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Abstract In this second paper in a series dedicated to characterizing shear layers via 2D hybrid (kinetic ions–fluid electrons) simulations, we study the dynamical role of nonthermal particles (cosmic rays, CRs), either spontaneously generated or preexisting. We initialize Kolmogorov-type sinusoidal velocity shear flows unstable to the Kelvin–Helmholtz instability, which evolve nonlinearly into turbulence. Particles with large gyroradii act as long-range messengers that promote momentum exchange between layers, hence introducing a form of CR viscosity. Even when not energetically dominant, increasing the CR energy density generally enhances momentum transfer, provided that their gyroradii are smaller than the shear length scale. We consider flows ranging from subsonic to supersonic and assess the rate of shear dissipation, the partition of the initial kinetic energy among heating, thermal ion acceleration, CR reacceleration, magnetic-field amplification, and the maximum energy attained by accelerated particles. 

Abstract Supersonic flows are ubiquitous in warm and cool media; their dissipation leads to heating, generation of nonthermal particles, and amplification of background magnetic fields. We present 2D hybrid (kinetic ions–fluid electrons) simulations of decaying shear flows across the subsonic-to-supersonic transition, finding that the canonical Kelvin–Helmholtz instability in subsonic cases gives way to the formation of shocklets in supersonic shears, where dissipation is faster and nonthermal particles are produced. We discuss the dependence on the flow Mach number of particle acceleration, the viscosity induced by kinetic effects, and the production of magnetic turbulence. We outline the potential impact of these findings for turbulence in the warm interstellar medium, for molecular clouds, and for accretion disks, leaving to a companion paper the discussion of the effects on the shear of self-generated and preexisting energetic particles. 

Abstract Collisionless, turbulent plasmas surround the Earth, from the magnetosphere to the intergalactic medium, and the fluctuations within them affect nearly every field in the space sciences, from space weather forecasts to theories of galaxy formation. Where turbulent motions become supersonic, their interactions can lead to the formation of shocks, which are known to efficiently energize ions to cosmic-ray energies. We present 2.5-dimensional hybrid-kinetic simulations of decaying, supersonic, nonrelativistic turbulence in a collisionless plasma using the codedHybridR. The ions are initially cold ( ${\beta }_i\simeq \frac{1}{16}$), although they heat rapidly at the onset of turbulence. Turbulence within these simulations is highly compressible; after accounting for this compression by taking the omnidirectional power spectrum of thedensity-weightedvelocity field, we find turbulent spectra with power-law slopes of $\alpha \approx -\frac{5}{3}$for low Mach numbers, in the inertial range, andα ≈ −2 for high Mach numbers. Ions embedded in the highly supersonic simulations are accelerated to high energies at efficiencies similar to those seen in shocks, despite being in a nonrelativistic regime and lacking the large-scale structure of a shock. We observe that particles are accelerated into a power-law spectrum, with a slope ofq ≈ 2.5 in (nonrelativistic) energy. We compare these results to those obtained from the theory and simulations of diffusive shock acceleration and discuss the astrophysical implications of this theoretical work. 

Abstract Galactic supernova remnants (SNRs) are thought to accelerate cosmic rays (CRs) to several PeV energies, but this has yet to be confirmed as general behavior. Although several sources show ∼100 TeVγ-rays, their hadronic origin is uncertain; a matching neutrino signal would provide definitive evidence. Using insight from the theory of diffusive shock acceleration, we evaluate the spectra and environments of the sample of Galactic SNRs to identify those most likely to be hadronic, categorizing them into a tiered catalog, depending on their likelihood to produce neutrinos detectable in the TeV–PeV range. We then calculate the estimated stacked sensitivity of IceCube for each tier using IceCube’s 10-year public data. Our results suggest that this strategy of stacking SNRs and carefully excluding leptonic sources by using theoretical arguments may allow for a detection of this source class that would otherwise be impossible. A follow-up analysis of these catalogs using TeV–PeV sensitive neutrino data from IceCube (or similar telescopes like KM3NeT/ARCA) offers the most decisive, near-future test for the hadronic nature of these SNRs and the maximum energies of their CR spectra. 

Abstract Detecting ultrahigh-energy neutrinos can take two complementary approaches with different trade-offs. 1) Wide and shallow: aim for the largest effective volume, and to be cost-effective, go for wide field-of-view but at the cost of a shallow instantaneous sensitivity — this is less complex conceptually, and has strong discovery potential for serendipitous events. However, it is unclear if any source can be identified, following detection. And 2) Deep and narrow: here one uses astrophysical and multi-messenger information to target the most likely sources and populations that could emit neutrinos — these instruments have deep instantaneous sensitivity albeit a narrow field of view. Such an astrophysically-motivated approach provides higher chances for detection of known/observed source classes, and ensures multi-messenger astronomy. However, it has less potential for serendipitous discoveries. In light of the recent progress in multi-messenger and time-domain astronomy, we assess the power of the deep and narrow instruments, and contrast the strengths and complementarities of the two detection strategies. We update the science goals and associated instrumental performances that envisioned projects can include in their design in order to optimize discovery potential. 

Abstract Merging galaxy clusters exhibit strong observational evidence for efficient particle acceleration in the intracluster medium (ICM), particularly in the form of synchrotron-emitting radio relics and halos. Cosmic-ray (CR) electrons are likely accelerated (or reaccelerated) at merger and accretion shocks via diffusive shock acceleration. However, in the presence of the large diffusion coefficients, one would naively expect in the rarefied, relatively unmagnetized ICM, this acceleration—in particular, the maximum proton energy ( $E_{\max }$)—is limited by long acceleration times. On the other hand, recent work on CR transport suggests that the diffusion coefficient can be suppressed in ICM-like environments. In this picture, deviations from local thermodynamic equilibrium can trigger the mirror instability, creating plasma-scale magnetic structures, or “micromirrors,” that efficiently scatter CRs. In this paper, we investigate the implications of micromirror confinement for shock acceleration in the ICM. We demonstrate that micromirrors enforce a minimum value of $E_{\max }\gtrsim 100$GeV that does not rely on CR-driven magnetic field amplification. We also discuss micromirror confinement in the context of cosmological simulations andγ-ray observations, and present a simulation of a Coma-like merging cluster that self-consistently includes CR acceleration at shocks, with an effective diffusion coefficient set by micromirrors. We show that the introduction of micromirrors yields simulated galaxy clusters that remain consistent withγ-ray observations. 

Abstract We investigate the process of diffusive shock acceleration of particles with mass number to charge number ratiosA/Q > 1, e.g., partially ionized heavy ions. To this end, we introduce helium- and carbon-like ions at solar abundances into two-dimensional hybrid (kinetic ions–fluid electrons) simulations of nonrelativistic collisionless shocks. This study yields three main results: (1) Heavy ions are preferentially accelerated compared to hydrogen. For typical solar abundances, the energy transferred to accelerated helium ions is comparable to, or even exceeds, that of hydrogen, thereby enhancing the overall shock acceleration efficiency. (2) Accelerated helium ions contribute to magnetic field amplification, which increases the maximum attainable particle energy and steepens the spectra of accelerated particles. (3) The efficient acceleration of helium significantly enhances the production of hadronicγ-rays and neutrinos, likely dominating the one due to hydrogen. These effects should be taken into account, especially when modeling strong space and astrophysical shocks. 

ABSTRACT Among more than 1000 known fast radio bursts (FRBs), only five sources – FRBs 20121102A, 20190520B, 20201124A, 20240114A, and 20190417A – have confirmed associations with persistent radio sources. The observed quasi-steady emission is consistent with synchrotron radiation from a composite of magnetar wind nebula and supernova (SN) ejecta. Using a phenomenological model that incorporates simplified treatments of the nebular dynamics and particle acceleration, we compute the synchrotron flux by solving kinetic equations for energized electrons, accounting for electromagnetic cascades of electron–positron pairs interacting with nebular photons. Within the framework of our model, the rotation-powered scenario requires a young neutron star (NS) with age $$t_{\rm age}\approx 20\, {\rm yr}$$, dipolar magnetic field $$B_{\rm dip}\approx (3{\!-\!}5)\times 10^{12}\, {\rm G}$$ and initial spin period $$P_i\approx 1.5{\!-\!}3\, {\rm ms}$$ in an ultra-stripped SN progenitor to account for emissions from FRBs 20121102A and 20190520B. In contrast, FRB 20201124A requires $$t_{\rm age}\approx 10\, {\rm yr}$$, $$B_{\rm dip}\approx 5.5\times 10^{13}\, {\rm G}$$, and $$P_i\approx 10\, {\rm ms}$$ in a conventional core-collapse SN progenitor. For the magnetar-flare-powered model, NS aged $$t_{\rm age} \approx 25\, /40\, {\rm yr}$$ in a USSN progenitor and $$t_{\rm age} \approx 12.5\, {\rm yr}$$ in a CCSN progenitor explains the observed flux for FRB 20121102A/20190520B and FRB 20201124A, respectively. Finally, we estimate a minimum NS age $$t_{\rm age,min} \sim 1{\!-\!}3\, {\rm yr}$$ based on the near-source plasma contribution to observed DM, and $$t_{\rm age,min} \sim 6.5{\!-\!}10\, {\rm yr}$$ from the absence of radio signal attenuation. 

Abstract Superheavy dark matter has been attractive as a candidate of particle dark matter. We propose a “natural” particle model, in which the dark matter serves as the inflaton in natural inflation, while decaying to high-energy particles at energies of 10⁹-10¹³GeV from the prediction of the inflation. A scalar field responsible for diluting the dark matter abundance revives the natural inflation either with or without the recent data from the Atacama Cosmology Telescope (ACT) and baryon acoustic oscillation results from Dark Energy Spectroscopic Instrument.Since the dark matter must be a spin-zero scalar, we carefully study the galactic dark matter 3-body decay into fermions and two body decays into a gluon pair, and point out relevant multi-messenger bounds that constrain these decay modes. Interestingly, the predicted energy scale may coincide with the AMATERASU event and/or the KM3NeT neutrino event, KM3-230213A. We also point out particle models with dark baryon to further alleviateγ-ray bounds. This scenario yields several testable predictions for the UHECR observations, including the highest-energy neutrons that are unaffected by magnetic fields, the tensor-to-scalar ratio, the running of spectral indices,α_s≳ 𝒪(0.001), and the existence of light new colored particles that could be accessible at future collider experiments.Further measurements of high-energy cosmic rays, including their components and detailed directions, may provide insight into not only the origin of the cosmic rays but also inflation. 

ABSTRACT The light curves of radioactive transients, such as supernovae and kilonovae, are powered by the decay of radioisotopes, which release high-energy leptons through $$\beta ^+$$ and $$\beta ^-$$ decays. These leptons deposit energy into the expanding ejecta. As the ejecta density decreases during expansion, the plasma becomes collisionless, with particle motion governed by electromagnetic forces. In such environments, strong or turbulent magnetic fields are thought to confine particles, though the origin of these fields and the confinement mechanism have remained unclear. Using fully kinetic particle-in-cell (PIC) simulations, we demonstrate that plasma instabilities can naturally confine high-energy leptons. These leptons generate magnetic fields through plasma streaming instabilities, even in the absence of pre-existing fields. The self-generated magnetic fields slow lepton diffusion, enabling confinement, and transferring energy to thermal electrons and ions. Our results naturally explain the positron trapping inferred from late-time observations of thermonuclear and core-collapse supernovae. Furthermore, they suggest potential implications for electron dynamics in the ejecta of kilonovae. We also estimate synchrotron radio luminosities from positrons for Type Ia supernovae and find that such emission could only be detectable with next-generation radio observatories from a Galactic or local-group supernova in an environment without any circumstellar material.