The Origin Of Cosmic High-energy Particles

The origin of cosmic-ray particles,or simply cosmic rays(CRs)for short,which in this thesis narrowly refer to charged particles traveling as fast as the light,is one of the essential problems in astrophysics.With one-hundred-year observational,theoretical and numerical work,it is widely accepted that Galactic CRs(GCRs)are from diffusive particle accelerations driven by large-scale shocks of supernova remnants(SNRs).Un-der the steady state from an injection-convection-diffusion balance,the diffusive shock acceleration(DSA)can achieve an(inverse)power-law energy spectrum of accelerated particles with the spectral index depending only on the shock compression ratio.Such a simple but effective particle acceleration mechanism is not only invoked in the hypoth-esis of the GCR origin from SNRs,but also widely used to give qualitative explanations for other astronomical high-energy phenomena.Recently,the classical qualitative model of the DSA and the Galactic diffusive propagation(GDP)of CRs is challenged by some precise observational results.Never-theless,so far it is hard to shake the dominant position of SNRs in the contribution to the GCR total flux,but more detailed physical processes need to be taken into account in the DSA-GDP scenario to fit these new observations.We study two subtle "anoma-lous" structures discovered by direct measurements of CR spectra,where the one is a spectral hardening around 200 GV for all particles,and the other one is the fact that above 45 GV the spectrum of protons is softer than those of other primary nuclei.In combination with the evolution of SNR radiation spectra implied by multi-wavelength observations,we suggest the model of time-dependent particle accelerations by SNRs.Accordingly,these anomalous structures are mainly explained via a superposition ef-fect of characteristic acceleration spectra from two distinct evolving stages of SNRs,and also affected by dependence of the DSA injection rigidity on the particle charge-to-mass ratio.This model can also naturally reproduce the spectral "knee" with prominent contributions of SNRs up to the "ankle",and predict a turbulent diffusion of particles in SNRs.These conclusions need to be verified by observations with higher precision and plasma numerical simulations.Another issue remains to be addressed is whether it is necessary,and how to appro-priately introduce nonlinear feedback of CRs into the DSA model.Although in physical principle the feedback must exist,in observations of SNRs there has been no strong ev-idence for nonlinear shock "precursors".In consideration of the overall conservation law,without constraints for effects of the fluid discontinuity on CRs,we study jump conditions between the shock up-and downstream,and then obtain a lower limit of the acceleration efficiency from the energy spectral index of accelerated CRs.On that ba-sis,taking no account of the thickness of the nonlinear diffusion region,we prove that the spectral index is positively,and the acceleration efficiency is negatively correlated with the shock speed.Qualitatively,the first and the second correlation is consistent with multi-wavelength observations of SNRs and expectations of the time-dependent particle acceleration model,respectively.For maintaining an acceleration efficiency of 10-50%,the injection rate and the maximum energy of the particle acceleration need to be positively correlated with the shock speed.We also show that,even constraining the CR distribution to be continuous at the fluid discontinuity,the escape of particles from the acceleration source is not necessary for the acceleration.This is a clarification on the ambiguous conclusion of some previous studies.We also solve the self-similar prob-lem for blast waves considering the CR feedback,and find that the blast-wave center is dominated by the ultra-relativistic pressure,thus the overall acceleration efficiency can be amplified.This verifies a solution found by the early work.Despite the fact that the power-law energy spectrum of particles largely deviates from the Maxwell distribution,and indeed may always be constructed with non-thermal transport theories,there seems no satisfactory answer to the question whether it can be incorporated into the scope of a thermodynamic equilibrium framework.Based on the wave-particle gyro-resonance regime,for a system governed by degrees of freedom of waves,we derive the power-law spectrum under the equilibrium state.Further studies are needed to show whether this theory can be used to explain "non-thermal" particles found in plasma numerical simulations and astronomical observations.