A review of the current status of the field of Ultra-High-Energy Cosmic Ray (UHECR) including a summary of remaining open questions waspresented in the white paper “Ultra-High Energy Cosmic Rays: at the Intersection of the Cosmic and Energy Frontiers” (Astropart. Phys. 147 (2023) 102794; 2205.05845). The authors concluded that two types of next-generation detectors are needed to answer these questions: high-accuracy instruments and detectors that maximize exposure at the highest energies. The Probe Of Extreme Multi-Messenger Astrophysics (POEMMA), a proposed dual-satellite observatory, exemplifies the latter class and is designed to increase statistics of the highest-energy cosmic rays and to detect very-high-energy neutrinos following multi-messenger alerts. POEMMA-Balloon with Radio (PBR) implements a compact, balloon-borne version of the POEMMA concept, adapted for a Super-Pressure Balloon flight from Wanaka, New Zealand, with an expected campaign exceeding 20 days. PBR couples a wide field-of-view Schmidt telescope and a hybrid optical focal surface with a dedicated radio instrument to deliver simultaneous, complementary measurements of extensive air showers. The mission will validate the fluorescence detection strategy from space and raise technology readiness for a POEMMA-like space mission by observing UHECR-induced fluorescence light from suborbital altitudes, obtaining the first simultaneous optical Cherenkov and radio observations of high-altitude horizontal air showers above the cosmic-ray knee (E > 3 PeV), enabling energy-spectrum and composition studies at the PeV scale, and performing follow-ups of multi-messenger alerts to search for very-high-energy neutrinos via upward-going air showers. This paper summarizes the PBR payload and its expected performance.

Very recently, a significant ∼ 20 GeV gamma-ray excess in the Milky Way halo has been reported anda dark matter origin has been suggested. The inferred dark matter parameters are m χ ∼ 0.5–0.8 TeV and 〈σv〉 ∼ (5–8) × 10-25 cm3 s-1 for the bb̅ channel. If correct, prominent antiproton emission is produced and can be directly tested by the AMS-02 data. In this work we calculate the corresponding antiproton emission and show that the expected flux at ∼ 100 GeV is already above the AMS-02 observation. A proper treatment on the antiproton background resulting from the high energy cosmic ray propagation would suggest an annihilation cross section of < 2 × 10-25 cm3 s-1, which is a few × 10 times lower than that needed to interpret the potential signal. We therefore conclude that the ∼ 20 GeV gamma-ray excess in the Milky Way halo is not a viable dark matter signal.

Abstract “PeVatrons” refer to astrophysical sources capable of accelerating particles to energies around 10 15 electron volts and higher, potentially contributing to the cosmic-ray spectrum in the knee region. Recently, the Large High Altitude Air Shower Observatory (LHAASO) has discovered a large number of PeVatrons, allowing us to investigate in greater depth the contributions of these sources to cosmic rays above the knee region. However, high-energy gamma rays undergo attenuation due to interactions with the interstellar radiation field and cosmic microwave background radiation, requiring corrections to restore the true spectral characteristics at the source. In this study, using the interstellar radiation field model extracted from the galprop code, we quantitatively calculated the spectral absorption effects of PeV sources listed in the first LHAASO source catalog, with some sources showing absorption reaching ∼30% at 100 TeV and ∼80% at 3 PeV. We also calculated the high-energy gamma-ray absorption effects of Galactic microquasars, which are potential PeVatrons. By calculating the absorption effects, it will help differentiate the radiation mechanisms of the acceleration sources, to investigate whether they are the origins of high-energy cosmic rays.

Observation of in-ice Askaryan radiation from high-energy cosmic rays

Gravitational-wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) is a constellation of all-sky monitors in hard X-ray and gamma-ray bands, primarily observing high-energy transients such as gamma-ray bursts, soft gamma-ray repeaters, solar flares, and terrestrial gamma-ray flashes. As GECAM has the highest temporal resolution (0.1 μs) among instruments of its kind, it can identify the so-called simultaneous events (STEs) that deposit signals in multiple detectors nearly at the same time (with a time window of 0.3 μs). However, the properties and origin of STEs have not yet been explored. In particular, STEs may impact the observation of high-energy transients. In this work, we present the first systematic study of the properties of STEs detected by GECAM, including the morphology, energy deposition, and the dependence on the geomagnetic latitude. Based on their properties, we suggest that these STEs probably result from direct interactions between high-energy charged cosmic rays and the satellite. GEANT4 Monte Carlo simulations using the GECAM spacecraft mass model were carried out to provide additional support for this interpretation. Our result indicates that GECAM could potentially detect and characterize the high-energy cosmic rays through STEs, thereby extending its scientific capability.

Fast simulation of muon detectors in high energy cosmic ray air showers

We investigate the propagation of ultraheavy (UH) nuclei as ultrahigh-energy cosmic rays (UHECRs). We show that their energy loss lengths at ≲300 EeV are significantly longer than those of protons and intermediate-mass nuclei, and that the highest-energy cosmic rays with energies beyond ∼100 EeV, including the Amaterasu particle, may be UH-UHECRs. For the first time, we derive constraints on the contribution of UH-UHECR sources, and find that the current data are consistent with energy generation rate densities of UHECRs from collapsars and neutron star mergers. Our model predicts that the mean value of the depth of shower maximum is lower than that for iron nuclei beyond 100EeV, which can be tested with future composition measurements, e.g., AugerPrime and the Global Cosmic Ray Observatory. In addition, the spectral tension between the Telescope Array (TA) and the Pierre Auger Observatory can be alleviated by considering the enhanced contribution of UHECRs-including UH nuclei-from a nearby transient.

Abstract In the standard Galactic cosmic-ray (CR) paradigm, protons are accelerated up to ∼1 PeV by Galactic sources. While supernova remnants have been traditionally considered as the primary accelerators, recent observations by LHAASO and HAWC have detected very-high-energy (VHE) gamma rays exceeding 100 TeV from several microquasars, suggesting that these X-ray binaries can accelerate CRs beyond 1 PeV. We investigate the escape process of CRs from microquasars, focusing on the energy-dependent transport mechanisms. High-energy CRs are likely to have long mean free paths and move ballistically on scales smaller than their mean free path, while lower-energy CRs undergo diffusive propagation. This transition results in a spectral break in the CR distribution around the microquasar. We calculate CR energy spectra within a 10–30 pc radius for various diffusion coefficients and timescales. Our model predicts a spectral break and hardening at E p ∼ 10–100 TeV when the standard diffusion coefficient for the interstellar space is assumed. However, current VHE gamma-ray observations do not show clear spectral breaks, suggesting that the diffusion coefficient may be significantly reduced near microquasars, possibly due to magnetic field amplification by CR-driven turbulence.

The high altitude detection of astronomical radiation (HADAR) project proposes the use of a refracting telescope composed of four 5.0 m diameter water lenses arranged in a square configuration (100 m × 100 m). This configuration features a wide field of view (FoV, up to 0.84 sr) and low-energy threshold characteristics for observing Cherenkov light generated by high-energy cosmic rays in atmospheric air showers. The Fresnel lens exhibits excellent imaging performance, lightweight characteristics, mature manufacturing processes, strong adaptability in high-altitude low-temperature environments, and facilitates array deployment. The lens has been validated through a series of pilot missions in the Joint Exploratory Missions for an Extreme Universe Space Observatory program, leading to the proposal of a telescope unit design that utilizes the Fresnel lens as an alternative to the water lens. This study simulates and examines the effects of parameters such as the radius of curvature, tooth width, and Fresnel lens thickness on the focal length and image spot ( ). To this end, five Fresnel lenses with the same focal length as the 5.0 m diameter water lens were designed, the best focusing positions under different incident angles were extracted, and the curved image surface was constructed through fitting. The results indicate that the imaging quality of the Fresnel lens depends on the radius of curvature. With increasing focal length, decreases gradually until it remains unchanged. The tooth width and thickness of the lens affect the structural complexity of the lens and have little impact on imaging quality. The curved image surface design can effectively suppress the aberrations and changes in the solid angle caused by increased incidence angles, maintaining an acceptance that is approximately consistent across different incidence angles. To meet the scientific objectives (wide FoV and low-energy threshold) consistent with HADAR and consider the engineering constraints (focal length 10 m), we select a Fresnel lens with a diameter of 2.0 m and a focal length of 5.3 m (FoV angle 29°, total acceptance 9.81 m2·sr) as the basic lens unit for subsequent array performance simulation. This is based on the premise that the total acceptance is not lower than that of the water lens unit (7.43 m2·sr), the on-axis imaging is less than 7.5 cm, and the FoV is as wide as possible.

Abstract Diffuse γ -ray emission is a key probe of cosmic-ray (CR) distribution within the Galaxy. However, the discrepancies between observations and theoretical model expectations highlight the need for refined uncertainty estimates. In the literature, spatial and temporal variability of lepton flux has been discussed as an uncertainty in diffuse γ -ray estimation. In the present work, we demonstrate that variability in the high-energy CR hadron flux is an important, yet previously underappreciated, source of uncertainty in diffuse γ -ray estimates. To assess this effect, we perform fully three-dimensional, time-dependent GALPROP simulations of CR protons injected from discrete Galactic sources. Our results reveal that the uncertainty in the hadronic component of diffuse γ rays is nonnegligible and can be comparable to, or even exceed, current experimental uncertainties at very high energies. This finding challenges the conventional assumption that only leptonic fluctuations are relevant to diffuse γ -ray modeling.

During the course of its mission, ESA's spacecraft has generated a map of the stars of the Galaxy of exquisite detail. While in its L2 orbit, the satellite has been exposed to high-energy cosmic rays and solar particles, which caused permanent damage to its CCDs. The main effect of radiation damage on data is the distortion of its images and spectra that is caused by the charge transfer inefficiency (CTI) of the CCDs during the readout process. When this is not taken into account, it can result in inaccurate measurements of a star's location and flux. We analysed and modelled the effect of CTI on the serial readout direction, which is larger than in the parallel due to the presence of CCD manufacturing defects. A pixel-based physically motivated CTI model, ţiPixel, was developed to characterise the damage in CCDs. The model was calibrated using dedicated serial CTI diagnostic data, taken every three to four months over the course of the mission. The model is shown to be a good representation of the observed signatures of CTI in the calibration datasets, and its parameters reveal significant insights into the nature of the CCD defects generated by space irradiation. The damage in the serial direction slightly increases linearly over time in general, with sudden step changes after strong solar flares and coronal mass ejections directed towards Earth. The serial CTI showed a further step increase as a consequence of the engineering CCD annealing experiment that was carried out after the completion of science observations. Gaia Gaia Gaia Gaia

Abstract “PeVatrons” refer to astrophysical sources capable of accelerating particles to energies of ∼PeV and higher, potentially contributing to the cosmic-ray spectrum in the knee region. Recently, the High-Altitude Water Cherenkov Observatory (HAWC) and the Large High Altitude Air Shower Observatory (LHAASO) have discovered a new type of PeVatron–X-ray binary, allowing us to investigate in greater depth the contributions of these sources to cosmic rays around the knee region. There are hundreds of X-ray binaries observed in our Galaxy that are potential PeVatrons. In this work, we derive the radial distribution of X-ray binaries in the Galaxy. Then we use the DRAGON package to calculate energy spectrum, anisotropy of cosmic rays, and the resulting diffuse gamma-ray emissions, after considering them as factories of cosmic rays in the knee energy bands. Our findings show that the contributions from X-ray binary PeVatrons may be dominant. More X-ray binary PeVatrons can be observed by LHAASO and HAWC in the future and will confirm the contribution of X-ray binaries to high-energy cosmic rays.

Abstract We present the discovery of floods of short duration transient radio events detected simultaneously by at least 3 of 7 small horn radio telescopes, when continuously observing the sky at 1421.5 ± 3.0 MHz. The goal of these observations is to detect transient radio flashes, which are expected when high-energy cosmic rays hit Earth’s atmosphere. We find a variety of radio flash patterns, including expected isolated radio flashes, a grouping of a few events, primarily seen near Sun transit, and floods of events, where thousands of flashes are seen over periods of hours. The origin of these flood events have not yet been determined, but are likely due to a variety of causes, including solar events, bursts of cosmic rays, human interference, weather, and (perhaps) communication from extraterrestrial sources. High school students and teachers are encouraged to participate in the building of electronics and telescopes for radio observations.

Fluctuations and structure across a wide range of spatial and temporal scales are frequently studied in the solar wind. The properties of the low-frequency fluctuations are of relevance to turbulent energy injection into the plasma and the transport of high-energy cosmic rays. Correlation analysis of decade-long intervals of interplanetary data permits study of fluctuations at time scales much longer than suitably defined correlation times, and therefore at frequencies well below those associated with the Kolmogorov inertial range of in situ turbulence. At the frequencies of interest, we study the familiar occurrence of the [Formula: see text] spectral signature. We also study point spectral features due to solar rotation and their relation with the [Formula: see text] signal. We report properties at timescales ranging from minutes up to years, using data selected by wind speed, phase of solar cycle, and cartesian components of the magnetic field. A surprising finding is that the power in solar rotation harmonics is consistent with an extension of the [Formula: see text] spectrum, down to frequencies as low as around [Formula: see text]. The presence of a broadband [Formula: see text] spectrum across different wind types supports the interpretation that [Formula: see text] signals may be related to or even originate from the solar dynamo.

Introduction: Radio emissions from extensive air showers can provide valuable insights into the properties of high-energy cosmic rays and the atmospheric conditions through which they propagate. While these emissions can be accurately modeled under fair-weather conditions, the presence of strong and complex atmospheric electric fields during thunderstorms can significantly affect the radio signal. Methods: Measuring these fields directly is challenging due to the instability and unpredictability of thunderclouds, making indirect methods essential. The macroscopic model MGMR3D (Macroscopic GeoMagnetic Radiation, three-dimensional model) offers a semi-analytic approach to reconstructing these atmospheric electric field structures, while the microscopic simulation CoREAS (CORSIKA-based Radio Emission from Air Showers) serves as a detailed benchmark. In this study, the outputs of MGMR3D were validated against CoREAS to assess its ability to reproduce radio emission patterns under multi-layer thunderstorm field configurations. Both models were used to determine the intensity, linear polarization, and circular polarization of radio emissions generated by extensive air showers as they traversed layered electric field structures, including complex four-layer models. Results: The comparison demonstrated that MGMR3D can reliably reproduce the results obtained from CoREAS for both simple electric field configurations as well as intricate four-layer structures. The good agreement between both models indicates that MGMR3D captures the essential physics governing radio emission in strong electric field conditions. Conclusion: The findings demonstrate that MGMR3D, through its optimization procedure, is an efficient tool for reconstructing the internal electric field structures of thunderclouds; consequently, the model is suitable for extracting detailed electric field profiles from experimental radio measurements. This represents an important step in the use of radio detection as a diagnostic tool for atmospheric electricity during thunderstorms.

NUSES is a planned space mission aiming to test new observational and technological approaches related to the study of low-energy cosmic rays, gamma rays, and high-energy astrophysical neutrinos. Two scientific payloads will be hosted onboard the NUSES space mission: Terzina and Zirè. Terzina will be an optical telescope readout by SiPM arrays for the detection and study of Cerenkov light emitted by Extensive Air Showers (EASs) generated by high-energy cosmic rays and neutrinos in the atmosphere. Zirè will focus on the detection of protons and electrons up to a few hundred MeV and 0.1–30 MeV photons and will include the Low-Energy Module (LEM). The LEM will be a particle spectrometer devoted to the observation of fluxes of low-energy electrons in the 0.1–7-MeV range and protons in the 3–50 MeV range in low Earth orbit (LEO) followed by the hosting platform. The detection of Particle Bursts (PBs) in this physics channel of interest could provide insights into understanding complex phenomena such as possible correlations between seismic events or volcanic activity with the collective motion of particles in the plasma populating Van Allen belts. With its compact size and limited acceptance, the LEM will allow the exploration of hostile environments such as the South Atlantic Anomaly (SAA) and the inner Van Allen belt, in which the anticipated electron fluxes are on the order of 106 to 107 electrons per square centimeter per steradian per second. Concerning the vast literature on space-based particle spectrometers, the innovative aspect of the LEM resides in its compactness, within 10×10×10 cm3, and in its “active collimation” approach to dealing with the problem of multiple scattering at these low energies. In this work, the geometry of the detector, its detection concept, its operation modes, and the hardware adopted will be presented. Some preliminary results from a Monte Carlo simulation (Geant4) will be shown.

Future space detectors for Ultra High Energy neutrinos and cosmic rays will utilize Cherenkov telescopes to detect forward-beamed Cherenkov light produced by charged particles in Extensive Air Showers (EASs). A Cherenkov detector can be equipped with an array of Silicon Photo-Multiplier (SiPM) pixels, which offer several advantages over traditional Photo-Multiplier Tubes (PMTs). SiPMs are compact and lightweight and operate at lower voltages, making them well-suited for space-based experiments. The SiSMUV (SiPM-based Space Monitor for UV-light) is developing a SiPM-based Cherenkov camera for PBR (POEMMA Baloon with Radio) at INFN Napoli. To understand the response of such an instrument, a comprehensive simulation of the response of individual SiPM pixels to incident light is needed. For the accurate simulation of a threshold trigger, this simulation must reproduce the current produced by a SiPM pixel as a function of time. Since a SiPM pixel is made of many individual Avalanche Photo-Diodes (APDs), saturation and pileup in APDs must also be simulated. A Gaussian mixture fit to ADC count spectrum of a SiPM pixel exposed to low levels of laser light at INFN Napoli shows a significant amount of samples between the expected PE (Photo Electron) peaks. Thus, noise sources such as dark counts and afterpulses, which result in partially integrated APD pulses, must be accounted for. With static, reasonable values for noise rates, the simulation chain presented in this work uses the characteristics of individual APDs to produce the aggregate current produced by a SiPM pixel. When many such pulses are simulated and integrated, the ADC spectra generated by low levels of laser light at the INFN Napoli SiSMUV test setup can be accurately reproduced.

The Large Magellanic Cloud as a source of the highest energy cosmic rays

We propose a particle acceleration mechanism driven by large-amplitude Alfvén waves in a strong magnetic field. The acceleration process proceeds through multiple stages triggered by counterpropagating wave-particle resonant acceleration (CWRA) via decay instability. Initially, parent and daughter Alfvén waves resonantly accelerate particles perpendicular to the ambient magnetic field. The resultant modulational instability generates electrostatic fields within the wave packet, which are locally amplified by the ponderomotive force of the Alfvén wave packet. These fields subsequently drive further acceleration within a few relativistic gyroperiods via gyroresonant surfing acceleration (GRSA). During this, the v×B force facilitates momentum transfer from the perpendicular to the parallel direction. In the later stage, particles become trapped by the parent wave and gain additional energy through single wave resonant acceleration (SWRA). Furthermore, the accumulation of accelerated particles induces electrostatic trailing fields behind and at the tail of the wave packet, which drive trailing-field acceleration (TFA) of other electrons. The combined effects of these mechanisms, CWRA followed by GRSA and SWRA, result in highly relativistic electron energy. The electron energy accelerated through the above process is higher than that accelerated through TFA. This multistage acceleration process provides insights into the generation of high energy cosmic rays in astrophysical environments.

Direct simulations of very high energy cosmic ray acceleration in 3D MHD model of a compact star cluster