1. Reconstructing air-shower observables using a universality-based modelMaximilian Stadelmaier, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2025, published scientific conference contribution Abstract: Air-Shower universality describes the regularity in the longitudinal, lateral, and energy distributions of electromagnetic shower particles, as motivated by solutions of the cascade equations. To reconstruct air-shower observables from ultra-high-energy cosmic rays, we employ a universality-based model of shower development that incorporates hadronic particle components. Depending on the input parameters, the model can be used, for example, to estimate the depth of the shower maximum or the number of muons on event level. In this context, we present the expected performance for the reconstruction using air-shower simulations and data from the Pierre Auger Observatory.
Keywords: ultra-high-energy cosmic rays, extensive air showers, air-shower universality, Pierre Auger Observatory, UHECR event reconstruction, air-shower maximum depth, muonic shower component
Published in RUNG: 09.06.2025; Views: 337; Downloads: 3
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2. Prospects of a composition-enhanced search for large-scale anisotropy with the Pierre Auger ObservatoryEdyvania Emily Martins, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2025, published scientific conference contribution Abstract: The Pierre Auger Observatory has been collecting data for over 19 years, reaching more than 135 000 sq. km yr sr of accumulated exposure, with the surface detectors spread over 3000 sq. km. A remarkable discovery is the large-scale dipole structure with a total amplitude of 7.4% for energies above 8 EeV. The observed modulation in right ascension has a statistical significance of 6.8 sigma. The dipolar pattern in the events with energies between 8 and 16 EeV has a statistical significance of over 5 sigma. The Pierre Auger Collaboration has also reported an increase in the dipole amplitude with energy. This anisotropy is understood to be of extragalactic origin, as the maximum of the dipolar component is located ∼ 115◦ away from the Galactic Center. In the same energy range, the observed evolution of the depth of maximum shower development with energy indicates a progression towards heavier composition of cosmic rays with increasing energy. This contribution presents a novel approach to a search for composition-enhanced large-scale anisotropy. On the one hand, lighter events have higher rigidity than their heavier counterparts with similar energy; therefore, their trajectories are less affected by magnetic fields. The expected effect is a higher anisotropy in the arrival direction of a subset of events with smaller mass and charge than the anisotropy in the overall flux of cosmic rays. On the other hand, the attenuation length is distinct for each mass group, leading to different horizon of cosmic ray sources for each of them. Under a source-agnostic model, we investigate the dipole amplitude as a function of rigidity. Using a simulation library, we analyze the possibility of measuring a separation in total dipole amplitude between two sub-populations distinct in mass of the Auger Phase I dataset.
Keywords: ultra-high-energy cosmic rays, Pierre Auger Observatory, UHECR arrival directions, UHECR mass composition, large-scale anisotropy
Published in RUNG: 29.05.2025; Views: 267; Downloads: 2
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3. Machine learning-based analyses using surface detector data of the Pierre Auger ObservatorySteffen Hahn, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2025, published scientific conference contribution Abstract: The Pierre Auger Observatory is the largest detector for the study of extensive air showers induced
by ultra-high-energy cosmic rays (UHECRs). Its hybrid detector design allows the simultaneous
observation of different parts of the shower evolution using various detection techniques. To
accurately understand the physics behind the origin of UHECRs, it is essential to determine their
mass composition. However, since UHECRs cannot be measured directly, estimating their masses
is highly non-trivial. The most common approach is to analyze mass-sensitive observables, such as
the number of secondary muons and the atmospheric depth of the shower maximum.
An intriguing part of the shower to estimate these observables is its footprint. The shower footprint is detected by ground-based detectors, such as the Water-Cherenkov detectors (WCDs) of the Surface Detector (SD) of the Observatory, which have an uptime of nearly 100%, resulting in a high number of observed events. However, the spatio-temporal information stored in the shower footprints is highly complex, making it very challenging to analyze the footprints using analytical and phenomenological methods. Therefore, the Pierre Auger Collaboration utilizes machine learning-based algorithms to complement classical methods in order to exploit the measured data with unprecedented precision. In this contribution, we highlight these machine learning-based analyses used to determine high-level shower observables that help to infer the mass of the primary particle, with a particular focus on analyses using the shower footprint detected by the WCDs and the Surface Scintillator Detectors (SSD) of the SD. We show that these novel methods show promising results on simulations and offer improved reconstruction performance when applied to measured data.
Keywords: ultra-high-energy cosmic rays (UHECRs), extensive air showers, Pierre Auger Observatory, surface detector, Water-Cherenkov detectors (WCDs), Surface Scintillator Detectors (SSDs), UHECR mass composition, air-shower footprint, machine learning
Published in RUNG: 16.05.2025; Views: 390; Downloads: 6
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4. The distribution of ultrahigh-energy cosmic rays along the supergalactic plane measured at the Pierre Auger ObservatoryA. Abdul Halim, P. Abreu, M. Aglietta, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2025, original scientific article Abstract: Ultrahigh-energy cosmic rays are known to be mainly of extragalactic origin, and their propagation is limited by energy losses, so their arrival directions are expected to correlate with the large-scale structure of the local Universe. In this work, we investigate the possible presence of intermediate-scale excesses in the flux of the most energetic cosmic rays from the direction of the supergalactic plane region using events with energies above 20 EeV recorded with the surface detector array of the Pierre Auger Observatory up to 2022 December 31, with a total exposure of 135,000 sq. km sr yr. The strongest indication for an excess that we find, with a posttrial significance of 3.1σ, is in the Centaurus region, as in our previous reports, and it extends down to lower energies than previously studied. We do not find any strong hints of excesses from any other region of the supergalactic plane at the same angular scale. In particular, our results do not confirm the reports by the Telescope Array Collaboration of excesses from two regions in the Northern Hemisphere at the edge of the field of view of the Pierre Auger Observatory. With a comparable integrated exposure over these regions, our results there are in good agreement with the expectations from an isotropic distribution.
Keywords: ultra-high-energy cosmic rays, UHECR propagation, large-scale structure, UHECR energy losses, UHECR deflections, supergalactic plane region, Centaurus region, Pierre Auger Observatory, Auger surface detector array
Published in RUNG: 06.05.2025; Views: 526; Downloads: 5
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5. Update on full-sky searches for large- and medium-scale anisotropies in the UHECR flux using the Pierre Auger Observatory and the Telescope ArrayGrigory I. Rubtsov, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2025, published scientific conference contribution Abstract: The flux of ultra-high-energy cosmic rays (UHECRs) is remarkably uniform across all directions in the sky. The only anisotropy detected with a significance greater than 5σ is a large-scale dipolar modulation in right ascension for energies above 8 EeV. To enhance our sensitivity to potential anisotropies, which may be obscured by significant deflections by magnetic fields, two strategies can be employed: (1) focusing on large-scale anisotropies, such as the dipole and quadrupole moments across various energy intervals, which are anticipated to be more resilient to magnetic deflections; or (2) focusing on the highest energies, where the background from distant sources is more attenuated. The unique aspect of our research is achieving full-sky coverage by combining data for the Pierre Auger Observatory and the Telescope Array, which would not be possible with a single detector array. This comprehensive coverage enables the application of analysis techniques that would otherwise require specific assumptions with partial sky coverage. Accounting for potential systematic effects in energy reconstruction is crucial to avoid spurious north–south anisotropies; the overlapping sky region observed by both arrays allows us to address this in an entirely data-driven manner. In this contribution, we present the latest results using the largest UHECR dataset collected to date, with events detected until December 2022 at the Pierre Auger Observatory and until May 2024 at the Telescope Array. It is shown that the dipolar modulation is the only anisotropy that is significantly (4.6σ) identified in the angular power spectrum. The hypothesis of correlations with the starburst galaxies is supported at the significance of 4.4σ.
Keywords: ultra-high-energy cosmic rays, Pierre Auger Observatory, UHECR arrival directions, UHECR large-scale anisotropies, UHECR medium-scale anisotropies, full-sky coverage, Telescope Array
Published in RUNG: 05.05.2025; Views: 485; Downloads: 6
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6. Amplifying UHECR arrival direction information using mass estimators at the Pierre Auger ObservatoryLorenzo Apollonio, A. Abdul Halim, P. Abreu, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2025, published scientific conference contribution Abstract: The origin of Ultra-High-Energy Cosmic Rays (UHECRs) is one of the biggest mysteries in modern astrophysics. Since UHECRs are deflected by Galactic and extragalactic magnetic fields, their arrival directions do not point to their sources. Previous analyses conducted on the arrival directions of high-energy events (E ≥ 32 EeV) recorded by the Surface Detector of the Pierre Auger Observatory have not shown significant anisotropies. The largest excess found in the first 19 years of data - at the 4.0 sigma level - is in the region around Centaurus A, and it is also the driving force of a correlation of UHECR arrival directions with a catalog of Starburst Galaxies, which is at the 3.8 sigma level. Since UHECRs are mostly nuclei, the lightest ones (least charged) are also the least deflected. While the mass of the events can be estimated better using the Fluorescence Detector of the Pierre Auger Observatory, the Surface Detector provides the necessary statistics needed for astrophysical studies. The introduction of novel mass-estimation techniques, such as machine learning models and an algorithm based on air-shower universality, will help identify high-rigidity events in the Surface Detector data of the Pierre Auger Observatory. With this work, we present how event-per-event mass estimators can help enhance the sensitivity in the search for anisotropies in the arrival directions of UHECRs at small and intermediate angular scales using simulations.
Keywords: ultra-high-energy cosmic rays (UHECRs), extensive air showers, Pierre Auger Observatory, UHECR propagation, UHECR arrival directions, UHECR mass composition, Centaurus A radio galaxy, starburst galaxies, air-shower universality
Published in RUNG: 30.04.2025; Views: 563; Downloads: 11
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7. Mass composition of ultrahigh energy cosmic rays from distribution of their arrival directions with the Telescope ArrayR. U. Abbasi, Y. Abe, T. Abu-Zayyad, M. Allen, Y. Arai, R. Arimura, E. Barcikowski, J. W. Belz, Douglas R. Bergman, Jon Paul Lundquist, 2024, original scientific article Abstract: We use a new method to estimate the injected mass composition of ultrahigh cosmic rays (UHECRs) at energies higher than 10 EeV. The method is based on comparison of the energy-dependent distribution of cosmic ray arrival directions as measured by the Telescope Array (TA) experiment with that calculated in a given putative model of UHECR under the assumption that sources trace the large-scale structure (LSS) of the Universe. As we report in the companion Letter, the TA data show large deflections with respect to the LSS which can be explained, assuming small extragalactic magnetic fields (EGMF), by an intermediate composition changing to a heavy one (iron) in the highest energy bin. Here we show that these results are robust to uncertainties in UHECR injection spectra, the energy scale of the experiment and galactic magnetic fields. The assumption of weak EGMF, however, strongly affects this interpretation at all but the highest energies E > 100 EeV, where the remarkable isotropy of the data implies a heavy injected composition even in the case of strong EGMF. This result also holds if UHECR sources are as rare as 2 × 10[sup]−5 Mpc[sup]−3, that is the conservative lower limit for the source number density.
Keywords: ultrahigh energy cosmic rays, large-scale structure, extragalactic magnetic fields, UHECR propagation, Telescope Array, UHECR mass composition, UHECR arrival directions
Published in RUNG: 23.04.2025; Views: 411; Downloads: 4
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8. Isotropy of Cosmic Rays beyond 10[sup]20 eV Favors Their Heavy Mass CompositionR. U. Abbasi, Jon Paul Lundquist, 2024, original scientific article Abstract: We report an estimation of the injected mass composition of ultrahigh energy cosmic rays (UHECRs) at energies higher than 10 EeV. The composition is inferred from an energy-dependent sky distribution of UHECR events observed by the Telescope Array surface detector by comparing it to the Large Scale Structure of the local Universe. In the case of negligible extragalactic magnetic fields (EGMFs), the results are consistent with a relatively heavy injected composition at E ∼ 10 EeV that becomes lighter up to E ∼ 100 EeV, while the composition at E > 100 EeV is very heavy. The latter is true even in the presence of highest experimentally allowed extragalactic magnetic fields, while the composition at lower energies can be light if a strong EGMF is present. The effect of the uncertainty in the galactic magnetic field on these results is subdominant.
Keywords: ultrahigh energy cosmic rays (UHECRs), Large Scale Structure, extragalactic magnetic fields, UHECR propagation, Telescope Array surface detector, UHECR mass composition, UHECR arrival directions
Published in RUNG: 23.04.2025; Views: 435; Downloads: 4
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9. The Pierre Auger Observatory open dataA. Abdul Halim, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2025, original scientific article Abstract: The Pierre Auger Collaboration has embraced the concept of open access to their research data since its foundation, with the aim of giving access to the widest possible community. A gradual process of release began as early as 2007 when 1% of the cosmic-ray data was made public, along with 100% of the space-weather information. In February 2021, a portal was released containing 10% of cosmic-ray data collected by the Pierre Auger Observatory from 2004 to 2018, during the first phase of operation of the Observatory. The Open Data Portal includes detailed documentation about the detection and reconstruction procedures, analysis codes that can be easily used and modified and, additionally, visualization tools. Since then, the Portal has been updated and extended. In 2023, a catalog of the highest-energy cosmic-ray events examined in depth has been included. A specific section dedicated to educational use has been developed with the expectation that these data will be explored by a wide and diverse community, including professional and citizen scientists, and used for educational and outreach initiatives. This paper describes the context, the spirit, and the technical implementation of the release of data by the largest cosmic-ray detector ever built and anticipates its future developments.
Keywords: ultra-high-energy cosmic rays (UHECRs), extensive air showers, Pierre Auger Observatory, open data, UHECR event data, space weather data, Auger Open Data Portal
Published in RUNG: 03.04.2025; Views: 635; Downloads: 11
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10. Mass composition of ultra-high-energy cosmic rays at the Pierre Auger ObservatoryThomas Fitoussi, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2025, published scientific conference contribution Abstract: For the past 20 years, the Pierre Auger Observatory has collected the largest dataset of
ultra-high-energy cosmic rays (UHECRs) ever achieved using a hybrid detector. The study
of this dataset has led to numerous unexpected discoveries that enhance our understanding
of the origins of UHECRs. One of the key points in this study is their mass composition.
In this work, we will present the most recent results regarding the mass composition of UHECRs at the Pierre Auger Observatory. In particular, we will focus on the measurement of the depth
of the maximum of air-shower profiles, denoted as Xmax . This determination has been achieved through both direct measurements from the Fluorescence Detector data and the application of machine learning for estimating Xmax on an event-by-event basis using the Surface Detector data. The latter has allowed us to extend the measurement to energies up
to 100 EeV and indicates a correlation between changes in composition and three features
of the energy spectrum (ankle, instep, steepening). Moreover, the results provide evidence
of a heavy and nearly pure primary beam for energies greater than 50 EeV that is independent
of the hadronic interaction model. The implications of these findings for astrophysics and
for modelling hadronic interactions will be discussed.
Keywords: ultra-high-energy cosmic rays, Pierre Auger Observatory, extensive air showers, UHECR mass composition
Published in RUNG: 28.03.2025; Views: 617; Downloads: 11
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