Cosmic Rays Research Articles

Prospects for Gamma-Ray Emission from Magnetar Regions in CTAO Observations

Abstract Recent multiwavelength observations have highlighted magnetars as significant sources of cosmic rays, particularly through their gamma-ray emissions. This study examines three magnetar regions—CXOU J171405.7–31031, Swift J1834−0846, and SGR 1806−20—known for emitting detectable electromagnetic signals. We assess the detectability of these regions using the upcoming Cherenkov Telescope Array Observatory (CTAO) by conducting an ON/OFF spectral analysis and compare the expected results with existing observations. Our findings indicate that CTAO will detect gamma-ray emissions from these three magnetar regions with significantly reduced emission flux errors compared to current instruments. In particular, the study shows that the CXOUJ1714−3810 and SwiftJ1834−0846 magnetar regions can be observed by the full southern and northern CTAO arrays in just 5 hr of observation, with mean significances above 10σ and 30σ, respectively. This paper discusses the regions analyzed, presents key results, and concludes with insights drawn from the study.

Improvements to monoscopic analysis for imaging atmospheric Cherenkov telescopes: Application to H.E.S.S.

Imaging atmospheric Cherenkov telescopes (IACTs) detect rays by measuring the Cherenkov light emitted by secondary particles in the air shower when the rays hit the atmosphere of the Earth. Given usual distances between telescopes in IACT arrays, at low energies (lesssim 100 GeV), the limited amount of Cherenkov light produced typically implies that the event is registered by one IACT only. Such events are called monoscopic events, and their analysis is particularly difficult. Challenges include the reconstruction of the event's arrival direction, energy, and the rejection of background events due to charged cosmic rays. Here, we present a set of improvements, including a machine-learning algorithm to determine the correct orientation of the image in the camera frame, an intensity-dependent selection cut that ensures optimal performance across all energies, and a collection of new image parameters. To quantify these improvements, we make use of simulations and data from the 28-m diameter central telescope of the IACT array. Knowing the correct image orientation, which corresponds to the arrival direction of the photon in the camera frame, is especially important for the angular reconstruction, which could be improved in resolution by 57% at 100,GeV. The event selection cut, which now depends on the total measured intensity of the events, leads to a reduction of the low-energy threshold for source analyses by ∼50%. The new image parameters characterize the intensity and time distribution within the recorded images and complement the traditionally used Hillas parameters in the machine learning algorithms. We evaluate their importance to the algorithms in a systematic approach and carefully evaluate associated systematic uncertainties. We find that including subsets of the new variables in machine-learning algorithms improves the reconstruction and background rejection, resulting in a sensitivity improved by 41% at the low-energy threshold. Finally, we apply the new analysis to data from the Crab Nebula and estimate systematic uncertainties introduced by the new method.

Assessment of Natural Radioactivity in a Residence in Belo Horizonte

Natural radioactivity comes from natural radionuclides present in rocks and soil and from cosmic rays. In addition to gamma-emitting radionuclides, radon stands out, which is a carcinogenic gas. In nature, radon occurs as radioisotopes: 222Rn (half-life time, t1/2,of 3.83 d), 219Rn (t1/2 = 3.92 s) and 220Rn (or thoron, t1/2 = 54.5 s). The World Health Organization (OMS) recommends that it be measured in homes, with the recommended limit being 100 Bq/m3 and the maximum limit being 300 Bq/m3. This work evaluates the origin of natural radiation in a residence in Belo Horizonte, Minas Gerais, Brazil, in which was observed radon concentrations above the limits established by the OMS. The detectors used were: nuclear trace detectors type CR-39, electrets (E-PERM), AlphaGUARD (AG) and the detector RS-230. The result of the 222Rn concentration measured in the electrets in the rooms of the residence varied from 14,8 Bq/m3 to 932 Bq/m3. At this highest point, the result of the average concentration of AlphaGUARD was from 4.78 kB/m3, with a maximum value of 8.5 kBq/m3. The concentration of 220Rn varied Bq/m3 Bq/m3. As this is a basement-type room, the bedroom next door was investigated, in which it is highlighted that the radon concentration of the detector CR-39 was from 445 Bq/m3, AG of 198.2 Bq/m3 with a maximum value of 350 Bq/m3, and electret of 29.6 Bq/m3. Gamma scanning with the RS-230 indicates higher counts on uncoated walls. Measurements of radon in the water showed background concentrations. It is concluded that poor ventilation of the location must be the biggest contributor to the high concentration of 222Rn, which originates mainly from the soil. The thoron concentration was higher in construction materials due to the measurement method. The instrumentation used allows a complete and comparative study between detectors in order to find the origin of natural radionuclides and subsequently suggest appropriate mitigation measures. In future studies, radon concentration in the soil and radon progeny will be measured.

Measurement of the depth of maximum of air-shower profiles with energies between 1018.5 and 1020 eV using the surface detector of the Pierre Auger Observatory and deep learning

We report an investigation of the mass composition of cosmic rays with energies from 3 to 100 EeV (1 EeV=1018 eV) using the distributions of the depth of shower maximum Xmax. The analysis relies on ∼50,000 events recorded by the surface detector of the Pierre Auger Observatory and a deep-learning-based reconstruction algorithm. Above energies of 5 EeV, the dataset offers a 10-fold increase in statistics with respect to fluorescence measurements at the Observatory. After cross-calibration using the fluorescence detector, this enables the first measurement of the evolution of the mean and the standard deviation of the Xmax distributions up to 100 EeV. Our findings are threefold: (i) The evolution of the mean logarithmic mass toward a heavier composition with increasing energy can be confirmed and is extended to 100 EeV. (ii) The evolution of the fluctuations of Xmax toward a heavier and purer composition with increasing energy can be confirmed with high statistics. We report a rather heavy composition and small fluctuations in Xmax at the highest energies. (iii) We find indications for a characteristic structure beyond a constant change in the mean logarithmic mass, featuring three breaks that are observed in proximity to the ankle, instep, and suppression features in the energy spectrum. Published by the American Physical Society 2025

Inference of the Mass Composition of Cosmic Rays with Energies from 1018.5 to 1020 eV Using the Pierre Auger Observatory and Deep Learning

We present measurements of the atmospheric depth of the shower maximum Xmax, inferred for the first time on an event-by-event level using the surface detector of the Pierre Auger Observatory. Using deep learning, we were able to extend measurements of the Xmax distributions up to energies of 100 EeV (1020 eV), not yet revealed by current measurements, providing new insights into the mass composition of cosmic rays at extreme energies. Gaining a 10-fold increase in statistics compared to the fluorescence detector data, we find evidence that the rate of change of the average Xmax with the logarithm of energy features three breaks at 6.5±0.6(stat)±1(syst) EeV, 11±2(stat)±1(syst) EeV, and 31±5(stat)±3(syst) EeV, in the vicinity to the three prominent features (ankle, instep, suppression) of the cosmic-ray flux. The energy evolution of the mean and standard deviation of the measured Xmax distributions indicates that the mass composition becomes increasingly heavier and purer, thus being incompatible with a large fraction of light nuclei between 50 and 100 EeV. Published by the American Physical Society 2025

Revealing an unexpectedly low electron injection threshold via reinforced shock acceleration

Collisionless shock waves, found in supernova remnants, interstellar, stellar, and planetary environments, and laboratories, are one of nature’s most powerful particle accelerators. This study combines in situ satellite measurements with recent theoretical developments to establish a reinforced shock acceleration model for relativistic electrons. Our model incorporates transient structures, wave-particle interactions, and variable stellar wind conditions, operating collectively in a multiscale set of processes. We show that the electron injection threshold is on the order of suprathermal range, obtainable through multiple different phenomena abundant in various plasma environments. Our analysis demonstrates that a typical shock can consistently accelerate electrons into very high (relativistic) energy ranges, refining our comprehension of shock acceleration while providing insight on the origin of electron cosmic rays.

Variation of the low-mass end of the stellar initial mass function with redshift and metallicity

Abstract We report the stellar mass functions obtained from 20 radiation hydrodynamical simulations of star cluster formation in 500 M⊙ molecular clouds with metallicities of 3, 1, 1/10 and 1/100 of the solar value, with the clouds subjected to levels of the cosmic microwave background radiation that are appropriate for star formation at redshifts z = 0, 3, 5, 7, and 10. The calculations include a thermochemical model of the diffuse interstellar medium and treat dust and gas temperatures separately. We find that the stellar mass distributions obtained become increasingly bottom light as the redshift and/or metallicity are increased. Mass functions that are similar to a typical Galactic initial mass function are obtained for present-day star formation (z = 0) independent of metallicity, and also for the lowest-metallicity (1/100 solar) at all redshifts up to z = 10, but for higher metallicities there is a larger deficit of brown dwarfs and low-mass stars as the metallicity and redshift are increased. These effects are a result of metal-rich gas being unable to cool to as lower temperatures at higher redshift due to the warmer cosmic microwave background radiation. Based on the numerical results we provide a parameterisation that may be used to vary the stellar initial mass function with redshift and metallicity; this could be used in simulations of galaxy formation. For example, a bottom-light mass function reduces the mass-to-light ratio compared to a typical Galactic stellar initial mass function, which may reduce the estimated masses of high-redshift galaxies.

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

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

Ultrarelativistic gravity and extremely energetic cosmic ray

Ultrarelativistic gravity and extremely energetic cosmic ray

Cosmic rays escape from their sources

Cosmic rays escape from their sources

Increased burstiness at high redshift in multiphysics models combining supernova feedback, radiative transfer, and cosmic rays

ABSTRACT We study star formation variability, or burstiness, as a method to constrain and compare different galaxy formation models at high redshift using the azahar simulation suite. The models range from magnetohydrodynamics with turbulence-driven star formation to more sophisticated set-ups incorporating radiative transfer and cosmic ray physics. Analysing a sample of galaxies at redshifts $z=4-10$, we find that including both radiative transfer and cosmic rays results in more regular star formation periodicity, as revealed by the Lomb–Scargle periodogram. While both radiative transfer and cosmic rays amplify star formation stochasticity, their combination leads to the largest scatter in burst intensity and the most pronounced deviations from the star-forming main sequence. To compare this comprehensive model against observations, we generate a mock spectrum of a low-mass galaxy during a mini-quenching event at $z=7.5$. The resulting spectrum aligns well with the low-mass quiescent galaxy JADES-GS-z7-01-QU observed at $z=7.3$, though discrepancies attributed to stellar metallicity suggest it may have a composite nature. Our findings highlight the importance of including complex physical processes like cosmic rays and radiative transfer in simulations to accurately capture the bursty nature of star formation in early galaxy formation. Future JWST observations, particularly of the scatter around the star-forming main sequence, might provide critical constraints for numerical models of galaxy formation at high redshift.

Detectability of the passage of the heliosphere through an interstellar cloud with cosmogenic nuclides in lunar soil

As the Sun traverses interstellar space it may encounter interstellar molecular clouds (IMCs) characterised by higher particle densities than in the ambient interstellar medium. These occurrences have, for example, been proposed to explain the increase of ^60Fe measured in sea sediments. Magnetohydrodynamic (MHD) simulations show that such IMC crossings effectively shrink the heliosphere, thereby reducing its ability to modulate the incident spectrum of galactic cosmic rays (GCRs). Therefore, the hallmark of such encounters in the past may be increased GCR intensities, which can be detected via analyses of cosmogenic nuclides in lunar regolith samples. The present study proposes a method for testing whether such IMC crossings have indeed occurred in the past, by analysing the rates at which the long-lived cosmogenic nuclide (lifetime 1.0 Myr) is formed in lunar soil samples. Cosmogenic nuclide production rates at varying depths in lunar soil are related to a corresponding GCR modulation potential, which in turn is related to a corresponding modulation boundary, and hence interstellar density, via a scaling relation based on published MHD simulation results. A lower limit for the detectability of past heliospheric crossings of IMCs is presented, governed by the amount of time spent in such a cloud: shorter passages may be undetectable, but longer passages would be clearly observable. However, we find no evidence of the Solar System encountering a cold, dense cloud. Lunar cosmogenic nuclides represent a powerful tool whereby the past modulation history of the heliosphere can be revealed over timescales of millions of years, which in turn can provide invaluable insights as to the past interstellar environment encountered by the Sun. However, techniques such as the one proposed here will benefit greatly from new, higher-precision analyses of existing lunar samples.

The Theory of Resonant Cosmic Ray–driven Instabilities—Growth and Saturation of Single Modes

Cosmic-ray (CR) feedback is critical for galaxy formation as CRs drive galactic winds, regularize star formation in galaxies, and escape from active galactic nuclei to heat the cooling cores of galaxy clusters. The feedback strength of CRs depends on their coupling to the background plasma and, as such, on the effective CR transport speed. Traditionally, this has been hypothesized to depend on the balance between the wave growth of CR-driven instabilities and their damping. Here, we study the physics of CR-driven instabilities from first principles, starting from a gyrotropic distribution of CR ions that stream along a background magnetic field. We develop a theory of the underlying processes that organize the particles’ orbits and in particular their gyrophases, which provides an intuitive physical picture of (i) wave growth as the CR gyrophases start to bunch up lopsidedly toward the local wave magnetic field, (ii) instability saturation as a result of CRs overtaking the wave and damping its amplitude without isotropizing CRs in the wave frame, and (iii) CR backreaction onto the unstable plasma waves as the CR gyrophases follow a pendulum motion around the wave magnetic field. Using our new fluid-particle-in-cell code fluid-SHARP, we validate our theory on the evolution and excitation of individual unstable modes, such as forward- and backward-propagating Alfvén and whistler waves. We show that these kinetic simulations support our theoretical considerations, thus potentially foreshadowing a revision of the theory of CR transport in galaxies and galaxy clusters.

A Cosmic Ray Acceleration Mechanism Based on Background Flow Velocity Inhomogeneities Yielding Power-law Spectra

In astrophysics, one significant challenge lies in understanding the acceleration of cosmic rays, which leads to the occurrence of a power law. In this article, momentum transport generated by the combined effects of pitch-angle diffusion and background flow velocity inhomogeneities is proposed to obtain a cosmic rays acceleration mechanism, starting from the well-known focused transport equation describing particle diffusion and acceleration. The inhomogeneities of background flow velocity are ubiquitous in the astrophysical environment. The equation for the isotropic part of the distribution function of charged energetic particles is derived, and its solution is obtained, demonstrating the form of momentum power laws of cosmic rays. In addition, if it is assumed that cosmic rays penetrate compressive MHD waves or turbulence, for quasi-steady states, the spectral index δ of the momentum power law spectrum of cosmic rays is found to be in the range [−5, −3], which includes the observed power law indices of galactic cosmic rays. The results obtained in this article demonstrate that the mechanism proposed in this article, along with shock acceleration, may also contribute to the acceleration of galactic cosmic rays. Furthermore, when momentum convection effect and higher-order momentum derivative terms are considered, the indices of power laws should be smaller than −5. This may explain the power laws of solar energetic particle events.

Imprints of PeV cosmic-ray sources on the diffuse gamma-ray emission

We present our new model for the description of the very high energy Galactic gamma-ray emission based on a discrete injection of cosmic rays by individual sources. We investigate the morphology of the very high energy gamma-ray sky, the detectability of cosmic-ray sources and the clumpiness of the diffuse gamma-ray flux, assuming two different scenarios for cosmic-ray propagation. Namely, a standard isotropic and homogeneous diffusion process and an isotropic and inhomogeneous diffusion process. We notably formulate a possible explanation to the small number of hadronic PeVatrons recently detected by LHAASO. In the case of the inhomogeneous diffusion process, we constrain the number of hadronic PeVatrons to be small. Finally, we give an argument that may explain the discrepancy between the interstellar gas density distribution and the very high energy diffuse gamma-ray flux.

Cosmic-ray acceleration and escape from supernova remnant W44 as probed by Fermi-LAT and MAGIC

Context. The supernova remnant (SNR) W44 and its surroundings are a prime target for studying the acceleration of cosmic rays (CRs). Several previous studies established an extended gamma-ray emission that is set apart from the radio shell of W44. This emission is thought to originate from escaped high-energy CRs that interact with a surrounding dense molecular cloud complex. Aims. We present a detailed analysis of Fermi-LAT data with an emphasis on the spatial and spectral properties of W44 and its surroundings. We also report the results of the observations performed with the MAGIC telescopes of the northwestern region of W44. Finally, we present an interpretation model to explain the gamma-ray emission of the SNR and its surroundings. Methods. We first performed a detailed spatial analysis of 12 years of Fermi-LAT data at energies above 1 GeV, in order to exploit the better angular resolution, while we set a threshold of 100 MeV for the spectral analysis. We performed a likelihood analysis of 174 hours of MAGIC data above 130 GeV using the spatial information obtained with Fermi-LAT. Results. The combined spectra of Fermi-LAT and MAGIC, extending from 100 MeV to several TeV, were used to derive constraints on the escape of CRs. Using a time-dependent model to describe the particle acceleration and escape from the SNR, we show that the maximum energy of the accelerated particles has to be ≃40 GeV. However, our gamma-ray data suggest that a small number of lower-energy particles also needs to escape. We propose a novel model, the broken-shock scenario, to account for this effect and explain the gamma-ray emission.

Predicting HCN, HCO+, multitransition CO, and dust emission of star-forming galaxies

The specific star formation rate of star-forming main sequence galaxies significantly decreased since z ∼ 1.5 because the molecular gas fraction and star formation efficiency decreased. The gas velocity dispersion decreased within the same redshift range and is apparently correlated with the star formation efficiency (inverse of the molecular gas depletion time). However, the radio–infrared (IR) correlation has not changed significantly since z ∼ 1.5. The theory of turbulent clumpy star-forming gas disks together with the scaling relations of the interstellar medium describes the large- and small-scale properties of galactic gas disks. We extend our previous work on the IR multitransition molecular line, and radio continuum emission of local and high-z star-forming and starburst galaxies to local and z ∼ 0.5 luminous IR galaxies. The model reproduces the IR luminosities, CO, HCN, and HCO+ line luminosities, and the CO spectral line energy distributions of these galaxies. We derived CO(1–0) and HCN(1–0) conversion factors for all galaxy samples. The relation between the star formation rate per unit area and the H2 surface density cannot be fit simply for all redshifts. The star formation efficiency, the product of the gas turbulent velocity dispersion, and the angular velocity of the galaxies are tightly correlated. Galaxies with lower stellar masses can in principle compensate their gas consumption via star formation by radial viscous gas accretion. The limiting stellar mass increases with redshift. The radio continuum emission is directly proportional to the density of cosmic-ray (CR) electrons, but the molecular line emission depends on the CR ionization rate via the gas chemistry. The normalization of the CR ionization rate we found for the different galaxy samples is higher by about a factor of three to five than the normalization for the solar neighborhood. This means that the mean yield of low-energy CR particles for a given star formation rate per unit area is higher by about three to ten times in external galaxies than was observed by Voyager I.

Radio emission from atmosphere-skimming cosmic ray showers in high-altitude balloon-borne experiments

Atmosphere-skimming air showers are initiated by cosmic rays with incoming directions that allow the cascade to develop entirely within the atmosphere, without reaching the ground. Radio pulses induced by this type of showers have already been observed in balloon-borne experiments such as ANITA, but a detailed characterisation of their properties is lacking. The extreme range of densities in which these cascades can develop gives rise to a wide range of shower profiles, with radio emission characteristics that can differ significantly from those of regular downward-going showers. In this work, we have used the ZHAireS-RASPASS program to characterise the expected radio emission from atmosphere-skimming air showers and its properties. We have studied the interplay between the magnetic field and atmospheric density profile in the expected radio signal, focusing on its detection aboard balloon-borne experiments. The almost horizontal geometry of the events gives rise to a significant refractive asymmetry in the spatial distribution of the electric field, due to the propagation of the radio signals across a gradient of index of refraction. In addition, a unique coherence asymmetry appears in the intensity of the signals, as a consequence of the cumulative effect of the Earth's magnetic field over the very long distances that these particle cascades traverse. The implications of the peculiar characteristics of the emission are discussed regarding their impact both on the interpretation of collected data and in the exposure of balloon-borne experiments.

Radiation Impact of the Halloween GLE Events During the October–November 2003 Period

AbstractFrom late October to early November 2003, one of the strongest recorded geomagnetic storms occurred due to heightened solar activity. Three ground‐level enhancement events (GLEs) took place during this period, GLE 65, 66, and 67, known as the Halloween events. In this work we expand upon and use, the newly developed and verified dosimetric model Oulu CRAC:DOMO (Cosmic Ray Atmospheric Cascade: Dosimetric Model) to compute the radiation impact of the Halloween events at an altitude typical for aviation, that is of 35 kft. Complex magnetospheric modeling was performed for each GLE event and the deep Forbush decrease present was reconstructed and accounted for. Using a conservative approach, the computed total effective dose for all three Halloween GLEs was 135 Sv in polar regions, an increase of 129% above the galactic cosmic ray background induced dose. While this is a significant increase, we conclude that the radiation hazard posed to aircrew and passengers was relatively minor. In addition, a comparison between flight dosimetric measurements taken during the Halloween events and CRAC:DOMO showed a good agreement.

Detectability of Self-lensing Flares of White Dwarfs with Compact Companions

Binaries containing compact objects, viewed nearly edge on, can produce periodic brightening events under certain conditions on the masses, radii, and binary separation. Such flares are caused by one object gravitationally lensing another, in what is known as self-lensing flares. We present a simulation tool that efficiently reproduces the main features of self-lensing flares and facilitates a detection sensitivity analysis for various sky surveys. We estimate the detection prospects for a handful of representative surveys when searching for systems of either two white dwarfs or a white dwarf with other compact objects, i.e., neutron stars and black holes. We find only a marginal ability to detect such systems in existing surveys. However, we estimate many such systems could be detectable by surveys in the near future, including the Vera Rubin Legacy Survey of Space and Time (LSST). We provide a quantitative analysis of the detectability of double-compact object self-lensing flares across the landscape of system parameters, and a qualitative discussion of survey and follow-up approaches to distinguish such flares from confounding events, such as stellar flares, satellite glints, and cosmic rays. We estimate 0.3, 3 and 247 double white dwarf systems could be detected by Transiting Exoplanet Survey Satellite, Zwicky Transient Facility, and LSST, respectively. A similar number of systems with a neutron star or black hole companion could be detected, but we caution that the number densities of such binaries is model dependent and so are our detection estimates. Such binaries can be used to constrain models of the end states of binary evolution.