1. 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: 99; Downloads: 0
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2. 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: 107; Downloads: 0
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3. 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: 348; Downloads: 8
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4. 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: 393; Downloads: 4
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5. 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: 318; Downloads: 7
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6. Inference of the Mass Composition of Cosmic Rays with Energies from 10[sup]18.5 to 10[sup]20 eV Using the Pierre Auger Observatory and Deep LearningA. Abdul Halim, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2025, original scientific article Abstract: We present measurements of the atmospheric depth of the shower maximum Xmax,
inferred for the first time on an event-by-event level using the Surface Detector
of the Pierre Auger Observatory. Using deep learning, we were able to extend
measurements of the Xmax distributions up to energies of 100 EeV (10[sup]20 eV),
not yet revealed by current measurements, providing new insights into the mass
composition of cosmic rays at extreme energies.
Gaining a 10-fold increase in statistics compared to the Fluorescence Detector data,
we find evidence that the rate of change of the average Xmax with the logarithm
of energy features three breaks at 6.5 ± 0.6 (stat) ± 1 (sys) EeV,
11 ± 2 (stat) ± 1 (sys) EeV, and 31 ± 5 (stat) ± 3 (sys) EeV, in the vicinity to the three
prominent features (ankle, instep, suppression) of the cosmic-ray flux.
The energy evolution of the mean and standard deviation of the measured Xmax
distributions indicates that the mass composition becomes increasingly heavier
and purer, thus being incompatible with a large fraction of light nuclei between
50 EeV and 100 EeV. Keywords: ultra-high-energy cosmic rays (UHECRs), extensive air showers, Pierre Auger Observatory, UHECR mass composition, depth of the shower maximum, fluorescence detector, surface detector, deep learning Published in RUNG: 20.01.2025; Views: 675; Downloads: 5
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7. Measurement of the depth of maximum of air-shower profiles with energies between ▫$10^{18.5} and 10^{20}$▫ eV using the surface detector of the Pierre Auger Observatory and deep learningA. 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: We report an investigation of the mass composition of cosmic rays with energies
from 3 to 100 EeV (1 EeV = 10[sup]18 eV) using the distributions of the depth of shower
maximum Xmax. The analysis relies on ∼50,000 events recorded by the surface detector
of the Pierre Auger Observatory and a deep-learning-based reconstruction algorithm.
Above energies of 5 EeV, the dataset offers a 10-fold increase in statistics with respect to
fluorescence measurements at the Observatory. After cross-calibration using
the fluorescence detector, this enables the first measurement of the evolution of the mean
and the standard deviation of the Xmax distributions up to 100 EeV.
Our findings are threefold: (i) The evolution of the mean logarithmic mass toward a heavier
composition with increasing energy can be confirmed and is extended to 100 EeV.
(ii) The evolution of the fluctuations of Xmax toward a heavier and purer composition
with increasing energy can be confirmed with high statistics. We report a rather heavy
composition and small fluctuations in Xmax at the highest energies.
(iii) We find indications for a characteristic structure beyond a constant change
in the mean logarithmic mass, featuring three breaks that are observed in proximity
to the ankle, instep, and suppression features in the energy spectrum. Keywords: ultra-high-energy cosmic rays, UHECRs, extensive air showers, Pierre Auger Observatory, UHECR mass composition, depth of shower maximum, fluorescence detector, surface detector, deep learning Published in RUNG: 20.01.2025; Views: 704; Downloads: 8
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8. Combined fit of spectrum and composition for FR0 radio-galaxy-emitted ultra–high energy cosmic rays with resulting secondary photons and neutrinosJon Paul Lundquist, Serguei Vorobiov, Lukas Merten, Anita Reimer, Margot Boughelilba, Paolo Da Vela, Fabrizio Tavecchio, Giacomo Bonnoli, Chiara Righi, 2025, original scientific article Abstract: This study comprehensively investigates the gamma-ray dim population of Fanaroff–Riley
Type 0 (FR0) radio galaxies as potentially significant sources of ultra–high energy cosmic rays
(UHECRs, E > 10[sup]18 eV) detected on Earth. While individual FR0 luminosities are relatively
low compared to the more powerful Fanaroff–Riley Type 1 and Type 2 galaxies, FR0s are
substantially more prevalent in the local universe, outnumbering the more energetic galaxies
by a factor of ∼5 within a redshift of z ≤ 0.05. Employing CRPropa3 simulations, we estimate
the mass composition and energy spectra of UHECRs originating from FR0 galaxies for energies
above 10[sup]18.6 eV. This estimation fits data from the Pierre Auger Observatory (Auger)
using three extensive air shower models; both constant and energy-dependent observed
elemental fractions are considered. The simulation integrates an approximately isotropic
distribution of FR0 galaxies, extrapolated from observed characteristics, with UHECR
propagation in the intergalactic medium, incorporating various plausible configurations of
extragalactic magnetic fields, both random and structured. We then compare the resulting
emission spectral indices, rigidity cutoffs, and elemental fractions with recent Auger results.
In total, 25 combined energy-spectrum and mass-composition fits are considered. Beyond
the cosmic-ray fluxes emitted by FR0 galaxies, this study predicts the secondary photon and
neutrino fluxes from UHECR interactions with intergalactic cosmic photon backgrounds.
The multimessenger approach, encompassing observational data and theoretical models,
helps elucidate the contribution of low-luminosity FR0 radio galaxies to the total cosmic-ray
energy density. Keywords: ultra-high-energy cosmic rays, UHECRs, UHECR energy spectrum, Pierre Auger Observatory, UHECR mass composition, UHECR sources, extragalactic magnetic fields, UHECR propagation, CRPropa tool Published in RUNG: 06.01.2025; Views: 644; Downloads: 10
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9. The depth of the shower maximum of air showers measured with AERABjarni Pont, Andrej Filipčič, Jon Paul Lundquist, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, Lukas Zehrer, 2023, published scientific conference contribution Abstract: The Auger Engineering Radio Array (AERA) is an array of 153 radio antennas spanning an area of 17 sq. km, currently the largest of its kind, that probes the nature of ultra-high energy cosmic rays at energies around the transition from Galactic to extra-galactic origin. It measures the MHz radio emission of extensive air showers produced by cosmic rays hitting our atmosphere. The elemental composition of cosmic rays is a crucial piece of information in determining what the sources of cosmic rays are and how cosmic rays are accelerated. This composition can be obtained from the mass-sensitive parameter Xmax, the depth of the shower maximum. We reconstruct Xmax with a likelihood analysis by comparing the measured radio footprint on the ground to an ensemble of footprints from Monte-Carlo CORSIKA/CoREAS air shower simulations. We compare our Xmax reconstruction with fluorescence Xmax measurements on a per-event basis, a setup unique to the Pierre Auger Observatory, and show the methods to be compatible. Furthermore, we extensively validate our reconstruction by identifying and correcting for systematic uncertainties. We determine the resolution of our method as a function of energy and reach a precision better than 15 g/cm[sup]2 at the highest energies. With a bias-free set of around 600 showers, we find a light to light-mixed composition at energies between 10[sup]17.5 to 10[sup]18.8 eV, also in agreement with Auger fluorescence measurements. Keywords: Pierre Auger Observatory, ultra-high-energy cosmic rays, extensive air showers, UHECR mass composition Published in RUNG: 03.10.2024; Views: 899; Downloads: 6
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10. Astrophysical interpretation of energy spectrum and mass composition of cosmic rays as measured at the Pierre Auger ObservatoryEleonora Guido, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2023, published scientific conference contribution Abstract: The combined interpretation of the spectrum and composition measurements plays a key role in the quest for the origin of ultra-high-energy cosmic rays (UHECRs). The Pierre Auger Observatory, thanks to its huge exposure, provides the most precise measurement of the energy spectrum of UHECRs and the most reliable information on their composition, exploiting the distributions of the depth of maximum of the showers in the atmosphere. A combined fit of a simple astrophysical model of UHECR sources to the spectrum and mass composition measurements is used to evaluate the constraining power of the data measured by the Pierre Auger Observatory on the source properties. We find that our data across
the “ankle” feature are well reproduced if two extragalactic populations of sources are considered, one emitting a very soft spectrum which dominates the region below the ankle, and the other taking over at energies above the ankle, with an intermediate mixed composition, a hard spectrum and a low rigidity cutoff. Interestingly, similar results can also be obtained if the medium-mass contribution at lower energy is provided by an additional galactic component. Keywords: Pierre Auger Observatory, ultra-high energy cosmic rays, UHECR energy spectrum, UHECR mass composition Published in RUNG: 24.01.2024; Views: 3131; Downloads: 19
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