A combined fit of energy spectrum, shower depth distribution and arrival directions to constrain astrophysical models of UHECR sources

The combined fit of the measured energy spectrum and distribution of depths of shower maximum of ultra-high-energy cosmic rays is known to constrain the parameters of astrophysical scenarios with homogeneous source distributions. Further measurements show that the cosmic-ray arrival directions agree better with the directions and fluxes of catalogs of starburst galaxies and active galactic nuclei than with isotropy. Here, we present a novel combination of both analyses. For that, a three-dimensional universe model containing a nearby source population and a homogeneous background source distribution is built, and its parameters are adapted using a combined fit of the energy spectrum, depth of shower maximum distribution and energy-dependent arrival directions. The model takes into account a symmetric magnetic field blurring, source evolution and interactions during propagation. We use simulated data, which resemble measurements of the Pierre Auger Observatory, to evaluate the method’s sensitivity. With this, we are able to verify that the source parameters as well as the fraction of events from the nearby source population and the size of the magnetic field blurring are determined correctly, and that the data is described by the fitted model including the catalog sources with their respective fluxes and three-dimensional positions. We demonstrate that by combining all three measurements we reach the sensitivity necessary to discriminate between the catalogs of starburst galaxies and active galactic nuclei

Performance of the 433 m surface array of the Pierre Auger Observatory

The Pierre Auger Observatory, located in western Argentina, is the world’s largest cosmic-ray observatory. While it was originally built to study the cosmic-ray flux above 1018.5 eV, several enhancements have reduced this energy threshold. One such enhancement is a surface array composed of a triangular grid of 19 water-Cherenkov detectors separated by 433 m (SD-433) to explore the energies down to about 1016 eV. We are developing two research lines employing the SD-433. Firstly, we will measure the energy spectrum in a region where previous experiments have shown evidence of the second knee. Secondly, we will search for ultra-high energy photons to study PeV cosmic-ray sources residing in the Galactic center. In this work, we introduce the SD-433 and we show that it is fully efficient above 5×1016 eV for hadronic primaries with θ < 45°. Using seven years of data, we present the parametrization of the lateral distribution function of measured signals. Finally, we show that an angular resolution of 1.8° (0.5°) can be attained at the lowest (highest) primary energies. Our study lays the goundmark for measurements in the energy range above 1016 eV by utilizing the SD-433 and thus expanding the scientific output of the Auger surface detector

Status and performance of the underground muon detector of the Pierre Auger Observatory

The Auger Muons and Infill for the Ground Array (AMIGA) is an enhancement of the Pierre Auger Observatory, whose purpose is to lower the energy threshold of the observatory down to 1016.5 eV, and to measure the muonic content of air showers directly. These measurements will significantly contribute to the determination of primary particle masses in the range between the second knee and the ankle, to the study of hadronic interaction models with air showers, and, in turn, to the understanding of the muon puzzle. The underground muon detector of AMIGA is concomitant to two triangular grids of water-Cherenkov stations with spacings of 433 and 750 m; each grid position is equipped with a 30 m2 plastic scintillator buried at 2.3 m depth. After the engineering array completion in early 2018 and general improvements to the design, the production phase commenced. In this work, we report on the status of the underground muon detector, the progress of its deployment, and the performance achieved after two years of operation. The detector construction is foreseen to finish by mid-2022

Constraining Lorentz Invariance Violation using the muon content of extensive air showers measured at the Pierre Auger Observatory

Lorentz Invariance (LI) implies that the space-time structure is the same for all observers. On the other hand, various quantum gravity theories suggest that it may be violated when approaching the Planck scale. At extreme energies, like those available in the collision of Ultra-High Energy Cosmic Rays (UHECRs) with atmosphere nuclei, one should also expect a change in the interactions due to Lorentz Invariance Violation (LIV). In this work, the effects of LIV on the development of Extensive Air Showers (EAS) have been considered. After having introduced LIV as a perturbation term in the single-particle dispersion relation, a library of simulated showers with different energies, primary particles and LIV strengths has been produced. Possible LIV has been studied using the muon content of air showers measured at the Pierre Auger Observatory. Limits on LIV parameters have been derived from a comparison between the Monte Carlo expectations and muon fluctuation measurements from the Pierre Auger Observatory

A search for ultra-high-energy photons at the Pierre Auger Observatory exploiting air-shower Universality

The Pierre Auger Observatory is the most sensitive detector to primary photons with energies above ∼ 0.2 EeV. It measures extensive air showers using a hybrid technique that combines a fluorescence detector (FD) with a ground array of particle detectors (SD). The signatures of a photon-induced air shower are a larger atmospheric depth at the shower maximum (Xmax) and a steeper lateral distribution function, along with a lower number of muons with respect to the bulk of hadron-induced background. Using observables measured by the FD and SD, three photon searches in different energy bands are performed. In particular, between threshold energies of 1–10 EeV, a new analysis technique has been developed by combining the FD-based measurement of Xmax with the SD signal through a parameter related to its muon content, derived from the universality of the air showers. This technique has led to a better photon/hadron separation and, consequently, to a higher search sensitivity, resulting in a tighter upper limit than before. The outcome of this new analysis is presented here, along with previous results in the energy ranges below 1 EeV and above 10 EeV. From the data collected by the Pierre Auger Observatory in about 15 years of operation, the most stringent constraints on the fraction of photons in the cosmic flux are set over almost three decades in energy

Event-by-event reconstruction of the shower maximum Xmax with the Surface Detector of the Pierre Auger Observatory using deep learning

The measurement of the mass composition of ultra-high energy cosmic rays constitutes a prime challenge in astroparticle physics. Most detailed information on the composition can be obtained from measurements of the depth of maximum of air showers, Xmax, with the use of fluorescence telescopes, which can be operated only during clear and moonless nights. Using deep neural networks, it is now possible for the first time to perform an event-by-event reconstruction of Xmax with the Surface Detector (SD) of the Pierre Auger Observatory. Therefore, previously recorded data can be analyzed for information on Xmax, and thus, the cosmic-ray composition. Since the SD operates with a duty cycle of almost 100% and its event selection is less strict than for the Fluorescence Detector (FD), the gain in statistics with respect to the FD is almost a factor of 15 for energies above 1019.5 eV. In this contribution, we introduce the neural network particularly designed for the SD of the Pierre Auger Observatory. We evaluate its performance using three different hadronic interaction models, verify its functionality using Auger hybrid measurements, and find that the method can extract mass information on an event level

Adjustments to Model Predictions of Depth of Shower Maximum and Signals at Ground Level using Hybrid Events of the Pierre Auger Observatory

We present a new method to explore simple ad-hoc adjustments to the predictions of hadronic interaction models to improve their consistency with observed two-dimensional distributions of the depth of shower maximum, Xmax, and signal at ground level, as a function of zenith angle. The method relies on the assumption that the mass composition is the same at all zenith angles, while the atmospheric shower development and attenuation depend on composition in a correlated way. In the present work, for each of the three leading LHC-tuned hadronic interaction models, we allow a global shift ΔXmax of the predicted shower maximum, which is the same for every mass and energy, and a rescaling RHad of the hadronic component at ground level which depends on the zenith angle. We apply the analysis to 2297 events reconstructed by both fluorescence and surface detectors at the Pierre Auger Observatory with energies 1018.5 − 1019.0 eV. Given the modeling assumptions made in this analysis, the best fit reaches its optimum value when shifting the Xmax predictions of hadronic interaction models to deeper values and increasing the hadronic signal at both extreme zenith angles. The resulting change in the composition towards heavier primaries alleviates the previously identified model deficit in the hadronic signal (commonly called the muon deficit), but does not remove it. Because of the size of the required corrections ΔXmax and RHad and the large number of events in the sample, the statistical significance of the corrections is large, greater than 5σstat even for the combination of experimental systematic shifts within 1σsys that are the most favorable for the models

Expected performance of the AugerPrime Radio Detector

The AugerPrime Radio Detector will significantly increase the sky coverage of mass-sensitive measurements of ultra-high energy cosmic rays with the Pierre Auger Observatory. The detection of highly inclined air showers with the world’s largest 3000 km2 radio-antenna array in coincidence with the Auger water-Cherenkov detector provides a clean separation of the electromagnetic and muonic shower components. The combination of these highly complementary measurements yields a strong sensitivity to the mass composition of cosmic rays. We will present the first results of an end-to-end simulation study of the performance of the AugerPrime Radio Detector. The study features a complete description of the AugerPrime radio antennas and reconstruction of the properties of inclined air showers, in particular the electromagnetic energy. The performance is evaluated utilizing a comprehensive set of simulated air showers together with recorded background. The estimation of an energy- and direction-dependent aperture yields an estimation of the expected 10-year event statistics. The potential to measure the number of muons in air showers with the achieved statistics is outlined. Based on the achieved energy resolution, the potential to discriminate between different cosmic-ray primaries is presented

Energy spectrum of cosmic rays measured using the Pierre Auger Observatory

We present the energy spectrum of cosmic rays measured at the Pierre Auger Observatory from 6 × 1015 eV up to the most extreme energies where the accumulated exposure reaches about 80 000 km2 sr yr. The wide energy range is covered with five different data sets: events detected by the surface detector (with separate reconstruction methods for zenith angles below and above 60◦), those collected by a denser array, a set of hybrid events simultaneously recorded by the surface and fluorescence detectors, and those events in which the signal is dominated by Cherenkov light registered by the high-elevation telescopes. In this contribution, we report updates of the analysis techniques and present the spectrum obtained by combining the five different measurements. Spectral features occurring in the wide energy range covered by the Observatory are discussed

Search for upward-going showers with the Fluorescence Detector of the Pierre Auger Observatory

Given its operation time and wide field of view, the Fluorescence Detector (FD) of the Pierre Auger Observatory is sufficiently sensitive to detect upward-going events when used in monocular mode. Upward-going air showers are a possible interpretation of the recent events reported by the ANITA Collaboration in the energy range above 1017 eV. The Pierre Auger FD data can be used to support or constrain this interpretation. If confirmed, it would require either new phenomena or significant modifications to the standard model of particle physics. To prepare this search, a set of quality selection criteria was defined by using 10% of the available FD data from 14 years of operation. This subset was mainly used to clean the data from improperly labelled laser events that had been used to monitor the quality of the atmosphere. The potential background for this search consists of cosmic-ray induced air showers with specific geometric configurations which, in a monocular reconstruction, can be reconstructed erroneously as upward-going events. To distinguish candidates from these false positives, to calculate the exposure, and to estimate the expected background, dedicated simulations for signal (upward-going events) and background (downward-going events) have been performed. The detector exposure is large enough to strongly constrain the interpretation of ANITA anomalous events. Preliminary results of the analysis after unblinding the data set are presented