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Register a new userReconstruction of inclined air showers detected with the Pierre Auger Observatory
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We describe the method devised to reconstruct inclined cosmic-ray air showers with zenith angles greater than $60^circ$ detected with the surface array of the Pierre Auger Observatory. The measured signals at the ground level are fitted to muon density distributions predicted with atmospheric cascade models to obtain the relative shower size as an overall normalization parameter. The method is evaluated using simulated showers to test its performance. The energy of the cosmic rays is calibrated using a sub-sample of events reconstructed with both the fluorescence and surface array techniques. The reconstruction method described here provides the basis of complementary analyses including an independent measurement of the energy spectrum of ultra-high energy cosmic rays using very inclined events collected by the Pierre Auger Observatory.
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With the Auger Engineering Radio Array (AERA) of the Pierre Auger Observatory, we have observed the radio emission from 561 extensive air showers with zenith angles between 60$^circ$ and 84$^circ$. In contrast to air showers with more vertical incidence, these inclined air showers illuminate large ground areas of several km$^2$ with radio signals detectable in the 30 to 80,MHz band. A comparison of the measured radio-signal amplitudes with Monte Carlo simulations of a subset of 50 events for which we reconstruct the energy using the Auger surface detector shows agreement within the uncertainties of the current analysis. As expected for forward-beamed radio emission undergoing no significant absorption or scattering in the atmosphere, the area illuminated by radio signals grows with the zenith angle of the air shower. Inclined air showers with EeV energies are thus measurable with sparse radio-antenna arrays with grid sizes of a km or more. This is particularly attractive as radio detection provides direct access to the energy in the electromagnetic cascade of an air shower, which in case of inclined air showers is not accessible by arrays of particle detectors on the ground.
The water-Cherenkov tanks of the Pierre Auger Observatory can detect particles at all zenith angles and are therefore well-suited for the study of inclined and horizontal air showers (60 - 90 degrees). Such showers are characterised by a dominance of the muonic component at ground, and by a very elongated and asymmetrical footprint which can even exhibit a lobular structure due to the bending action of the geomagnetic field. Dedicated algorithms for the selection and reconstruction of such events, as well as the corresponding acceptance calculation, have been set up on basis of muon maps obtained from shower simulations.
A measurement of the cosmic-ray spectrum for energies exceeding $4{times}10^{18}$ eV is presented, which is based on the analysis of showers with zenith angles greater than $60^{circ}$ detected with the Pierre Auger Observatory between 1 January 2004 and 31 December 2013. The measured spectrum confirms a flux suppression at the highest energies. Above $5.3{times}10^{18}$ eV, the ankle, the flux can be described by a power law $E^{-gamma}$ with index $gamma=2.70 pm 0.02 ,text{(stat)} pm 0.1,text{(sys)}$ followed by a smooth suppression region. For the energy ($E_text{s}$) at which the spectral flux has fallen to one-half of its extrapolated value in the absence of suppression, we find $E_text{s}=(5.12pm0.25,text{(stat)}^{+1.0}_{-1.2},text{(sys)}){times}10^{19}$ eV.
In this paper we introduce the concept of Lateral Trigger Probability (LTP) function, i.e., the probability for an extensive air shower (EAS) to trigger an individual detector of a ground based array as a function of distance to the shower axis, taking into account energy, mass and direction of the primary cosmic ray. We apply this concept to the surface array of the Pierre Auger Observatory consisting of a 1.5 km spaced grid of about 1600 water Cherenkov stations. Using Monte Carlo simulations of ultra-high energy showers the LTP functions are derived for energies in the range between 10^{17} and 10^{19} eV and zenith angles up to 65 degs. A parametrization combining a step function with an exponential is found to reproduce them very well in the considered range of energies and zenith angles. The LTP functions can also be obtained from data using events simultaneously observed by the fluorescence and the surface detector of the Pierre Auger Observatory (hybrid events). We validate the Monte-Carlo results showing how LTP functions from data are in good agreement with simulations.
Astrophysical observations of ultra-high-energy cosmic rays with the Pierre Auger Observatory
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