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Description
It is generally accepted that in supernova remnants (SNRs), charged particles are accelerated to cosmic-ray energies through the mechanism of diffusive shock acceleration (DSA). The most compelling observational evidence supporting this model is related to the electron component: relativistic electrons with energies of about $\sim 10$ GeV, accelerated in SNRs, generate nonthermal radio emission, which is one of the main observational manifestations of SNRs. The mechanism of DSA in supernova remnants has been extensively studied, especially in relation to young SNRs, as a significant amount of observational data is available for them.
A model of the evolution of radio emission from shell-type supernova remnants (SNRs) has been proposed, based on the mechanism of diffusive shock acceleration of electrons at the outer shock wave in the test-particle regime. It has been shown that during the evolution of an SNR in a uniform medium, the shock wave formed by the supernova explosion remains a continuous source of cosmic rays until the Mach number of the outer shock wave reaches $M\simeq2$. It is important to note that the self-sustaining nature of DSA process helps to accelerate particles even at such low intensities of the outer shock wave, provided that the interstellar medium (ISM) in which the SNR evolves is relatively uniform and sufficiently ionized.
It is known that during its evolution, an SNR undergoes several stages: free expansion, the adiabatic Sedov-Taylor stage, and the radiative phase. Particle acceleration occurs according to the standard diffusive shock acceleration scenario in the region of the outer shock wave front, whose size is significantly smaller than all other characteristic scales, such as the cooling length, gradients of changing physical quantities, etc.
Our model of shell-type SNR radio emission is in very good agreement with SNR statistics, which can be considered as evidence supporting the idea that shock waves, weakened to low Mach numbers, continue to accelerate electrons to energies sufficient for GHz radio wave emission.
