Energy Conservation in the thin layer approximation: V. The surface
  brightness in supernova remnants
  Two new equations of motion for a supernova remnant (SNR) are derived in the
framework of energy conservation for the thin-layer approximation. The first
one is based on an inverse square law for the surrounding density and the
second one on a non-cubic dependence of the swept mass. Under the assumption
that the observed radio-flux scales as the flux of kinetic energy, two scaling
laws are derived for the temporal evolution of the surface brightness of SNRs.
The astrophysical applications cover two galactic samples of surface brightness
and an extragalactic one.
