1. 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: 231; 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 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: 261; Downloads: 4
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3. 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: 322; Downloads: 6
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4. 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: 245; Downloads: 2
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5. 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: 247; Downloads: 2
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6. 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: 459; Downloads: 8
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7. 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: 415; Downloads: 9
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8. Testing the declination dependency of the spectrum measured by the Pierre Auger ObservatoryDiego Ravignani, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2025, published scientific conference contribution Abstract: The distribution of the arrival directions of cosmic rays observed by the Pierre Auger Observatory has a dipolar component that implies a flux dependence on declination. Previously, we showed that the spectrum built from events arriving with a zenith angle less than 60° is qualitatively consistent with the dipole. In this work, we go one step further and show that the Auger spectrum cannot reject the hypothesis of a declination-independent flux. By using events of up 80°, we extend the previous survey from +25° of declination to +45°, thus covering 85% of the sky.
Keywords: ultra-high-energy cosmic rays, Pierre Auger Observatory, UHECR arrival directions, large-scale dipolar anisotropy
Published in RUNG: 28.03.2025; Views: 433; Downloads: 7
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9. A new view of UHECRs with the Pierre Auger ObservatoryDenise Boncioli, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2025, published scientific conference contribution Abstract: In its Phase I, the Pierre Auger Observatory has led to several observations, driving the field of ultra- high-energy cosmic ray (UHECR) research over the last 20 years. Major achievements obtained so far include the unprecedented precise energy spectrum and its features, the observables linked to the UHECR mass composition and the distribution of arrival directions of the most energetic events. These results, together with the non-observation of high-energy neutrinos and photons, strongly disfavor the pre-Auger pure-proton paradigm.
In this talk, we will provide an overview on the main results of the Observatory, and describe possible astrophysical scenarios for their interpretation. The prospects of improving the current understanding about UHECR characteristics during the Phase II of the Observatory will be also shown.
Keywords: ultra-high-energy cosmic rays, Pierre Auger Observatory, UHE neutrinos, UHE photons, Auger Phase I, AugerPrime upgrade, Auger Phase II, UHECR mass composition, UHECR energy spectrum, UHECR anisotropy studies, UHECR astrophysical scenarios, UHECR data interpretation
Published in RUNG: 24.03.2025; Views: 453; Downloads: 4
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10. Astrophysical models to interpret the Pierre Auger Observatory dataJuan Manuel González, 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 measured the spectrum of ultra-high-energy cosmic rays with unprecedented precision, as well as the distribution of the depths of the maximum of the shower development in the atmosphere, which provide a reliable estimator of the mass composition. The measurements above 10[sup]17.8 eV can be interpreted assuming two populations of uniformly distributed sources, one with a soft spectrum dominating the flux below few EeV, and another one with a very hard spectrum dominating above that energy. When considering the presence of intense extragalactic magnetic fields between our Galaxy and the closest sources and a high-energy population with low spatial density, a magnetic horizon appears, suppressing the cosmic ray's flux at low-energies, which could explain the very hard spectrum observed at Earth. The distribution of arrival directions, which at energies above 32 EeV shows indications of a correlation with a population of starburst galaxies or the radio galaxy Centaurus A (Cen A), are also important to constrain the sources. It is shown that adding a fractional contribution from these sources of about 20% on top of an homogeneous background leads to an improvement of the model likelihood.
Keywords: ultra-high-energy cosmic rays, UHECR energy spectrum, UHECR mass composition, UHECR anisotropies, UHECR propagation, UHECR data interpretation, extragalactic magnetic fields, starburst galaxies, Centaurus A, Pierre Auger Observatory
Published in RUNG: 24.03.2025; Views: 382; Downloads: 7
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