Using a semianalytical approach based on the thin-shell approximation, we calculate the long-term evolution of supernova remnants (SNRs) while also accounting for the cosmic rays (CRs) accelerated at ...their blast waves. Our solution reproduces the results of state-of-the-art fluid simulations across the adiabatic and radiative stages for the gas-only case, and it predicts that typical CR acceleration efficiencies (≈10%) can boost SNR momentum deposition by a factor of 2-3. This enhancement can become as large as an order of magnitude in environments in which the gas experiences more severe radiative losses. This result may have a crucial impact on modeling the effect of supernova feedback on star formation and galaxy evolution.
Collisionless plasma shocks are efficient sources of nonthermal particle acceleration in space and astrophysical systems. We use hybrid (kinetic ion-fluid electron) simulations to examine the ...nonlinear feedback of the self-generated energetic particles (cosmic rays, CRs) on the shock hydrodynamics. When CR acceleration is efficient, we find evidence of both an upstream precursor, where the inflowing plasma is compressed and heated, and a downstream postcursor, where the energy flux in CRs and amplified magnetic fields play a dynamical role. For the first time, we assess how nonlinear magnetic fluctuations in the postcursor preferentially travel away from the shock at roughly the local Alfvén speed with respect to the downstream plasma. The drift of both magnetic and CR energy with respect to the thermal plasma substantially increases the shock compression ratio with respect to the standard prediction, in particular exceeding 4 for strong shocks. Such modifications also have implications for the spectrum of the particles accelerated via diffusive shock acceleration, a significant result detailed in a companion paper.
It has been proposed that ultra-high-energy cosmic rays (UHECRs) up to 1020 eV could be produced in the relativistic jets of powerful active galactic nuclei (AGNs) via a one-shot reacceleration of ...lower-energy CRs produced in supernova remnants (the espresso mechanism). We test this theory by propagating particles in realistic 3D magnetohydrodynamic simulations of ultrarelativistic jets and find that about 10% of the CRs entering the jet are boosted by at least a factor of ∼Γ2 in energy, where Γ is the jet's effective Lorentz factor, in agreement with the analytical expectations. Furthermore, about 0.1% of the CRs undergo two or more shots and achieve boosts well in excess of Γ2. Particles are typically accelerated up to the Hillas limit, suggesting that the espresso mechanism may promote galactic-like CRs to UHECRs even in AGN jets with moderate Lorentz factors, and not in powerful blazars only. Finally, we find that the sign of the toroidal magnetic field in the jet and in the cocoon controls the angular distribution of the reaccelerated particles, leading to a UHECR emission that may be either quasi-isotropic or beamed along the jet axis. These findings strongly support the idea that espresso acceleration in AGN jets can account for the UHECR spectra, chemical composition, and arrival directions measured by Auger and Telescope Array.
Using a semianalytic model of nonlinear diffusive shock acceleration, we model the total spectrum of cosmic ray (CR) electrons accelerated by supernova remnants (SNRs). Because electrons experience ...synchrotron losses in the amplified magnetic fields characteristic of SNRs, they exhibit substantially steeper spectra than protons. In particular, we find that the difference between the electron and proton spectral index (power law slope) ranges from 0.1 to 0.4. Our findings must be reckoned with theories of Galactic CR transport, which often assume that electrons and protons are injected with the same slope, and may especially have implications for the observed "positron excess".
Abstract
In the
espresso
scenario, ultra-high-energy (UHE) cosmic rays (CRs) are produced via a one-shot reacceleration of galactic-like CRs in the relativistic jets of active galactic nuclei, ...independently of the scattering rate dictated by magnetic fluctuations. In Mbarek & Caprioli (2019) we traced test-particle CRs in high-resolution magnetohyrodynamic (MHD) jet simulations and found that the associated spectral slope, chemical composition, and anisotropy are consistent with UHECR phenomenology. In this work, we extend such an analysis by including subgrid pitch-angle scattering to model small-scale magnetic turbulence that cannot be resolved by MHD simulations. We find that a large scattering rate unlocks stochastic acceleration and fosters the energization of lower-energy CRs, which eventually leads to harder UHECR spectra. Yet, the particles that achieve the highest energies (up to the Hillas limit) are invariably produced by
espresso
acceleration and their spectrum is independent of the assumed subgrid scattering rate.
We suggest that ultra-high-energy (UHE) cosmic rays (CRs) may be accelerated in ultra-relativistic flows via a one-shot mechanism, the “espresso” acceleration, in which already-energetic particles ...are generally boosted by a factor of ∼Γ2 in energy, where Γ is the flow Lorentz factor. More precisely, we consider blazar-like ultra-relativistic jets propagating into a halo of “seed” CRs produced in supernova remnants, which can accelerate UHECRs up to 1020 eV. Such a re-acceleration process naturally accounts for the chemical composition measured by the Pierre Auger Collaboration, which resembles the one around and above the knee in the CR spectrum, and is consistent with the distribution of potential sources in the local universe; particularly intriguing is the coincidence of the powerful blazar Mrk 421 with the hotspot reported by the Telescope Array Collaboration.
We study diffusive shock acceleration (DSA) of electrons in nonrelativistic quasi-perpendicular shocks using self-consistent one-dimensional particle-in-cell simulations. By exploring the parameter ...space of sonic and Alfvénic Mach numbers we find that high Mach number quasi-perpendicular shocks can efficiently accelerate electrons to power-law downstream spectra with slopes consistent with DSA prediction. Electrons are reflected by magnetic mirroring at the shock and drive nonresonant waves in the upstream. Reflected electrons are trapped between the shock front and upstream waves, and undergo multiple cycles of shock-drift acceleration before the injection into DSA. Strong current-driven waves also temporarily change the shock obliquity and cause mild proton pre-acceleration even in quasi-perpendicular shocks, which otherwise do not accelerate protons. These results can be used to understand nonthermal emission in supernova remnants and intracluster medium in galaxy clusters.
Diffusive shock acceleration is a prominent mechanism for producing energetic particles in space and in astrophysical systems. Such energetic particles have long been predicted to affect the ...hydrodynamic structure of the shock, in turn leading to CR spectra flatter than the test-particle prediction. However, in this work along with a companion paper, we use self-consistent hybrid (kinetic ion-fluid electron) simulations to show for the first time how CR-modified shocks actually produce steeper spectra. The steepening is driven by the enhanced advection of CRs embedded in magnetic turbulence downstream of the shock, in what we call the "postcursor." These results are consistent with multiwavelength observations of supernovae and supernova remnants and have significant phenomenological implications for space/astrophysical shocks in general.
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