Recently, the KM3NeT collaboration detected an astrophysical neutrino event, KM3-230213A, with an energy of approximately 220 PeV, providing unprecedented insights into the ultra-high-energy Universe. In this study, we introduce a novel likelihood framework designed to leverage this event to constrain the properties of superheavy dark matter (SHDM) decay. Our approach systematically integrates multi-messenger constraints from galactic and extragalactic neutrino flux measurements by IceCube, the absence of comparable neutrino events at IceCube and Auger observatories, and the latest gamma-ray experiment upper limits. Our findings impose the most stringent constraints to date, placing a lower bound on the SHDM lifetime at >= 5 & times; 1029-1030 s. Importantly, we identify, for the first time, the significant potential of galactic neutrino flux measurements in advancing dark matter research. Future investigations targeting astrophysical neutrinos originating from the Galactic Center at energies above 10 PeV will be crucial, not only for understanding the origin of the cosmic-ray knee but also for exploring the possible contributions of superheavy dark matter to our Universe.

In a magnetic field with a complex topology, as can be the Galactic magnetic field, cosmic ray transport cannot simply be described by diffusion parallel and perpendicular to magnetic field lines, because the gradients and curvature of the large-scale magnetic field induce drift motions. These effects become especially important at high energies. Here we revisit the possibility that the competition between diffusion and drifts may lead to a knee in the cosmic ray spectrum. We carry out test-particle simulations of cosmic ray transport in a mock Galactic magnetic field made of a regular large scale component, with a non-trivial topology and a homogeneous and isotropic turbulent magnetic field, with a spectrum that is assumed to be Kolmogorov-like in the basic setup. These simulations are used to infer the escape time and the grammage accumulated by cosmic rays with energy in the TeV–10 PeV energy range. In the case of a large scale magnetic field with a purely azimuthal structure, the drift due to the curvature of magnetic field lines produces a knee in the PeV range, but the model fails to reproduce the grammage, due to the exceedingly low value of the perpendicular diffusion coefficient. If the large scale magnetic field acquires a component perpendicular to the Galactic disc, the parallel diffusion coefficient becomes quickly dominant in terms of particle escape, and drifts are unable to compete. A knee structure does not appear in such a scenario. However, if the parallel diffusion coefficient becomes energy independent at E≳ 1 TeV, a knee may arise around PeV energies due to drift dominance. We discuss two cases in which this situation may occur.

The mass composition of ultra-high-energy cosmic rays (UHECRs) is commonly inferred from the first two moments of the depth of shower maximum, X-max, measured by fluorescence and hybrid detectors. Such analyses require fast and accurate mappings between the moments of X-max and those of the logarithmic mass, ln A, based on realistic air-shower simulations. In this work we provide updated parametrizations of the X-max moments and distributions for air showers initiated by nuclei from proton to iron, simulated with CONEX for three state-of-the-art hadronic interaction models: E-pos-LHC, S-IBYLL,2.3(E), and QGSJ(ET)-III-01. We parametrize the mean depth (X-max) and the variance sigma(2)(X-max) as functions of energy and mass. For the variance we compare a second-order polynomial model with an exponential model. In addition, we model the full Xmax distributions with a three-parameter generalized Gumbel function. The Gumbel parameters are fitted using an unbinned likelihood and are validated by comparing the implied mean and variance with the raw CONEX samples and with the moment parametrizations. Across the full energy range considered, residuals between the parametrizations (or the Gumbel representation) and the simulations are at the level of a few g cm(-2) for the mean and a few (g cm(-2))(2) for the variance, making these parametrizations suitable for precision UHECR composition studies and forward-folding analyses of X-max distributions.

TeV halos are extended very-high-energy gamma-ray sources found around some middle-aged pulsars. The emission spanning several tens of parsecs suggests an efficient confinement of the ultra-relativistic lepton pairs produced by pulsars in their vicinity. The physical mechanism responsible for this suppressed transport has not yet been identified. In some scenarios, pair confinement may be linked to the medium the pulsars are located in. We aim at understanding which type of medium pulsars are probing over their lifetime. We developed a model for the environment probed by moving pulsars, from their birth in core-collapse explosions, where they receive a natal kick, until their entry into the interstellar medium. The model involves: (i) a Monte-Carlo sampling of the properties of the massive-star progenitors of pulsars; (ii) a calculation of the structure of the surrounding medium shaped by these progenitors, for the two cases of isolated stars and star clusters; (iii) a computation of the evolution of supernova remnants in these parent environments. Ultimately, from a distribution of neutron star kick velocities, we assess in which medium pulsars are located as a function of time. We first derive the statistical properties of a fully synthetic Galactic population, and then apply the model to a selection of known pulsars to assess the likely nature of their environment. We show that pulsars escape into the ISM at around 200 kyr, significantly later than the values most commonly used in the literature. The majority of known pulsars with a confirmed TeV halo have high probabilities of still being in their parent environment, which suggests that efficient pair confinement is connected to the region influenced by progenitor stars. In order to test this idea, we provide the probability of still residing in the parent environment for a list of known pulsars.

Quantum backreaction effects may quench Hawking evaporation through a “memory burden”, allowing primordial black holes (PBHs) with formation masses well below 10^15 g to survive to the present and contribute to the dark matter. We show that ultra-high-energy cosmic rays (UHECRs) provide a powerful and previously unexplored probe of this scenario. We compute the proton and neutron emission from memory-burdened PBHs, including the Galactic-halo contribution and the extragalactic proton component, and confront it with the Pierre Auger Observatory proton spectrum and its EeV neutron limits from the Galactic plane. This yields new constraints on the PBH dark-matter fraction as a function of the PBH formation mass and the evaporation-suppression parameter k. For k≳ 3 the non-observation of ultra-high-energy protons leads to bounds competitive with those from UHE gamma rays, while neutron limits remain comparable to high-energy neutrino constraints. Our results highlights the key role of multi-messenger astronomy in constraining beyond-the-standard-model scenarios.

The origin of ultra-high-energy cosmic rays (UHECRs) remains an open question. Extragalactic magnetic fields can modify their propagation and, at sufficiently low energies, suppress the observed flux through the magnetic horizon (MH) effect.} {We quantify the impact of the MH on the propagation of UHECR protons using cosmological simulations and a dedicated numerical framework that follows cosmic rays in a time-evolving background.} {We use \texttt{UMAREL}, a parallel code developed for this study, to propagate UHECR protons through a cosmological volume simulated with ENZO. The magnetic-field configurations are chosen to be consistent with recent radio constraints on magnetic fields in cosmic-web filaments. Unlike stationary approaches, we follow particle trajectories through a sequence of time-evolving snapshots and compare the resulting arrival properties with those in an unmagnetised reference model.} {We find that observationally motivated extragalactic magnetic fields progressively suppress the flux of arriving protons below \(E \lesssim 3 \times 10^{19}\,\mathrm{eV}\) through an effective Magnetic Horizon (MH). We estimate \(R_{\mathrm{MH}} \sim 50\,\mathrm{Mpc}\) for protons with \(E = 10^{18}\,\mathrm{eV}\) and \(R_{\mathrm{MH}} \sim 150\,\mathrm{Mpc}\) for protons with \(E = 10^{19}\,\mathrm{eV}\).} {The MH generated by extragalactic magnetic fields must be taken into account when modelling UHECR propagation and interpreting the spectrum observed in the local Universe.}

Aims. The interpretation of the ultrahigh-energy cosmic-ray (UHECR) spectrum and composition suggests a suppression of the flux below approximate to 1 EeV, as observed by the Pierre Auger Observatory and Telescope Array. A natural explanation for this phenomenon involves magnetic confinement effects. We investigate the possibility that UHECRs self-generate the magnetic turbulence necessary for this confinement via current-driven plasma instabilities.Methods. Specifically, we show that the electric current produced by escaping UHECRs can excite a nonresonant streaming instability in the surrounding plasma. This instability reduces the diffusion coefficient in the source environment and effectively traps particles with energies E less than or similar to 1 EeV & Laplacetrf;443/4 RMpc-3/2 lambda 30 L 44 3 / 4 R Mpc - 3 / 2 lambda 30 $ \mathcal{L}_{44}<^>{3/4} R_{\mathrm{Mpc}}<^>{-3/2} \lambda_{30} $ for times that exceed the age of the Universe. Here, & Laplacetrf;44 is the source luminosity in units of 1044 erg/s, RMpc is the radial size in Mpc, and lambda 30 is the intergalactic magnetic field coherence length in units of 30 Mpc. We discuss the conditions in detail (the source luminosity, the initial magnetic field, and the environment in which this complex phenomenon occurs) that need to be fulfilled in order for self-confinement to take place near a source of UHECRs, and we also discuss the caveats that affect our conclusions. We emphasize that these conclusions were derived within a simplified model framework; their wider applicability requires that the assumptions hold in realistic source environments.Results. By modeling a population of UHECR sources with a luminosity function typical of extragalactic gamma-ray sources, we connected the spectrum of escaping particles to the luminosity distribution. Furthermore, we calculated the contribution of these confined particles to cosmogenic neutrino production and found consistency with current observational constraints. Our results suggest that self-induced turbulence might play an important role in shaping the UHECR spectrum. In particular, it might account for the flux suppression near their sources. This offers a promising framework for interpreting current observations.

The broad shape of the Galactic cosmic-ray knee challenges source models in which all supernova remnants share a nearly universal, sharp maximum rigidity. We investigate whether the knee can instead arise as a population effect, produced by source-to-source variations in the maximum energy of Galactic supernova remnants. We derive the population-averaged spectrum for sources with sharp individual cutoffs and distributed E_max, showing that it is given by an underlying propagated power law multiplied by the survival probability of the cutoff distribution. A lognormal distribution of E_max naturally produces a smooth, continuously curving knee, while a power-law tail gives an approximately constant post-knee steepening. We then connect the lognormal width to supernova-remnant physics through maximum-energy scalings with explosion energy and ambient density, finding that the expected variance is mainly driven by the spread in explosion energies. Fitting the measured proton spectrum with a two-component lognormal-cutoff model, we find that the PeV component requires σ_log_10E_max≃ 0.24. This width is substantially smaller than the variance expected for the full Galactic remnant population, indicating that the PeV component must originate from a more restricted and comparatively homogeneous subset of remnants. Our results show that the knee can be understood as the gradual exhaustion of a heterogeneous population of PeV-capable supernova remnants, without requiring a universal maximum rigidity.

The spectrum and composition of ultra-high-energy cosmic rays (UHECRs) suggest that the population dominating above the ankle releases particles with an unusual hard spectrum at low rigidity, below the EV scale. In self-confinement scenarios, such an apparent hardening arises from transport: escaping UHECRs generate magnetic turbulence that delays their own release from the magnetized environments surrounding their sources. We extend the self-confinement scenario based on the non-resonant streaming instability to a mixed nuclear composition. We describe the confinement region with an effective leaky-box model including escape, photodisintegration, and secondary production. We then compare the resulting spectrum and composition with Auger measurements and compute the associated cosmogenic neutrino and gamma-ray emission. We find that self-generated turbulence can suppress the escaping flux below the EV scale for source luminosities and magnetic-field coherence lengths compatible with UHECR sources hosted in galaxy clusters and propagating through cosmic filaments. During confinement, heavy nuclei efficiently photodisintegrate, producing secondary protons that contribute below the ankle and help account for the observed composition. The predicted neutrino flux remains compatible with current limits, while the diffuse gamma-ray background provides a potentially strong constraint on the most extreme configurations.

The interpretation of the ultra-high-energy cosmic ray spectrum (UHECRs) and composition suggests a suppression of the flux below ∼1 EeV, as observed by the Pierre Auger Observatory and Telescope Array. A natural explanation for this phenomenon involves magnetic confinement effects. We investigate the possibility that UHECRs self-generate the magnetic turbulence necessary for such confinement via current-driven plasma instabilities. Specifically, we show that the electric current produced by escaping UHECRs can excite a non-resonant streaming instability in the surrounding plasma. This instability reduces the diffusion coefficient in the source environment, effectively trapping particles with energies E ≲ 0.6 EeV ℒ_45^1/2 R_Mpc^-1 λ_10^2 for times exceeding the age of the Universe. Here, ℒ_45 is the source luminosity in units of 10^45 erg/s, R_Mpc is the radial size in Mpc, and λ_10 is the intergalactic magnetic field coherence length in units of 10 Mpc. We discuss in detail the conditions, in terms of source luminosity, initial magnetic field, and the environment in which this phenomenon occurs, that need to be fulfilled in order for self-confinement to take place near a source of UHECRs. By modeling a population of UHECR sources with a luminosity function typical of extragalactic gamma-ray sources, we connect the spectrum of escaping particles to the luminosity distribution. Furthermore, we calculate the contribution of these confined particles to cosmogenic neutrino production, finding consistency with current observational constraints. Our results suggest that self-induced turbulence may play an important role in shaping the UHECR spectrum and, in particular, may account for the flux suppression near their sources, offering a promising framework for interpreting current observations.