed approach for reconstructing the core location of an EAS can significantly increase the sensitive area of a surface array.     

The observation of extensive air showers from an Earth-orbiting satellite

We review the main issues that are relevant for the observation of extensive air showers from an Earth-orbiting satellite. Extensive air showers are produced by the interaction of ultra-high energy cosmic particles with the atmosphere and can be observed by an orbiting telescope detecting the air scintillation light.We provide the main analytical formulas and semi-analytical results needed to optimize the design of a suitable telescope and estimate the best-expected performance and the minimal necessary requirements for the observation.While we have in mind an EUSO-like general-purpose experiment, the results presented in this paper are useful for any kind of space-based experiment.     

A Heitler model of extensive air showers

A simple, semi-empirical model is used to develop the hadronic portion of air showers in a manner analogous to the well-known Heitler splitting approximation of electromagnetic cascades. Various characteristics of EAS are plainly exhibited with numerical predictions in good accord with detailed Monte Carlo simulations and with data. Results for energy reconstruction, muon and electron sizes, the elongation rate, and for the effects of the atomic number of the primary are discussed.     

Time structure of individual extensive air showers

Extensive air showers detected by the GREX array have been sampled by means of highly segmented 8 m² bakelite RPC in the GREX/COVER_PLASTEX experiment. Delay distributions of particles with respect to the first arriving particle in the EAS front at PeV energies have been analysed for individual events in the core distance range of 0–100 m. It is shown that both mean arrival time and EAS front thickness in individual showers fluctuate strongly and cannot be a good measure of the distance from the EAS axis in a 0–100 m core distance interval.

Individual distributions have been compared with integrated inclusive distributions measured in the same experiment. Results indicate that the width of the individual distribution is systematically less than that of the inclusive distribution. It means that the bulk of particles in individual showers arrive as a relatively compact group delayed by different time intervals from the first arriving particle. Such fluctuations of the arrival time for most of the shower particles may be the consequence of large fluctuations in the shower longitudinal development.

Comparison with CORSIKA Monte Carlo simulations confirmed the difference between the mean width of inclusive and individual arrival time distribution. It revealed also the presence in the experiment of the excessive train of delayed particles near the shower core. This train is obviously due to the non-relativistic low energy hadrons most abundant in the shower core region.     

Atmospheric effects on extensive air showers observed with the surface detector of the Pierre Auger observatory

Atmospheric parameters, such as pressure (P), temperature (T) and density (ρ∝P/T), affect the development of extensive air showers initiated by energetic cosmic rays. We have studied the impact of atmospheric variations on extensive air showers by means of the surface detector of the Pierre Auger Observatory. The rate of events shows a 10% seasonal modulation and 2% diurnal one. We find that the observed behaviour is explained by a model including the effects associated with the variations of P and ρ. The former affects the longitudinal development of air showers while the latter influences the Molière radius and hence the lateral distribution of the shower particles. The model is validated with full simulations of extensive air showers using atmospheric profiles measured at the site of the Pierre Auger Observatory.     

Time structure of the extensive air shower front

The GREX/COVER_PLASTEX experiment has measured the temporal and spatial fine structure of the EAS disc at sea level in a new and original way, using resistive plate counter detectors for direct measurements of the arrival time of each particle crossing the detector. Data were taken at EAS core distances up to 100 m for shower size N > 10⁵ (PeV energy range). Arrival times of shower particles were measured with nanosecond accuracy. More than 450000 air shower events have been included in this analysis.     

Energy release in air showers

A simulation study of the energy released in air due to the development of an extensive air shower has been carried out using the CORSIKA code. The contributions to the energy release from different particle species and energies as well as the typical particle densities are investigated. Special care is taken of particles falling below the energy threshold of the simulation which contribute about 10% to the total energy deposition. The dominant contribution to the total deposition stems from electrons and positrons from sub-MeV up to a few hundred MeV, with typical transverse distances between particles exceeding 1 mm for 10 EeV showers.     

One-dimensional hybrid approach to extensive air shower simulation

An efficient scheme for one-dimensional extensive air shower simulation and its implementation in the program conex are presented. Explicit Monte Carlo simulation of the high-energy part of hadronic and electro-magnetic cascades in the atmosphere is combined with a numeric solution of cascade equations for smaller energy sub-showers to obtain accurate shower predictions. The developed scheme allows us to calculate not only observables related to the number of particles (shower size) but also ionization energy deposit profiles which are needed for the interpretation of data of experiments employing the fluorescence light technique. We discuss in detail the basic algorithms developed and illustrate the power of the method. It is shown that Monte Carlo, numerical, and hybrid air shower calculations give consistent results which agree very well with those obtained within the corsika program.     

Underground water Cherenkov muon detector array with the Tibet air shower array for gamma-ray astronomy in the 100 TeV region

We propose to build a large water-Cherenkov-type muon-detector array (Tibet MD array) around the 37 000 m² Tibet air shower array (Tibet AS array) already constructed at 4300 m above sea level in Tibet, China. Each muon detector is a waterproof concrete pool, 6 m wide × 6 m long × 1.5 m deep in size, equipped with a 20 inch-in-diameter PMT. The Tibet MD array consists of 240 muon detectors set up 2.5 m underground. Its total effective area will be 8640 m² for muon detection. The Tibet MD array will significantly improve gamma-ray sensitivity of the Tibet AS array in the 100 TeV region (10–1000 TeV) by means of gamma/hadron separation based on counting the number of muons accompanying an air shower. The Tibet AS+MD array will have the sensitivity to gamma rays in the 100 TeV region by an order of magnitude better than any other previous existing detectors in the world. The Tibet ASγ Collaboration.     

Can intergalactic suprathermal grains produce extensive air showers?

The events following the impact of intergalactic suprathermal grains with atmosphere are examined, and some similarity is found between the expected air shower and observations of largest cosmic ray showers. It is concluded that the largest air showers are, in any case, initiated by primaries of intergalactic origin. Whether the primaries are suprathermal dust grains or single nuclei is inconclusive.     

Macroscopic treatment of radio emission from cosmic ray air showers based on shower simulations

We present a macroscopic calculation of coherent electro-magnetic radiation from air showers initiated by ultra-high energy cosmic rays, based on currents obtained from Monte Carlo simulations of air showers in a realistic geo-magnetic field. We can clearly relate the time signal to the time dependence of the currents. We find that the most important contribution to the pulse is related to the time variation of the currents. For showers forming a sufficiently large angle with the magnetic field, the contribution due to the currents induced by the geo-magnetic field is dominant, but neither the charge excess nor the dipole contribution can be neglected. We find a characteristic bipolar signal. In our calculations, we take into account a realistic index of refraction, whose importance depends on the impact parameter and the inclination. Also very important is the role of the positive ions.     

Distortions of experimental muon arrival time distributions of extensive air showers by the observation conditions

Event-by-event measured arrival time distributions of extensive air shower (EAS) muons are affected and distorted by various interrelated effects which originate from the time resolution of the timing detectors, from fluctuations of the reference time and the number (multiplicity) of detected muons spanning the arrival time distribution of the individual EAS events. The origin of these effects is discussed, and different correction procedures, which involve detailed simulations, are proposed and illustrated. The discussed distortions are relevant for relatively small observation distances (R_μ<200 m) from the EAS core. Their significance decreases with increasing observation distance and increasing primary energies. Local arrival time distributions which refer to the observed arrival time of the first local muon prove to be less sensitive to the mass of the primary. This feature points to the necessity of arrival time measurements with additional information on the curvature of the EAS disk.