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1. Hadronic versus Leptonic Origin of Gamma-Ray Emission from Supernova Remnants

   https://doi.org/10.3847/1538-4357/ace699  

   Corso, Nicholas J. ; Diesing, Rebecca ; Caprioli, Damiano ( August 2023 , The Astrophysical Journal)

   GeV and TeV emission from the forward shocks of supernova remnants (SNRs) indicates that they are capable particle accelerators, making them promising sources of Galactic cosmic rays (CRs). However, it remains uncertain whether thisγ-ray emission arises primarily from the decay of neutral pions produced by very-high-energy hadrons, or from inverse-Compton and/or bremsstrahlung emission from relativistic leptons. By applying a semi-analytic approach to non-linear diffusive shock acceleration, and calculating the particle and photon spectra produced in different environments, we parameterize the relative strength of hadronic and leptonic emission. We show that even if CR acceleration is likely to occur in all SNRs, the observed photon spectra may primarily reflect the environment surrounding the SNR: the emission is expected to look hadronic unless the ambient density is particularly low (with proton number density ≲0.1 cm⁻³) or the photon background is enhanced with respect to average Galactic values (with radiation energy densityu_rad≳ 10 eV cm⁻³). We introduce a hadronicity parameter to characterize how hadronic or leptonic we expect a source to look based on its environment, which can be used to guide the interpretation of currentγ-ray observations and the detection of high-energy neutrinos from SNRs.

    

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2. Diffusive shock re-acceleration

   https://doi.org/10.1017/S0022377818000478  

   Caprioli, Damiano ; Zhang, Horace ; Spitkovsky, Anatoly ( June 2018 , Journal of Plasma Physics)

   We have performed two-dimensional hybrid simulations of non-relativistic collisionless shocks in the presence of pre-existing energetic particles (‘seeds’); such a study applies, for instance, to the re-acceleration of galactic cosmic rays (CRs) in supernova remnant (SNR) shocks and solar wind energetic particles in heliospheric shocks. Energetic particles can be effectively reflected and accelerated regardless of shock inclination via a process that we call diffusive shock re-acceleration. We find that re-accelerated seeds can drive the streaming instability in the shock upstream and produce effective magnetic field amplification. This can eventually trigger the injection of thermal protons even at oblique shocks that ordinarily cannot inject thermal particles. We characterize the current in reflected seeds, finding that it tends to a universal value $J\simeq en_{\text{CR}}v_{\text{sh}}$ , where $en_{\text{CR}}$ is the seed charge density and $v_{\text{sh}}$ is the shock velocity. When applying our results to SNRs, we find that the re-acceleration of galactic CRs can excite the Bell instability to nonlinear levels in less than ${\sim}10~\text{yr}$ , thereby providing a minimum level of magnetic field amplification for any SNR shock. Finally, we discuss the relevance of diffusive shock re-acceleration also for other environments, such as heliospheric shocks, galactic superbubbles and clusters of galaxies. 

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3. The Maximum Energy of Shock-accelerated Cosmic Rays

   https://doi.org/10.3847/1538-4357/ad00b1  

   Diesing, Rebecca ( November 2023 , The Astrophysical Journal)

   Identifying the accelerators of Galactic cosmic ray (CR) protons with energies up to a few PeV (10¹⁵eV) remains a theoretical and observational challenge. Supernova remnants (SNRs) represent strong candidates because they provide sufficient energetics to reproduce the CR flux observed at Earth. However, it remains unclear whether they can accelerate particles to PeV energies, particularly after the very early stages of their evolution. This uncertainty has prompted searches for other source classes and necessitates comprehensive theoretical modeling of the maximum proton energy,$E_{\max }$, accelerated by an arbitrary shock. While analytic estimates of$E_{\max }$have been put forward in the literature, they do not fully account for the complex interplay between particle acceleration, magnetic field amplification, and shock evolution. This paper uses a multizone, semianalytic model of particle acceleration based on kinetic simulations to place constraints on$E_{\max }$for a wide range of astrophysical shocks. In particular, we develop relationships between$E_{\max }$, shock velocity, size, and ambient medium. We find that SNRs can only accelerate PeV particles under a select set of circumstances, namely, if the shock velocity exceeds ∼10⁴km s⁻¹and escaping particles drive magnetic field amplification. However, older and slower SNRs may still produce observational signatures of PeV particles due to populations accelerated when the shock was younger. Our results serve as a reference for modelers seeking to quickly produce a self-consistent estimate of the maximum energy accelerated by an arbitrary astrophysical shock.¹¹

   Presented as a thesis to the Department of Astronomy and Astrophysics, The University of Chicago, in partial fulfillment of the requirements for a Ph.D. degree.

    

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4. An Unusual Energetic Particle Flux Enhancement Associated with Solar Wind Magnetic Island Dynamics

   Zhao, L.-L. ; Zank, G. P. ; Khabarova, O. ; Du, S. ; Chen, Y. ; Adhikari, L. ; Hu, Q. ( September 2018 , The Astrophysical journal. Letters to the editor)

   The possibility that charged particles are accelerated statistically in a “sea” of small-scale interacting magnetic flux ropes in the supersonic solar wind is gaining credence. In this Letter, we extend the Zank et al. statistical transport theory for a nearly isotopic particle distribution by including an escape term corresponding to particle loss from a finite acceleration region. Steady-state 1D solutions for both the accelerated particle velocity distribution function and differential intensity are derived. We show Ulysses observations of an energetic particle flux enhancement event downstream of a shock near 5 au that is inconsistent with the predictions of classical diffusive shock acceleration (DSA) but may be explained by local acceleration associated with magnetic islands. An automated Grad-Shafranov reconstruction approach is employed to identify small-scale magnetic flux ropes behind the shock. For the first time, the observed energetic particle “time-intensity” profile and spectra are quantitatively compared with theoretical predictions. The results show that stochastic acceleration by interacting magnetic islands accounts successfully for the observed (i) peaking of particle intensities behind the shock instead of at the shock front as standard DSA predicts; (ii) increase in the particle flux amplification factor with increasing particle energy; (ii) increase in distance between the particle intensity peak and the shock front with increasing energy; and (iv) hardening of particle power-law spectra with increasing distance downstream of the shock. 

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5. An Unusual Energetic Particle Flux Enhancement Associated with Solar Wind Magnetic Island Dynamics

   Zhao, L.-L. ; Zank, G.P. ; Khabarova, O. ; Du, S. ; Chen, Y. ; Adhikari, L. ; Hu, Q. ( September 2018 , Astrophysical journal)

   The possibility that charged particles are accelerated statistically in a “sea” of small-scale interacting magnetic flux ropes in the supersonic solar wind is gaining credence. In this Letter, we extend the Zank et al. statistical transport theory for a nearly isotopic particle distribution by including an escape term corresponding to particle loss from a finite acceleration region. Steady-state 1D solutions for both the accelerated particle velocity distribution function and differential intensity are derived. We show Ulysses observations of an energetic particle flux enhancement event downstream of a shock near 5 au that is inconsistent with the predictions of classical diffusive shock acceleration (DSA) but may be explained by local acceleration associated with magnetic islands. An automated Grad-Shafranov reconstruction approach is employed to identify small-scale magnetic flux ropes behind the shock. For the first time, the observed energetic particle “time-intensity” profile and spectra are quantitatively compared with theoretical predictions. The results show that stochastic acceleration by interacting magnetic islands accounts successfully for the observed (i) peaking of particle intensities behind the shock instead of at the shock front as standard DSA predicts; (ii) increase in the particle flux amplification factor with increasing particle energy; (ii) increase in distance between the particle intensity peak and the shock front with increasing energy; and (iv) hardening of particle power-law spectra with increasing distance downstream of the shock. 

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