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1.
The distribution of ultrahigh-energy cosmic rays along the supergalactic plane measured at the Pierre Auger Observatory
A. 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: 207; Downloads: 2
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2.
Update on full-sky searches for large- and medium-scale anisotropies in the UHECR flux using the Pierre Auger Observatory and the Telescope Array
Grigory 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: 241; Downloads: 4
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3.
Improved calibration methods and reconstruction of the underground muon detector of the Pierre Auger Observatory
Joaquín De Jesús, 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: As part of the upgrade of the Pierre Auger Observatory, known as AugerPrime, the Underground Muon Detector is being deployed in the low-energy extension of the Surface Detector. It comprises an array of 30 m[sup]2 plastic scintillator muon counters, buried 2.3 meters underground near the water-Cherenkov detectors, allowing for direct measurement of the muonic component of air showers in the energy range of 10[sup]16.5 − 10[sup]19 eV. To achieve an extended dynamic range, the detector operates in two modes: the binary mode, which is optimized for low muon densities, and the ADC mode, designed for high muon densities. In this contribution, we present the latest improvements to the calibration procedure of the ADC mode and to the data reconstruction of the binary mode. We assess their performance with simulations.
Keywords: ultra-high-energy cosmic rays (UHECRs), extensive air showers, Pierre Auger Observatory, AugerPrime upgrade, Auger underground muon detector (UMD), muonic air-shower component, detector calibration, data reconstruction
Published in RUNG: 30.04.2025; Views: 415; Downloads: 5
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4.
Amplifying UHECR arrival direction information using mass estimators at the Pierre Auger Observatory
Lorenzo 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: 311; Downloads: 6
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5.
An extremely energetic cosmic ray observed by a surface detector array
R. U. Abbasi, M. Allen, R. Arimura, J. W. Belz, Douglas R. Bergman, S. A. Blake, K. Shin, I. J. Buckland, B. G. Cheon, Jon Paul Lundquist, 2023, original scientific article

Abstract: Cosmic rays are energetic charged particles from extraterrestrial sources, with the highest-energy events thought to come from extragalactic sources. Their arrival is infrequent, so detection requires instruments with large collecting areas. In this work, we report the detection of an extremely energetic particle recorded by the surface detector array of the Telescope Array experiment. We calculate the particle’s energy as 244 +- 29 (stat.) +51,-76 (syst.) exa–electron volts (~40 joules). Its arrival direction points back to a void in the large-scale structure of the Universe. Possible explanations include a large deflection by the foreground magnetic field, an unidentified source in the local extragalactic neighborhood, or an incomplete knowledge of particle physics.
Keywords: ultra-high-energy cosmic rays, telescope array, extremely energetic cosmic-ray event
Published in RUNG: 23.04.2025; Views: 239; Downloads: 2
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6.
Multimessenger studies with the Pierre Auger Observatory
Jon Paul Lundquist, Andrej Filipčič, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2025, published scientific conference contribution

Abstract: The Pierre Auger Observatory, the world’s largest ultra-high-energy (UHE) cosmic ray (CR) detector, plays a crucial role in multi-messenger astroparticle physics with its high sensitivity to UHE photons and neutrinos. Recent Auger Observatory studies have set stringent limits on the diffuse and point-like fluxes of these particles, enhancing constraints on dark-matter models and UHECR sources. Although no temporal coincidences of neutrinos or photons with LIGO/Virgo gravitational wave events have been observed, competitive limits on the energy radiated in these particles have been established, particularly from the GW170817 binary neutron star merger. Additionally, correlations between the arrival directions of UHECRs and high-energy neutrinos have been explored using data from the IceCube Neutrino Observatory, ANTARES, and the Auger Observatory, providing additional neutrino flux constraints. Efforts to correlate UHE neutron fluxes with gamma-ray sources within our galaxy continue, although no significant excesses have been found. These collaborative and multi-faceted efforts underscore the pivotal role of the Auger Observatory in advancing multi-messenger astrophysics and probing the most extreme environments of the Universe.
Keywords: high-energy particle physics, astrophysics, ultra-high energy cosmic rays, neutrinos, gravitional waves
Published in RUNG: 22.04.2025; Views: 255; Downloads: 0
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7.
The Pierre Auger Observatory open data
A. 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: 454; Downloads: 8
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8.
Search for the anomalous events detected by ANITA using the Pierre Auger Observatory
A. Abdul Halim, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2025, original scientific article

Abstract: A dedicated search for upward-going air showers at zenith angles exceeding 110° and energies E>0.1  EeV has been performed using the Fluorescence Detector of the Pierre Auger Observatory. The search is motivated by two “anomalous” radio pulses observed by the ANITA flights I and III that appear inconsistent with the standard model of particle physics. Using simulations of both regular cosmic-ray showers and upward-going events, a selection procedure has been defined to separate potential upward-going candidate events and the corresponding exposure has been calculated in the energy range [0.1–33] EeV. One event has been found in the search period between January 1, 2004, and December 31, 2018, consistent with an expected background of 0.27 ± 0.12 events from misreconstructed cosmic-ray showers. This translates to an upper bound on the integral flux of (7.2±0.2)×10[sup]−21  cm[sup]−2 sr[sup]−1 y[sup]−1 and (3.6±0.2)×10−20  cm[sup]−2 sr[sup]−1 y[sup]−1 for an E[sup]−1 and E[sup]−2 spectrum, respectively. An upward-going flux of showers normalized to the ANITA observations is shown to predict over 34 events for an E[sup]−3 spectrum and over 8.1 events for a conservative E[sup]−5 spectrum, in strong disagreement with the interpretation of the anomalous events as upward-going showers.
Keywords: ultra-high-energy cosmic rays, extensive air showers, upward-going air showers, Pierre Auger Observatory, Fluorescence Detector, anomalous ANITA events
Published in RUNG: 28.03.2025; Views: 438; Downloads: 5
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9.
Subluminal pulses in the surface-scintillator detectors of AugerPrime
Tobias Schulze, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2025, published scientific conference contribution

Abstract: In extensive air showers, the signals from the electromagnetic and muonic components typically span a few microseconds in scintillation detectors. Neutrons are the only stable neutral hadrons over the timescale of air showers. They lose energy exclusively through hadronic interactions and quasi-elastic scattering, which results in their high abundance at ground level. These neutrons can produce delayed pulses in scintillation detectors, appearing up to several milliseconds after the primary shower signal. This allows us to probe hadronic interactions in the development of air showers. In this study, we characterize such subluminal pulses using the first measurements from the scintillator surface detectors of the AugerPrime upgrade of the Pierre Auger Observatory.
Keywords: ultra-high-energy cosmic rays, Pierre Auger Observatory, extensive air showers, AugerPrime upgrade
Published in RUNG: 28.03.2025; Views: 429; Downloads: 7
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10.
Highlights from the Auger Engineering Radio Array
Bjarni Pont, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2025, published scientific conference contribution

Abstract: The Auger Engineering Radio Array (AERA) at the Pierre Auger Observatory is an array of 153 radio-antenna stations that measure the 30−80 MHz radio emission produced in extensive air showers in the energy range between 0.1 and 10 EeV. It has been taking data for over a decade. In this contribution, we present the recent results of AERA. We show the measurements of the depths of the shower maxima (Xmax) using the radio footprint and using interferometry, demonstrating compatibility and competitiveness with the established fluorescence detection method. We also show the measurement of the stability of the radio signal over close to a decade determined using the Galactic radio background as a calibration source, demonstrating that a radio detector can be used to lower systematic uncertainties on the energy scale of, for example, fluorescence and water-Cherenkov detectors.
Keywords: ultra-high-energy cosmic rays, Pierre Auger Observatory, extensive air showers, radio emission
Published in RUNG: 28.03.2025; Views: 423; Downloads: 5
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