Radio emission from shell-type supernova remnants

The evolution of the radio emission of shell-type Supernova remnants (SNRs) is modeled within the framework of the simple and commonly used assumptions that the mechanism of diffusive shock acceleration (DSA) is responsible for generating radio emitting electrons and that the magnetic field is the typical interstellar field compressed at the shock. It is considered that electrons are injected into the mechanism in test-particle regime directly from the high energy tail of the downstream Maxwellian distribution function. The model can be applied to most of the observed SNRs. It is shown that the model successfully explains the many averaged observational properties of evolved shell-type SNRs. In particular, the radio surface brightness ($\Sigma$) evolves with diameter as $\sim D^{-(0.3 \div 0.5)}$, while the bounding shock is strong (Mach number is ${\mathcal M} \geq10$), followed by steep decrease (steeper than $\sim D^{-4.5}$) for ${\cal M}