arXiv subjects

Foteini Oikonomou

Publications and source records attributed to Foteini Oikonomou.

At least 19 recordsLinked to original sources

arXiv2026

GRANDlib: A simulation pipeline for the Giant Radio Array for Neutrino Detection (GRAND)

The operation of upcoming ultra-high-energy cosmic-ray, gamma-ray, and neutrino radio-detection experiments, like the Giant Radio Array for Neutrino Detection (GRAND), poses significant computational challenges involving the production of numerous simulations of particle showers and their detection, and a high data throughput. GRANDlib is an open-source software tool designed to meet these challenges. Its primary goal is to perform end-to-end simulations of the detector operation, from the interaction of ultra-high-energy particles, through -- by interfacing with external air-shower simulations -- the ensuing particle shower development and its radio emission, to its detection by antenna arrays and its processing by data-acquisition systems. Additionally, GRANDlib manages the visualization, storage, and retrieval of experimental and simulated data. We present an overview of GRANDlib to serve as the basis of future GRAND analyses.

astro-ph.IM↗

arXiv2026

Anisotropy-driven constraints on the transition from Galactic to extragalactic cosmic-ray sources

The spatial distribution of the cosmic-ray (CR) flux above $8\,\mathrm{EeV}$ exhibits a significant dipole in agreement with expectations from extragalactic sources following the large-scale structure. At the ankle, the direction changes suddenly and is compatible with the Galactic center for energies $\lesssim4\,\mathrm{EeV}$. We leverage the direction and strength of the dipole anisotropy as diagnostics of the possible sources of the intermediate-mass nuclei below the ankle that cannot be associated with the continuation of the Peters cycle of lower-energy Galactic CRs. Using 3D-simulations of CR propagation in up-to-date Galactic magnetic field models, we show that the dipole anisotropy of both continuous and transient individual Galactic sources and source distributions exceeds the measurements by $>3σ$. An extragalactic origin of the intermediate nuclei below the ankle instead agrees with all data, where the observed change in dipole direction is guided by the fading heavy Galactic component dominant at the second knee.

astro-ph.HE↗

arXiv2026

Search for neutrino emission from blazar $γ$-ray flares accounting for possible neutrino time delays

We report the results of the search for the high-energy neutrino emission associated with blazar flares, accounting for a possible lag of neutrinos with respect to the electromagnetic emission, either due to the slowness of the proton energy losses in $pγ$ collisions and/or proton acceleration. We perform two tests, cross-matching neutrinos with energies $E_ν \gtrsim 100$ TeV from the public catalogue of neutrino alerts IceCat-1 with active galactic nuclei from two source samples based on 1) the MOJAVE database and 2) the CGRaBS catalogue, and utilising Fermi-LAT light curves from the public light curve repository. We scan over a wide range of values of the jet-frame time delay $t^{\prime}_{\mathrm{delay}}$ between the neutrino arrival and the time of the prior major $γ$-ray flare and find a pre-trial $\sim 2σ$ correlation at $t^{\prime}_{\mathrm{delay}} \sim 10^{3}$ d, which is consistent ($p_{\mathrm{post-trial}} \sim 0.1$) with expectations under the null hypothesis after trial correction.

astro-ph.HE↗

arXiv2026

Leptonic and hadronic models of high-energy nebula around V4641 Sgr

A prominent, 200-pc-scale high-energy nebula surrounding the microquasar V4641 Sgr is the brightest known gamma-ray source in the Southern sky at $E > 100\,\mathrm{TeV}$. In this paper, we develop self-consistent leptonic, hadronic, and leptohadronic models that reproduce both the observed spectrum and morphology of the source. Purely leptonic models are energetically more favorable yet they require rather specific morphological assumptions. The gamma-ray morphology of the source can be better explained within a hadronic scenario based on the identification of cold gas structures spatially correlated with the observed gamma-ray emission. However, a purely hadronic model for the source emission requires a substantial energy reservoir in protons and fails to reproduce the extended x-ray emission recently detected by XRISM. We show that emission including a combination of leptonic and hadronic components can reproduce both the spectral and morphological properties of the source. We provide predictions for the x-ray and neutrino spectra of the nebula that can discriminate the hadronic and leptonic contributions to the overall source signal.

astro-ph.HE↗

arXiv2026

Constraining the contribution of Seyfert galaxies to the diffuse neutrino flux in light of point source observations

Recently, the IceCube Collaboration reported evidence for TeV neutrino emission from several nearby Seyfert galaxies, with the highest significance found for NGC 1068. Assuming stochastic proton acceleration in magnetized turbulence inside the corona, we model the neutrino emission of Seyfert galaxies as a function of their X-ray luminosity. Applying our model to NGC 1068, we obtain a good fit to the public IceCube data and constrain the coronal radius to $\lesssim 5 R_S$ by comparing our MeV $γ$-ray predictions to Fermi-LAT observations. Extending to the full Seyfert population, we estimate their diffuse neutrino contribution and find that they can explain a significant fraction of the observed flux below $10\,\mathrm{TeV}$. However, scenarios with highly turbulent coronae and high cosmic-ray pressure across the population are ruled out. In particular, if all sources shared the best-fit parameters obtained for NGC 1068, their cumulative neutrino emission would exceed current upper limits at TeV energies by $3.8σ$. Our results, informed by both neutrino and $γ$-ray data, show that those Seyfert galaxies that emerge as neutrino point sources must be exceptionally efficient neutrino emitters and are not representative of the broader population.

astro-ph.HE↗

arXiv2026

Towards the Giant Radio Array for Neutrino Detection (GRAND): the GRANDProto300 and GRAND@Auger prototypes

The Giant Radio Array for Neutrino Detection (GRAND) is a proposed multi-messenger observatory of Ultra-High-Energy (UHE) particles of cosmic origin. Its main goal is to find the long-sought origin of UHE cosmic rays by detecting large numbers of them and the secondary particles created by their interactions like gamma rays and neutrinos. The GRAND Collaboration plans to achieve this using large arrays of radio antennas that look for the radio signals emitted by the air showers initiated by the interactions of the UHE particles in the atmosphere. Since 2023, three small-scale prototype GRAND arrays have been in operation: GRAND@Nançay in France, GRAND@Auger in Argentina, and GRANDProto300 in China. Together, their goal is to validate the detection principle of GRAND under prolonged field conditions, achieving efficient, autonomous radio-detection of air showers. We describe the hardware, software, layout, and operation of the GRAND prototypes. Using their data, we show a first characterization of the local electromagnetic environment of each site and a measurement of the Galactic synchrotron emission. Despite challenges, the successful operation of the prototypes confirms that the GRAND instrumentation is apt to address the goals of the experiment and lays the groundwork for its ensuing stages.

astro-ph.IM↗

arXiv2026

Testing the Association of Supermassive Black Hole Infrared Flares and High-energy Neutrinos

The physical origin of the observed cosmic neutrinos remains an open question and the subject of active research. While matter accretion onto supermassive black holes is long thought to accelerate particles to high energies, it has recently been suggested that tidal disruption events, and accretion flares in general, with prominent IR echoes can account for a fraction of the diffuse high-energy neutrino signal. Motivated by this result, we compile a sample of nearby accretion flares detected in the NEOWISE survey featuring strong IR echoes, and we cross-match it with the latest catalog of neutrino alerts, IceCat-1. We recover only a single spatial coincidence between the two catalogs, consistent with a chance coincidence. We find no temporal and spatial coincidences between the two samples, which, given the properties of our sample, appears to challenge previous conclusions. We discuss the physical implications of our results and potential future explorations.

astro-ph.HE↗

arXiv2025

A Comprehensive Hadronic Code Comparison for Active Galactic Nuclei

We perform the first dedicated comparison of five hadronic codes (AM$^3$, ATHE$ν$A, B13, LeHa-Paris, and LeHaMoC) that have been extensively used in modeling of the spectral energy distribution (SED) of jetted active galactic nuclei. The purpose of this comparison is to identify the sources of systematic errors (e.g., implementation method of proton-photon interactions) and to quantify the expected dispersion in numerical SED models computed with the five codes. The outputs from the codes are first tested in synchrotron self-Compton scenarios that are the simplest blazar emission models used in the literature. We then compare the injection rates and spectra of secondary particles produced in pure hadronic cases with monoenergetic and power-law protons interacting on black-body and power-law photon fields. We finally compare the photon SEDs and the neutrino spectra for realistic proton-synchrotron and leptohadronic blazar models. We find that the codes are in excellent agreement with respect to the spectral shape of the photons and neutrinos. There is a remaining spread in the overall normalization that we quantify, at its maximum, at the level of $\pm 40\%$. This value should be used as an additional, conservative, systematic uncertainty term when comparing numerical simulations and observations.

astro-ph.HE↗

arXiv2025

Insights from leptohadronic modelling of the brightest blazar flare

The blazar 3C 454.3 experienced a major flare in November 2010, making it the brightest $γ$-ray source in the sky of the Fermi Large Area Telescope (LAT). We obtain seven daily consecutive spectral-energy distributions (SEDs) of the flare in the infrared, optical, ultraviolet, X-ray and $γ$-ray bands with publicly available data. We simulate the physical conditions in the blazar and show that the observed SEDs are well reproduced in the framework of a "standing feature" where the position of the emitting region is almost stationary, located beyond the outer radius of the broad-line region and into which fresh blobs of relativistically moving magnetised plasma are continuously injected. Meanwhile, a model with a single "moving blob" does not describe the data well. We obtain a robust upper limit to the amount of high-energy protons in the jet of 3C 454.3 from the electromagnetic SED. We construct a neutrino light curve of 3C 454.3 and estimate the expected neutrino yield at energies $\geq 100$ TeV for 3C 454.3 to be up to $6 \times 10^{-3}$ $ν_μ$ per year. Finally, we extrapolate our model findings to the light curves of all Fermi-LAT flat-spectrum radio quasars. We find that next-generation neutrino telescopes are expected to detect approximately one multimessenger ($γ+ ν_μ$) flare per year from bright blazars with neutrino peak energy in the hundreds TeV -- hundreds PeV energy range and show that the electromagnetic flare peak can precede the neutrino arrival by months to years.

astro-ph.HE↗

arXiv2025

Beyond first light: Global monitoring for high-energy neutrino astronomy

Decades of progress have culminated in first light for high-energy neutrino astronomy: the identification of the first astrophysical sources of TeV-PeV neutrinos by the IceCube neutrino telescope, the active galactic nuclei NGC 1068 and TXS 0506+056. Today, the prospect of going beyond first light to build high-energy neutrino astronomy in earnest by discovering many more neutrino sources is hampered by the relatively low rate of neutrino detection and the limited view of the sky afforded by IceCube, the single cubic-kilometer-scale neutrino telescope in operation. Yet, this will not stand for much longer. Already today, and over the next 10-20 years, the combined observations of new neutrino telescopes, larger and distributed around the world, will have the potential for transformative progress. Together, they will increase the global rate of neutrino detection by up to 30 times and continuously monitor the entire sky. Within a new joint analysis network - the Planetary Neutrino Monitoring network (PLEnuM) - we make detailed forecasts for the discovery of steady-state astrophysical sources of high-energy neutrinos. We show that a combined analysis of global data will expedite source discovery - in some cases, by decades - and enable the detection of fainter sources anywhere in the sky, discovering up to tens of new neutrino sources.

astro-ph.HE↗

arXiv2025

The Giant Radio Array for Neutrino Detection (GRAND) Collaboration -- Contributions to the 39th International Cosmic Ray Conference (ICRC 2025)

The Giant Radio Array for Neutrino Detection (GRAND) is an envisioned observatory of ultra-high-energy particles of cosmic origin, with energies in excess of 100 PeV. GRAND uses large surface arrays of antennas to look for the radio emission from extensive air showers that are triggered by the interaction of ultra-high-energy cosmic rays, gamma rays, and neutrinos in the atmosphere or underground. In particular, for ultra-high-energy neutrinos, the future final phase of GRAND aims to be sensitive enough to detect them in spite of their plausibly tiny flux. Three prototype GRAND radio arrays have been in operation since 2023: GRANDProto300, in China, GRAND@Auger, in Argentina, and GRAND@Nançay, in France. Their goals are to field-test the GRAND detection units, understand the radio background to which they are exposed, and develop tools for diagnostic, data gathering, and data analysis. This list of contributions to the 39th International Cosmic Ray Conference (ICRC 2025) presents an overview of GRAND, in its present and future incarnations, and a first look at data collected by GRANDProto300 and GRAND@Auger, including the first cosmic-ray candidates detected by them.

astro-ph.IM↗

arXiv2025

Ultra-high-energy cosmic rays from ultra-fast outflows of active galactic nuclei

We investigate ultra-fast outflows (UFOs) in active galactic nuclei (AGN) as potential sources of ultra-high-energy cosmic rays (UHECRs). We focus on cosmic-ray nuclei, an aspect not explored previously. These large-scale, mildly-relativistic outflows, characterised by velocities up to half the speed of light, are a common feature of AGN. We study the cosmic-ray spectrum and maximum energy attainable in these environments with 3D CRPropa simulations and apply our method to 86 observed UFOs. Iron nuclei can be accelerated up to $\sim10^{20}\,$eV at the wind-termination shock in some UFOs, but the escaping flux is strongly attenuated due to photonuclear interactions with intense AGN photon fields. The maximum energy of nuclei escaping a typical UFO is limited by photodisintegration to below $\sim 10^{17}\,$eV. However, in the most extreme $5-10\%$ of UFOs, helium (nitrogen) [iron] nuclei can escape with energy exceeding $10^{17.4}$ ($10^{17.8}$) $[10^{18.4}]\,$eV. Protons and neutrons, either primaries or by-products of photodisintegration, escape UFOs with little attenuation, with half of the observed UFOs reaching energies exceeding $10^{18}\,$eV. Thus, UFOs emerge as viable sources of the diffuse cosmic-ray flux between the end of the Galactic cosmic-rays and the highest-energy extragalactic flux. For a few UFOs in our sample, nuclei escape without photodisintegration with energy up to $10^{19.8}\,$eV. This occurs during low-emission states of the AGN, which would make UFOs intermittent sources of UHECR nuclei up to the highest observed energies. The role of UFOs as UHECR sources is testable with neutrino telescopes due to a substantial accompanying flux of PeV neutrinos.

astro-ph.HE↗

arXiv2025

KM3-230213A: An Ultra-High Energy Neutrino from a Year-Long Astrophysical Transient

The Km3NET collaboration has recently reported the detection of a neutrino event with energy in excess of 100 PeV. This detection is in 2.5-3$σ$ tension with the upper limit on the neutrino flux at this energy imposed by IceCube and the Pierre Auger Observatory, if the event is considered part of the diffuse all-sky neutrino flux. We explore an alternative possibility that the event originates from a flare of an isolated source. We show that the data of Km3NET, IceCube and the Pierre Auger Observatory are consistent with the possibility of a source flare of duration $T \lesssim 2$ yr with muon neutrino flux $F \approx 3\times 10^{-10}(1\mbox{ yr }/ T)$ erg cm$^{-2}$ s$^{-1}$. Constraints on the neutrino spectrum indicate that the protons responsible for the neutrino emission have a very hard spectrum in the $E_p\gtrsim 10^{19}$ eV energy range, or otherwise that the neutrinos are produced by photohadronic interactions with infrared photons. The all-sky rate of similar neutrino flaring sources is constrained to be $R\lesssim 0.4/$ yr.

astro-ph.HE↗

arXiv2025

Ideas and Requirements for the Global Cosmic-Ray Observatory (GCOS)

After a successful kick-off meeting in 2021. two workshops in 2022 and 2023 on the future Global Cosmic-Ray Observatory (GCOS) focused mainly on a straw man design of the detector and science possibilities for astro- and particle physics. About 100 participants gathered for in-person and hybrid panel discussions. In this report, we summarize these discussions, present a preliminary straw-man design for GCOS and collect short write-ups of the flash talks given during the focus sessions.

astro-ph.IM↗

arXiv2024

High-energy neutrino transients and the future of multi-messenger astronomy

The recent discovery of high-energy astrophysical neutrinos and first hints of coincident electromagnetic and neutrino emission herald the beginning of the era of multi-messenger astronomy. Due to their high power, transient sources are expected to supply a significant fraction of the observed energetic astroparticles, through enhanced particle acceleration and interactions. Here, we review theoretical expectations of neutrino emission from transient astrophysical sources and the current and upcoming experimental landscape, highlighting the most promising channels for discovery and specifying their detectability.

astro-ph.HE↗

arXiv2024

Multi-messenger signature of cosmic rays from the microquasar V4641 Sgr propagating along a Galactic Magnetic Field line

The recently detected extended, very-high-energy gamma-ray emission from the microquasar V4641 Sgr reveals a puzzling 200-parsec-long jet-like structure significantly misaligned with its radio jet. We propose that this gamma-ray structure is produced by high-energy cosmic-ray particles escaping from the microquasar along ordered field lines of the Galactic Magnetic Field and interacting with the interstellar medium. We show that if the gamma-ray emission is produced by interactions of high-energy cosmic ray nuclei, the system is detectable by future multi-km3 neutrino detectors. We argue that gamma-ray observations of jet-like features adjacent to high-energy sources in the Milky Way provide a new method to measure the regular and turbulent components of the Galactic magnetic field at different locations in the Milky Way.

astro-ph.HE↗

arXiv2024

The Giant Radio Array for Neutrino Detection (GRAND) Collaboration -- Contributions to the 38th International Cosmic Ray Conference (ICRC 2023)

The Giant Radio Array for Neutrino Detection (GRAND) is an envisioned observatory of ultra-high-energy particles of cosmic origin, with energies in excess of 100 PeV. GRAND uses large surface arrays of autonomous radio-detection units to look for the radio emission from extensive air showers that are triggered by the interaction of ultra-high-energy cosmic rays, gamma rays, and neutrinos in the atmosphere or underground. In particular, for ultra-high-energy neutrinos, the future final phase of GRAND aims to be sensitive enough to discover them in spite of their plausibly tiny flux. Presently, three prototype GRAND radio arrays are in operation: GRANDProto300, in China, GRAND@Auger, in Argentina, and GRAND@Nancay, in France. Their goals are to field-test the design of the radio-detection units, understand the radio background to which they are exposed, and develop tools for diagnostic, data gathering, and data analysis. This list of contributions to the 38th International Cosmic Ray Conference (ICRC 2023) presents an overview of GRAND, in its present and future incarnations, and a look at the first data collected by GRANDProto13, the first phase of GRANDProto300.

hep-ex↗

arXiv2024

The Giant Radio Array for Neutrino Detection (GRAND) Collaboration -- Contributions to the 10th International Workshop on Acoustic and Radio EeV Neutrino Detection Activities (ARENA 2024)

This is an index of the contributions by the Giant Radio Array for Neutrino Detection (GRAND) Collaboration to the 10th International Workshop on Acoustic and Radio EeV Neutrino Detection Activities (ARENA 2024, University of Chicago, June 11-14, 2024). The contributions include an overview of GRAND in its present and future incarnations, methods of radio-detection that are being developed for them, and ongoing joint work between the GRAND and BEACON experiments.

astro-ph.IM↗