High Energy Cosmic Rays Research Articles

A Possible Common Physics Picture Reflected by the Gamma-Ray Emission of the Galactic Center

Abstract Long-term observations of the Galactic center by Fermi and HESS have revealed a novel phenomenon: the high-energy gamma-ray spectrum from the gamma-ray source HESS J1745-290 exhibits a double power-law structure. In this study, we propose a new explanation for this phenomenon. We suggest that the low-energy (GeV) power-law spectrum originates from interactions between trapped background “sea” cosmic ray particles and the dense gaseous environment near the Galactic center. In contrast, the bubble-like structure in the high-energy (TeV) spectrum is produced by protons accelerated during active phases of the Galactic center, through the same physical process. Based on this framework, we first calculate the gamma-ray emission generated by cosmic ray protons accelerated in the Galactic center. Then, using a spatially dependent cosmic ray propagation model, we compute the energy spectrum of background “sea” cosmic ray protons and their associated diffuse gamma-ray emission in the Galactic center region. The results closely reproduce the observations from Fermi-LAT and HESS, suggesting that their long-term data support this picture: high-energy cosmic rays in the local region originate from nearby cosmic ray sources, while low-energy cosmic rays are a unified contribution from distant cosmic ray sources. We anticipate that this double power-law structure may be widely present in the halo of a Galactic cosmic-ray source or a slow-diffusion region. We hope that future observations will detect more such sources, allowing us to further test and validate our model.

Stabilizing an Ultracold Fermi Gas against Fermi Acceleration to Superdiffusion through Localization

Anderson localization, i.e., destructive quantum interference of multiple-scattering paths, halts transport entirely. Contrarily, time-dependent random forces expedite transport via Fermi acceleration, proposed as a mechanism for high-energy cosmic rays. Their competition creates interesting dynamics, but experimental observations are scarce. Here, we experimentally study the expansion of an ultracold Fermi gas inside time-dependent disorder and observe distinct regimes from sub- to superdiffusion. Unexpectedly, quantum interference counteracts acceleration in strong disorder before a transition to a diffusive state occurs in the driven system. Our system enables the investigation of Fermi acceleration in the quantum-transport regime. Published by the American Physical Society 2025

On Acceleration of Highest-energy Cosmic Rays in a Novel Scenario of Magnetar Transients

Abstract Transient phenomena in magnetars have been considered as possible acceleration sites of ultra-high-energy cosmic rays (CRs), whose energy reaches ∼200 EeV, such as the Amaterasu particle. However, the process of CR acceleration and the trigger mechanism of magnetar transients remain unclear. A recently suggested scenario for the activity predicts that the magnetar’s rotation axis suddenly flips due to the “Dzhanibekov effect,” resulting in a sudden rise of the Euler force. The material in the outer layer plastically flows due to the force and finally fractures in this scenario. We study the possibilities of ion acceleration along with this scenario. If the degenerate electrons burst open from the fractured region like a balloon burst, the pair plasma formation can be ignited inside the crust. We find that such pair plasma can emit photons similar to the observed bursts from magnetars. We also find that the electron stream at the beginning of the burst phenomenon possibly induces a strong electric field for a moment, resulting in the acceleration of ∼1 ZeV ion within a timescale of ∼1 ps. The nuclear spallation reactions limit this timescale, and therefore, high-energy CR “neutrons” from the parenteral nuclei become proper observational predictions of this scenario: their arrival time and direction will be correlated with the bursting photon emissions of the host magnetars. The nuclear spallation of ∼1 ZeV nuclei is preferred to explain ≳10 PeV neutrino events observed by IceCube and KM3Net.

Geant4 Simulations of a Scintillator Cosmic-Ray Detector

Reliable cosmic-ray measurements require a thorough understanding of the detector used. It is especially important when detectors are very simple like the scintillator detectors considered in this work, which provide only information about the amplitude of the signal generated by a detected particle. Arrays of these devices can work in coincidental setups to detect Extensive Air Showers caused by high-energy primary cosmic rays. Due to their low cost and simple design, they can be used as elements of large detector networks needed for the search for global correlations in the cosmic rays. To be able to interpret data collected by those arrays, extensive simulations of such detectors are necessary to determine their efficiency of detection of different types of particles. This work presents the results of analysis of such simulations performed using the Geant4 software (v1.1.2). The analysis results lead to the conclusion that detectors feature almost maximal (close to 100%) efficiency for the detection of cosmic-ray muons and electrons with momenta greater than 0.03 GeV/c. Their sensitivity to low-energy electrons and photons is lower but not negligible and has to be properly taken into account during the interpretation of collected data.

Ultra high energy cosmic rays in light of the Lorentz invariance violation effects within the proton sector

Tiny Lorentz invariance violation (LIV) effects, potentially arising from quantum gravity-induced spacetime structures, may also manifest in the proton sector, offering a plausible pathway to test Planck-scale physics through high-energy cosmic phenomena. Our analysis reveals that even minuscule LIV effects in the proton sector can significantly elevate the photon threshold energy for photopion production to O (0.1 to 103 eV), orders of magnitude higher than in Lorentz-symmetric scenarios. Consequently, protons in ultra-high-energy cosmic rays (UHECRs) can propagate for very long distances without significant energy loss via photopion interactions with cosmic microwave background (CMB) photons. This suppression of attenuation may provide a plausible explanation for the observed cosmic-ray events exceeding the Greisen–Zatsepin–Kuzmin (GZK) cutoff energy. We further demonstrate that when both leading-order and next-to-leading-order LIV effects are considered, higher-order LIV contributions induce discontinuous transitions in the GZK cutoff energy spectrum. Observations of proton-dominated UHECRs beyond the GZK threshold could provide constraints the LIV energy scale. Offering insights into the ultraviolet regime of LIV theories near the Planck scale, UHECRs may serve as a sensitive probe of LIV and provide a means to test quantum gravity predictions by constraining deviations from Lorentz symmetry in extreme-energy regimes.

From Space to Soil: Revealing Environmental Water with Cosmic-Ray Neutron Sensing

The Cosmic-ray neutron sensing (CRNS) technique is a modern technological solution that facilitates continuous measurement of the average water content in the environment, including soil, snow, and vegetation. The sensor monitors an area of 10 to 15 hectares and soil depths up to 50 centimeters. This configuration enables the non-invasive and field-scale measurement of the soil water content or snow water equivalent that is insensitive to small-scale heterogeneities. The method has the potential to serve as an alternative to conventional in-situ sensors or costly soil or snow samples, while also demonstrating the capability to bridge the gap between point measurements and remote sensing data in both horizontal and vertical directions. The passive CRNS technology measures the natural cosmogenic background radiation and detects changes in the moderation of cosmic-ray induced fast neutrons by hydrogen atoms. The neutron radiation measured with CRNS originates primarily from high-energy cosmic rays that interact with nuclei in the Earth's atmosphere, producing secondary particles such as neutrons. These secondary neutrons penetrate the atmosphere and interact with the soil, where their moderation is influenced by the hydrogen content, including soil moisture. CRNS is flexible and robust because it operates non-invasively and requires no direct soil contact, making it suitable for diverse terrains and conditions. Additionally, it is largely unaffected by small-scale soil heterogeneity and provides continuous, real-time data with minimal maintenance. The CRNS can also be employed on mobile platforms for the purpose of on-demand soil moisture mapping at the field, regional, or even national scale. Presently, a network of CRNS stations being established on global scale, with several hundred sensors already in operation. In this presentation, we will examine recent experiments and prospective future applications of CRNS, with the objective of extending the boundaries of this technology.

Next frontiers for magnetic monopole searches

Magnetic monopoles (MMs) are well-motivated hypothetical particles whose discovery would symmetrize Maxwell equations, explain quantization of electric charge, and probe the gauge structure of the unified theory. Recent models predict MMs with low masses, reinvigorating searches at colliders. However, most theories predict composite MMs, whose production in parton-parton collisions is expected to be suppressed. The Schwinger process, whereby MM pairs tunnel through the vacuum barrier in the presence of a strong magnetic field, is not subject to this limitation. Additionally, the Schwinger cross section can be calculated nonperturbatively. Together, these make it a golden channel for low-mass MM searches. We investigate the Schwinger production of MMs in heavy-ion collisions at future colliders, in collisions of cosmic rays with the atmosphere, and in decay of magnetic fields of cosmic origin. We find that a next-generation collider would provide the best sensitivity, allowing one to discover or exclude MMs with TeV-scale masses. At the same time, exploiting the infrastructure of industrial ore extraction and Antarctic ice drilling could advance the field at a faster timescale and with only a modest investment. In particular, we show with detailed calculations that the proposed experiments will be sensitive to fluxes of low-mass MMs as low as a few units of 10−22 cm−2 s−1 sr−1 in a wide range of Lorentz factors. We also propose deploying dedicated MM detectors in conjunction with cosmic ray observatories to directly investigate if the unexplained, highest-energy cosmic rays are MMs. Together, the proposed efforts would define the field of MM searches in the next decades. Published by the American Physical Society 2025

High-energy Gamma-Ray and Neutrino Emissions from Interacting Supernovae Based on Radiation Hydrodynamic Simulations: A Case of SN 2023ixf

Abstract Recent observations of core-collapse supernovae revealed that the existence of dense circumstellar matter (CSM) around their progenitors is ubiquitous. Interaction of supernova ejecta with such a dense CSM is a potential production site of high-energy cosmic rays (CRs), gamma rays, and neutrinos. We estimate the gamma-ray and neutrino signals from SN 2023ixf, a core-collapse supernova occurred in a nearby galaxy M101, which exhibits signatures of the interaction with the confined, dense CSM. Using a radiation-hydrodynamic simulation model calibrated by the optical and ultraviolet observations of SN 2023ixf, we find that the CRs cannot be accelerated in the early phase because the sharp velocity jump at the shock disappears due to strong radiation pressure. Roughly 4 days after the explosion, the collisionless subshock is formed in the CSM, which enables the CR production and leads to gamma-ray and neutrino emissions. The shock sweeps up the entire dense CSM roughly 9 days after the explosion, which ceases the high-energy radiation. Based on this scenario, we calculate the gamma-ray and neutrino signals, which have a peak around 9 days after the explosion. We can constrain the CR production efficiency to be less than 10% by comparing our prediction to the Fermi Large Area Telescope upper limits. Future multimessenger observations with an enlarged sample of nearby supernovae will provide a better constraint on the CR production efficiency in the early phases of supernovae.

Chaotic Behavior of Trapped Cosmic Rays

Abstract Recent experimental results on the arrival direction of high-energy cosmic rays have motivated studies to understand their propagating environment. The observed anisotropy is shaped by interstellar and local magnetic fields. In coherent magnetic structures, such as the heliosphere, or due to magnetohydrodynamic turbulence, magnetic mirroring can temporarily trap particles, leading to chaotic behavior. In this work, we develop a new method to characterize cosmic rays’ chaotic behavior in magnetic systems using finite-time Lyapunov exponents. This quantity determines the degree of chaos and adapts to transitory behavior. We study particle trajectories in an axial-symmetric magnetic bottle to highlight mirroring effects. By introducing time-dependent magnetic perturbations, we study how temporal variations affect chaotic behavior. We tailor our model to the heliosphere; however, it can represent diverse magnetic configurations exhibiting mirroring phenomena. Our results have three key implications. (1) Theoretical: We find a correlation between the finite-time Lyapunov exponent and the particle escape time from the system, which follows a power law that persists even under additional perturbations. This power law may reveal intrinsic system characteristics, offering insight into propagation dynamics beyond simple diffusion. (2) Simulation: Chaotic effects play a role in cosmic-ray simulations and can influence the resulting anisotropy maps. (3) Observational: Arrival maps display areas where the chaotic properties vary significantly; these changes can be the basis for time variability in the anisotropy maps. This work lays the framework for studying the effects of magnetic mirroring of cosmic rays within the heliosphere and the role of temporal variability in the observed anisotropy.

Generation of Cosmic-Ray Trajectories by a Diffusion Model Trained on Test Particles in 3D Magnetohydrodynamic Turbulence

Abstract Models for the transport of high-energy charged particles through strong magnetic turbulence play a key role in space and astrophysical studies, such as describing the propagation of solar energetic particles and high-energy cosmic rays. Inspired by the recent advances in high-performance machine learning techniques, we investigate the application of generative diffusion models to synthesizing test particle trajectories obtained from a turbulent magnetohydrodynamics simulation. We consider velocity increment, spatial transport, and curvature statistics, and find excellent agreement with the baseline trajectories for fixed particle energies. Additionally, we consider two synthetic turbulence models for comparison. Finally, challenges toward an application-ready transport model based on our approach are discussed.

Let There Be Neutrons! Hadronic Photoproduction from a Large Flux of High-energy Photons

Abstract We propose that neutrons may be generated in high-energy, high-flux photon environments via photo-induced reactions on pre-existing baryons. These photohadronic interactions are expected to occur in astrophysical jets and surrounding material. Historically, these reactions have been attributed to the production of high-energy cosmic rays and neutrinos. We estimate the photoproduction off of protons in the context of gamma-ray bursts, where it is expected there will be sufficient baryonic material that may be encompassing or entrained in the jet. We show that typical stellar baryonic material, even material completely devoid of neutrons, can become inundated with neutrons in situ via hadronic photoproduction. Consequently, this mechanism provides a means for collapsars and other astrophysical sites containing substantial flux of high-energy photons to be favorable for neutron-capture nucleosynthesis.

Constraining the Cosmic-Ray Energy Based on Observations of Nearby Galaxy Clusters by LHAASO

Abstract Galaxy clusters act as reservoirs of high-energy cosmic rays (CRs). As CRs propagate through the intracluster medium, they generate diffuse γ-rays detectable by arrays such as LHAASO. These γ-rays result from proton–proton (pp) collisions of very high-energy cosmic rays or inverse Compton (IC) scattering of positron-electron pairs created by pγ interactions of ultra-high-energy cosmic rays (UHECRs). We analyzed diffuse γ-ray emission from the Coma, Perseus, and Virgo clusters using LHAASO data. Diffuse emission was modeled as a disk of radius R 500 for each cluster while accounting for point sources. No significant diffuse emission was detected, yielding 95% confidence level (C.L.) upper limits on the γ-ray flux: for WCDA (1–25 TeV) and KM2A (>25 TeV), less than (49.4, 13.7, 54.0) and (1.34, 1.14, 0.40) × 10−14 ph cm−2 s−1 for Coma, Perseus, and Virgo, respectively. The γ-ray upper limits can be used to derive model-independent constraints on the integral energy of cosmic ray protons above 10 TeV (corresponding to the LHAASO observational range >1 TeV under the pp scenario) to be less than (1.96, 0.59, 0.08) × 1061 erg. The absence of detectable annuli/ring-like structures, indicative of cluster accretion or merging shocks, imposes further constraints on models in which the UHECRs are accelerated in the merging shocks of galaxy clusters.

Binary Neutron Star Mergers as the Source of the Highest Energy Cosmic Rays.

We propose that ultrahigh energy cosmic rays (UHECRs) are produced in binary neutron star (BNS) mergers. This scenario can account for the heretofore inexplicable narrow rigidity range of UHECRs because the jets of BNS mergers are generated by a gravitationally driven dynamo and thus are nearly identical due to the narrow range of BNS masses. Observed UHECRs with energies well beyond 100EeV can be explained as r-process nuclei, without invoking an exotic source class. Evidence for this mechanism, and its prediction of coincidences between neutrinos above 10PeV and gravitational waves, are discussed.

Ultra high energy cosmic rays versus models of high energy hadronic interactions

We evaluate the consistency of hadronic interaction models in the CORSIKA simulation package with publicly available fluorescence telescope data from the Pierre Auger Observatory. By comparing the first few central moments of the extensive air shower depth maximum distributions, as extracted from measured events, to those predicted by the best-fit inferred compositions, we derive a statistical measure of the consistency of a given hadronic model with data. To mitigate possible systematic biases, we include all primaries up to iron, compensate for the differences between the measured and simulated energy spectra of cosmic rays and account for other known systematic effects. Additionally, we study the effects of including higher central moments in the fit and project our results to larger statistics.

The iron and nickel spectra measured with CALET on the international space station

Recent direct measurements of the energy spectra of the charged cosmic ray have revealed unexpected spectral features, most notably the onset of a progressive hardening at few hundreds of GeV/n not only of proton and He spectra but also observable for heavier nuclei. Thus, the study of the spectra behavior of heavy elements may shed light on understanding propagation and acceleration phenomena in our Galaxy. In particular, Fe and Ni provide favorable conditions for observations thanks to the low background contamination from spallation of higher mass elements they are affected by. The CALorimetric Electron Telescope, CALET, has been measuring high-energy cosmic rays on the International Space Station since October 2015. The instrument consists of two layers of segmented plastic scintillators, a 3 radiation length thick tungsten-scintillating fiber imaging calorimeter and a 27 radiation length thick PWO calorimeter. It identifies the charge of individual elements up to Ni and beyond and it measures the energy of cosmic-ray nuclei providing a direct measurement of their spectra. In this contribution, the iron and nickel spectra, resulted after 5 years of data acquisition, are presented in the energy range between 10 and 2000 GeV/n and between 8.8 and 240 GeV/n, respectively. The analysis procedure and the assessment of systematic errors are detailed, in addition to the ratio between the two fluxes. Both spectra show similar shape and energy dependence.

Angular Power Spectrum of TeV–PeV Cosmic-Ray Anisotropies

Abstract Simulations of the cosmic-ray (CR) anisotropy down to TeV energies are presented, using turbulence parameters consistent with those inferred from observations of the interstellar medium. We compute the angular power spectra C ℓ of the CR anisotropy obtained from the simulations. We demonstrate that the amplitude of the large-scale gradient in the CR density profile affects only the overall normalization of the C ℓ values, without affecting the shape of the angular power spectrum. We show that the power spectrum depends on CR energy and that it is sensitive to the location of the observer at small ℓ. It is found to flatten at large ℓ and can be modeled by a broken power law, exhibiting a break at ℓ ≈ 4. Our computed power spectrum at ~10 TeV fits well HAWC and IceCube measurements. Moreover, we calculate all coefficients of the spherical harmonics and compute the component of the angular power spectrum projected onto the direction of the local magnetic field line. We find that deviations from gyrotropy become increasingly important at higher CR energies and larger values of ℓ.

Dual-Loop Charge Management for Space-Based Gravitational Wave Detection

In space-based gravitational wave detection missions, inertial sensors act as the inertial reference, requiring the test mass (TM) to maintain exceptionally low residual acceleration noise. High-energy particles, cosmic rays, and ion pumps during ground tests can quickly lead to charge accumulation on the TM surface, necessitating a charge management system to regulate surface charges. To manage the TM surface potential, this paper develops a mathematical model of the charge management system using established ultraviolet (UV) discharge simulation methods. The model describes how photoelectron emission or absorption on the TM surface varies with UV light-emitting diodes (LEDs) power and bias voltage. For the first time, a dual-loop control method is implemented for charge control, highlighting its practical significance. The controller precisely regulates the TM surface potential and residual charges to specified values, achieving a control accuracy of 1.1×106e, meeting the stringent requirements of space-based gravitational wave detection missions. This approach offers a closed-loop charge management solution and provides valuable insights for designing future charge management systems.

Cosmic rays cannot explain the high ionisation rates in the Galactic centre

The Htwo ionisation rate in the central molecular zone, located in the Galactic centre, is estimated to be ζ∼2 s based on observations of H_3^+ lines. This value is two to three orders of magnitude larger than that measured anywhere else in the Galaxy. Due to the high density of the gas in the central molecular zone, UV and X-ray photons do not penetrate this region. Hence, cosmic rays are expected to be the exclusive agents of ionisation. A high cosmic-ray density has been invoked to explain the unusually high ionisation rate. However, this excess is not seen in the γ-ray emission from this region, which is produced by high-energy cosmic rays. Therefore, an excess is expected only in the low-energy cosmic-ray spectrum. Here, we derive constraints on this hypothetical low-energy component in the cosmic-ray spectra, and we question its plausibility. To do so, we numerically solved the cosmic-ray transport equation in the central molecular zone, considering spatial diffusion, advection in the Galactic wind, re-acceleration in the ambient turbulence, and energy losses due to interactions with matter and radiation in the interstellar medium. We derived stationary solutions under the assumption that cosmic rays are continuously injected by a source located in the Galactic centre. The high-energy component in the cosmic-ray spectrum was then fitted to available γ-ray and radio data, and a steep low-energy component was added to the cosmic-ray spectrum to explain the large ionisation rates. We find that injection spectra of p^-7 for protons below p_ enh,p c≃780 MeV and p^-5.2 for electrons below p_ enh,e c=1.5 GeV are needed to reach the observed ionisation rates. This corresponds to a cosmic-ray power of the order of ∼10^ erg s injected at the Galactic centre. Not only is this unrealistic, but it is also impossible to reproduce a constant ionisation rate across the region, as observations suggest. We conclude that cosmic rays alone cannot explain the high ionisation rates in the Galactic centre.

Constraints on the maximal number of dark degrees of freedom from black hole evaporation, cosmic rays, colliders, and supernovae

A dark sector with a very large number of massive degrees of freedom is generically constrained by radiative corrections to Newton’s constant. However, there are caveats to this statement, especially if the degrees of freedom are light or massless. Here, we examine in detail and update a number of constraints on the possible number of dark degrees of freedom, including from black hole evaporation, from perturbations to systems including an evaporating black hole, from direct gravitational production at colliders, from high-energy cosmic rays, and from supernovae energy losses. Published by the American Physical Society 2025

On the challenging problem to estimate the energy of the Ultra High Energy Cosmic Rays

Ultra High Energy Cosmic Rays (UHECRs) are rare nuclei that collide in the atmosphere with energy larger than 1018 eV. In this contribution we critically review the experimental techniques for estimating their energy, trying to address current limitations and potential improvements that can be developed in the coming years.