arXiv subjects

J. Biteau

Publications and source records attributed to J. Biteau.

At least 19 recordsLinked to original sources

arXiv2026

Study of blazar variability through their flux distribution for CTAO long-term monitoring

The CTAO Key Science Project on Active Galactic Nuclei (AGN) includes a long-term monitoring (LTM) program of blazars. The probability distribution function (PDF) of the flux extracted from unbiased blazar lightcurves can provide important insight into the physical processes taking place in relativistic jets and their connection to observed variability. The aim of this work is to define an optimal observational strategy for the CTAO LTM program using the flux distributions as a quantitative metric. To achieve this goal, we develop a simulation-based analysis framework that enables a systematic comparison of different observational strategies. Light curves were simulated as would be observed with the future CTAO for a representative set of AGN sources, following four different observational strategies within a fixed time budget. For each simulated light curve, different PDF models were fitted and their goodness of fit was evaluated. Based on the best-fit parameters, Monte Carlo simulations were generated to assess the ability of the method to recover the underlying PDF model. Since the true model used to generate the simulated light curves is known, this procedure allows us to quantify the discriminatory power of the algorithm and to identify which observational strategy maximizes model discrimination. Additionally, the algorithm has the potential of discrimination against different models for the flux distributions, such as Gaussian and lognormal. Once validated on simulations, the method is applied to archival lightcurves from existing gamma-ray instruments to extract flux PDFs and study the physical processes driving blazar variability. The analysis framework has been developed and tested on simulated CTAO-like data, showing good stability and robustness. A study to optimize the observing strategy for the CTAO LTM is presented.

astro-ph.HE↗

arXiv2026

CtaAgnVar : a Pipeline for Variability Studies with the Cherenkov Telescope Array Observatory

Blazars are active galactic nuclei whose relativistic jet points toward the observer, accelerating particles that radiate across the electromagnetic spectrum. Characterizing their flux and spectral variability constrains the physics of the central black hole and its jet, making blazars a key science case for the Cherenkov Telescope Array Observatory (CTAO). The CTAO, with two sites and a sensitivity five to ten times better than current Imaging Atmospheric Cherenkov Telescopes, should reconstruct AGN variability at very high energies with unprecedented precision. To quantify these capabilities for blazars and transients, we developed CtaAgnVar, a pipeline based on Gammapy, the open-source Python package for gamma-ray analysis selected as core library of the CTAO Science Analysis Tools. It covers the full chain from a time-dependent spectral energy distribution to the simulation of a realistic observation sequence, accounting for source visibility, and to the reconstruction of spectra and light curves. It provides variability estimators, including a new statistical estimator quantifying the detection of hysteresis patterns in hardness-ratio diagrams. Released as open source, it lets the instrument enter only through the response functions and array location, so its use is not restricted to the CTAO. We simulated over 15 years of CTAO observations of PG 1553+113, whose high-energy emission shows a periodicity in Fermi-LAT data. Assuming the very-high-energy emission follows the high-energy one, the CTAO would recover this periodicity above 5 sigma with 15 years of 30 min observations on a weekly cadence, the period determination being then limited by the light-curve sampling rather than by photon statistics.

astro-ph.HE↗

arXiv2026

Observation of the Moon and Sun shadows with cosmic rays at an average energy of $\text{7}{\times}\text{10}^\text{17}\,$eV

Interactions of cosmic rays with the Moon and the Sun produce deficits in their arrival-direction distributions relative to an isotropic flux. Such shadows have been observed previously at energies between $10^{12}\,$eV and $10^{16}\,$eV. We report the first observation of the Moon and Sun shadows at cosmic-ray energies larger than about $10^{16}\,$eV (average energy of $7\times10^{17}\,$eV), using data collected by the Pierre Auger Observatory. We employ data from three detector arrays covering $3000\,\text{km}^2$, $27\,\text{km}^2$, and $2\,\text{km}^2$, with spacings of $1500\,\text{m}$, $750\,\text{m}$, and $433\,\text{m}$, respectively. The data amount to over 10.6 million events. The Moon and Sun shadows are detected with a combined significance of approximately $3σ$. These observations confirm the pointing accuracy of the Surface Detector of the Pierre Auger Observatory using celestial bodies. From the combined Sun and Moon shadows, we infer an overall angular resolution of $(0.59^{+0.15}_{-0.11})^\circ$.

astro-ph.HE↗

arXiv2026

Search for active-sterile neutrino transitions using Pierre Auger Observatory data

We investigate the sensitivity of the Pierre Auger Observatory to physics beyond the Standard Model arising from magnetic-moment-induced transitions between active and heavy sterile neutrinos. Such dipole portal interactions can enhance neutrino-nucleon cross sections above a kinematic threshold set by the sterile neutrino mass, leading to observable modifications of neutrino detection rates at ultrahigh energies (UHE). We estimate the impact of these interactions on both down-going and Earth-skimming neutrino detection channels, the contrasting responses of which enable discrimination between an enhanced neutrino flux and a modified interaction cross section. Using the non-observation of UHE neutrino candidates, we derive neutrino-flux-dependent constraints with 90% confidence-level on the transition magnetic moment for sterile neutrino masses in the range 1 TeV-100 TeV. Under the assumed flux scenarios, the resulting flavor-independent limits extend existing bounds into previously unexplored parameter space.

hep-ph↗

arXiv2026

Proton-air interaction properties at $\sqrt{s} \simeq 100$ TeV from shower-depth measurements with the Pierre Auger Observatory and their connection to the Muon Puzzle

Hybrid measurements at the Pierre Auger Observatory indicate that most high-energy hadronic interaction models underestimate the average depth of the shower maximum, $\langle X_{\max} \rangle$, at a center-of-mass energy of $\sqrt{s}=97.7 \pm 0.4^{+6.6}_{-6.2}\,\mathrm{TeV}$. In this Letter, the hadronic interaction models are shown to follow a universal relation between the predicted $\langle X_{\max} \rangle$ and the mean values of variables characterizing the energy spectra of secondary particles produced in the first interaction of proton-induced air showers. Assuming the validity of these relations in Nature, we map the values of $\langle X_{\max} \rangle$ favored by Auger data into mean values of these variables. All models favor an increase in the mean elasticity and in the fraction of hadronic energy in proton--air interactions. The latter must be amplified by a factor of $2.8$ to $4.6$ to account for the muon puzzle.

hep-ex↗

arXiv2026

Bounds on Lorentz invariance violation from muon fluctuations at the Pierre Auger Observatory

Quantum gravity theories often modify spacetime symmetries. In particular, Lorentz invariance may be violated when approaching the Planck scale. Although the scales at which interactions occur in extensive air showers induced by ultra-high-energy cosmic rays in the atmosphere are many orders of magnitude below the Planck scale, these violations might still be observable. In this work, the fluctuations in the number of muons in the extensive air showers measured at the Pierre Auger Observatory are exploited, for the first time, to constrain Lorentz invariance violations. The bounds derived in the hadronic sector are the strongest ever obtained, and do not rely on assumptions about the mass composition of ultra-high-energy cosmic rays. The fluctuations in the number of muons constitute a new and powerful observable to further explore Lorentz invariance in a region of the parameter space not accessible to other observables.

astro-ph.HE↗

arXiv2026

Synergy between the Cherenkov Telescope Array Observatory and the Vera C. Rubin Observatory

The Cherenkov Telescope Array Observatory (CTAO) and the Vera C. Rubin Observatory are set to transform our understanding of the universe over the next decade. These two observatories have multiple areas of complementarity in their scientific applications, ranging from constraints on cosmological parameters to studies of asteroid occultations. The most opportune area of synergy probably lies in the field of time-domain astronomy. Due to their sensitivity and saturation limits, it will be difficult for the two observatories to conduct joint studies of variable and transient sources in the Milky Way. However, they could offer a fresh and rich perspective on non-thermal extragalactic sources, in particular gamma-ray bursts, active galactic nuclei and jetted tidal disruption events. Among these sources lie the best candidates for multi-messenger research into the origin of TeV-PeV neutrinos and multi-EeV cosmic rays. Thus, combined with multi-wavelength observations by X-ray satellites and wide-field gamma-ray instruments, the synergy between Rubin and the CTAO could provide answers to some of the most important questions in astroparticle physics. This scientific potential comes with a challenge: selecting a few alerts from the ten million issued by Rubin each night to repoint the CTAO telescopes. We use the variability of blazars over timescales ranging from a few days to several years as a case study to demonstrate how to address this challenge using the Fink broker of Rubin.

astro-ph.HE↗

arXiv2026

Depth of Maximum of Air-Shower Profiles above 10^17.7 eV Measured with the Fluorescence Detector of the Pierre Auger Observatory

We present measurements of the depth of shower maximum, Xmax, for cosmic-ray-induced extensive air showers recorded by the fluorescence detector of the Pierre Auger Observatory over 17 years. The data set covers primary energies from 10^17.7 eV to beyond 10^19.6 eV. With improved event reconstruction and an exposure 2.4 times larger than in our previous analysis, this work confirms and refines our conclusions on the mass composition at ultra-high energies. The energy evolution of the mean Xmax exhibits a pronounced break at around 10^18.4 eV, providing direct, model-independent evidence for a change in the evolution of the mass composition. Independently, the observed decrease of the Xmax fluctuations with energy indicates a transition toward a heavier and less diverse primary mass composition. No statistically significant declination dependence of the Xmax distributions is observed within the exposure of the Observatory, indicating an isotropic mass composition. The mean and standard deviation of the Xmax distributions, interpreted with air-shower simulations, yield the energy dependence of the average and variance of the logarithmic mass of cosmic rays arriving at Earth. Furthermore, energy-dependent fractional abundances of four representative primary-mass groups (p, He, CNO, Fe) are obtained by fitting the observed Xmax distributions in each energy bin with a weighted sum of elemental templates. These results provide strong evidence against a long-standing assumption that ultra-high-energy cosmic rays are predominantly protons: above ~10^18.4 eV, the average cosmic-ray mass increases, accompanied by a steadily decreasing diversity in the elemental composition.

astro-ph.HE↗

arXiv2026

Search for ultra-high energy neutrons from Galactic sources with the Pierre Auger Observatory

Deflections in the propagation of charged ultra-high-energy cosmic rays (UHECRs) caused by magnetic fields make the identification of their sources challenging. On the other hand, the arrival directions at Earth of neutrons point directly to their origin. The emission of UHECRs from a source is expected to be accompanied by the production of neutrons in its vicinity through interactions with ambient matter and radiation. Since free neutrons travel a mean distance $d/\text{kpc}=9.2(E/\text{EeV})$ before decaying, a neutron flux in the EeV range could be detected on Earth from sources of UHECRs in our Galaxy. Using cosmic-ray data from the Phase\,I of the Surface Detector of the Pierre Auger Observatory, we search for neutron fluxes from Galactic candidate sources. We select more than 1000 objects of astrophysical interest, stacking them into target sets. The targets all have declinations within the exposure of the Observatory, ranging from $-90^\circ$ up to $+45^\circ$ for energies above 1 EeV (and up to $+20^\circ$ for energies down to 0.1 EeV). Given that a neutron air shower is indistinguishable from a proton one, there is a significant background due to cosmic rays. A neutron flux from the direction of a candidate source would be identified by a celestial density of events that significantly exceeds the expected density of cosmic rays for that direction. No significant excess is found at any tested target direction, and an upper limit on the neutron flux is calculated for each candidate source.

astro-ph.HE↗

arXiv2026

Investigating Active Galactic Nuclei variability with the Cherenkov Telescope Array Observatory

Blazars, a type of active galactic nuclei (AGN) with relativistic jets pointed at the observer, exhibit flux variability across the electromagnetic spectrum due to particle acceleration in their jets. Power spectral density (PSD) studies show breaks at specific frequencies, particularly in X-rays, linked to the accretion regime and black hole mass. However, very-high-energy gamma-ray PSD breaks remain unexplored due to current instrument limitations. The Cherenkov Telescope Array Observatory (CTAO), with up to ten times greater sensitivity compared to current generation instruments, will allow precise PSD reconstruction and unprecedented study of blazar flares. These flares reveal key insights into particle acceleration, photon production, and jet properties. The AGN monitoring and flare programs in CTAO's Key Science Project aim to deepen our understanding of blazar emissions.

astro-ph.HE↗

arXiv2026

Measuring the muon content of inclined air showers using AERA and the water-Cherenkov detectors of the Pierre Auger Observatory

We present a novel approach for assessing the muon content of air showers with large zenith angles on a combined analysis of their radio emission and particle footprint. We use the radiation energy reconstructed by the Auger Engineering Radio Array (AERA) as an energy estimator and determine the muon number independently with the water-Cherenkov detector array of the Pierre Auger Observatory, deployed on a 1500 m grid. We focus our analysis on air showers with primary energy above 4 EeV to ensure full detection efficiency. Over approximately ten years of accumulated data, we identify a set of 40 high-quality events that are used in the analysis. The estimated muon contents in data are compatible with those for iron primaries as predicted by current-generation hadronic interaction models. This result can be interpreted as a deficit of muons in simulations as a lighter mass composition has been established from Xmax measurements. This muon deficit was already observed in previous analyses of the Auger Collaboration and is confirmed using hybrid events that include radio measurements for the first time.

astro-ph.HE↗

arXiv2025

Stellar physics at sub-nanoradian angular resolution

Many stars -- if they could be imaged with enough angular resolution -- would exhibit features expected from theory but not possible to extract from spectra. We may group these by increasing complexity as follows. First, smooth variations in brightness across the surface, resembling solar limb darkening but much more prominent and involving more processes in stars with fast spin or external tides. Next, there are periodic features: not only oscillations, but also convective cells and starspots, which appear to transit across a star as its spins, and exoplanets that really do transit across the star. Then, there are transients like flares. Current optical interferometers provide synthetic apertures of a few hundred metres and angular resolutions down to about nanoradian ($\simeq 0.2\,$milliarcsecond), enough to resolve some of the above features on the nearest upper main-sequence stars, giants and supergiants. Ongoing projects aims to km-scale synthetic apertures, enough to measure the radius of the nearest white dwarf. In this White Paper we briefly discuss what could be observed with synthetic apertures over $\sim20\,$km -- resolving detail on white dwarfs at the level currently possible on supergiants.

astro-ph.IM↗

arXiv2025

Long-term calibration and validation of stability of the Auger Engineering Radio Array using the diffuse Galactic radio emission

The Auger Engineering Radio Array (AERA) measures radio emission from high-energy extensive air showers. Consisting of 153 autonomous radio-detector stations spread over $17$\,km$^2$, it detects radio waves in the frequency range of $30$ to $80$\,MHz. Accurate characterization of the detector response is crucial for proper interpretation of the collected data. Previously, this was achieved through laboratory measurements of the analog chain and simulations and measurements of the antenna's directional response. In this paper, we perform an absolute calibration using the continuously monitored sidereal modulation of the diffuse Galactic radio emission. Calibration is done by comparing the average frequency spectra recorded by the stations with predictions from seven different models of the full radio sky, accounting for the system response, which includes the antenna, filters, and amplifiers. The analysis of the calibration constants over a period of seven years shows no relevant and no significant ageing effect in the AERA antennas. This result confirms the long-term stability of the detector stations and demonstrates the possibility for a radio detector to effectively monitor ageing effects of other detectors operating over extended periods.

astro-ph.IM↗

arXiv2025

Quasi-constant time gap in multiple rings of elves

We present evidence that the time delay between the multiple rings of elves is not caused by the ground reflection of the electromagnetic pulse produced by intracloud lightning. To investigate temporal differences of multi-elves, we analyzed data from four storms occurring at various times and distances from the Pierre Auger Observatory in Malargüe, Argentina. The Auger fluorescence detector's high temporal resolution of 100 ns enabled the frequent observation of multi-elves, accounting for approximately 23% of the events. By examining the traces of 70 double and 24 triple elves, we demonstrate that the time delay between the rings remains relatively constant regardless of the arc distance to the lightning. These results deviate from the trend expected from the electromagnetic pulse (EMP) ground reflection model, which predicts a decreasing time delay with increasing arc distance from an intracloud lightning at a given height. The first emission ring is due to a direct path of the EMP to the ionosphere, with the reflected EMP creating the second ring. Simulations conducted with this model demonstrate that short energetic in-cloud pulses can generate four-peak elves, and a temporal resolution of at least 25 $μ$s is required to separate them. Therefore, temporal resolution is crucial in the study of multi-elves. Our observations in the Córdoba province, central Argentina, indicate that the current understanding of the mechanism generating these phenomena may be incomplete, and further studies are needed to assess whether multi-elves are more likely related to the waveform shape of the lightning than to its altitude.

physics.space-ph↗

arXiv2025

Search for a diffuse flux of photons with energies above tens of PeV at the Pierre Auger Observatory

Diffuse photons of energy above 0.1 PeV, produced through the interactions between cosmic rays and either interstellar matter or background radiation fields, are powerful tracers of the distribution of cosmic rays in the Galaxy. Furthermore, the measurement of a diffuse photon flux would be an important probe to test models of super-heavy dark matter decaying into gamma-rays. In this work, we search for a diffuse photon flux in the energy range between 50 PeV and 200 PeV using data from the Pierre Auger Observatory. For the first time, we combine the air-shower measurements from a 2 km$^2$ surface array consisting of 19 water-Cherenkov surface detectors, spaced at 433 m, with the muon measurements from an array of buried scintillators placed in the same area. Using 15 months of data, collected while the array was still under construction, we derive upper limits to the integral photon flux ranging from 13.3 to 13.8 km$^{-2}$ sr$^{-1}$ yr$^{-1}$ above tens of PeV. We extend the Pierre Auger Observatory photon search program towards lower energies, covering more than three decades of cosmic-ray energy. This work lays the foundation for future diffuse photon searches: with the data from the next 10 years of operation of the Observatory, this limit is expected to improve by a factor of $\sim$20.

astro-ph.HE↗

arXiv2025

The distribution of ultra-high-energy cosmic rays along the supergalactic plane measured at the Pierre Auger Observatory

Ultra-high-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 31 December 2022, with a total exposure of 135,000 km^2 sr yr. The strongest indication for an excess that we find, with a post-trial 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.

astro-ph.HE↗

arXiv2025

Radio Measurements of the Depth of Air-Shower Maximum at the Pierre Auger Observatory

The Auger Engineering Radio Array (AERA), part of the Pierre Auger Observatory, is currently the largest array of radio antenna stations deployed for the detection of cosmic rays, spanning an area of $17$ km$^2$ with 153 radio stations. It detects the radio emission of extensive air showers produced by cosmic rays in the $30-80$ MHz band. Here, we report the AERA measurements of the depth of the shower maximum ($X_\text{max}$), a probe for mass composition, at cosmic-ray energies between $10^{17.5}$ to $10^{18.8}$ eV, which show agreement with earlier measurements with the fluorescence technique at the Pierre Auger Observatory. We show advancements in the method for radio $X_\text{max}$ reconstruction by comparison to dedicated sets of CORSIKA/CoREAS air-shower simulations, including steps of reconstruction-bias identification and correction, which is of particular importance for irregular or sparse radio arrays. Using the largest set of radio air-shower measurements to date, we show the radio $X_\text{max}$ resolution as a function of energy, reaching a resolution better than $15$ g cm$^{-2}$ at the highest energies, demonstrating that radio $X_\text{max}$ measurements are competitive with the established high-precision fluorescence technique. In addition, we developed a procedure for performing an extensive data-driven study of systematic uncertainties, including the effects of acceptance bias, reconstruction bias, and the investigation of possible residual biases. These results have been cross-checked with air showers measured independently with both the radio and fluorescence techniques, a setup unique to the Pierre Auger Observatory.

astro-ph.HE↗

arXiv2025

Demonstrating Agreement between Radio and Fluorescence Measurements of the Depth of Maximum of Extensive Air Showers at the Pierre Auger Observatory

We show, for the first time, radio measurements of the depth of shower maximum ($X_\text{max}$) of air showers induced by cosmic rays that are compared to measurements of the established fluorescence method at the same location. Using measurements at the Pierre Auger Observatory we show full compatibility between our radio and the previously published fluorescence data set, and between a subset of air showers observed simultaneously with both radio and fluorescence techniques, a measurement setup unique to the Pierre Auger Observatory. Furthermore, we show radio $X_\text{max}$ resolution as a function of energy and demonstrate the ability to make competitive high-resolution $X_\text{max}$ measurements with even a sparse radio array. With this, we show that the radio technique is capable of cosmic-ray mass composition studies, both at Auger and at other experiments.

astro-ph.HE↗