Highlights of the expansion of SN1993J

Paper in proceeding, 2012

SN 1993J in M 81, a powerful radio supernova high in the northern sky, has been carefully monitored with VLBI throughout its lifetime. Its extremely circular shell-like radio structure has expanded over 15 years in a rather self-similar way in reasonable agreement with Chevalier’s model. An extension of Chevalier’s model simultaneously accounts for all the light-curve and VLBI results. Beyond the first year a single deceleration parameter, mtrue = 0.87± 0.02, characterizes the shock expansion. However, at short wavelengths and beyond 5 years, an enhanced deceleration, mshortwavelengths = 0.79±0.01, is measured. This enhancement is interpreted as due to a combination of effects ranging from varying free-free opacity in the supernova ejecta at the short wavelengths to the radial shape of the intensity of the magnetic fields in the emitting region. The final radio-loud stage is characterized by an abrupt decrease of radio emission. This large flux-density decay rate can be explained as due to the supernova shock surpassing the outer boundary of the circumstellar medium generated by the supernova progenitor. Presently, the supernova expansion beyond the circumstellar/interstellar boundary is rather radio silent, although there are indications that the supernova remnant could be studied by LOFAR and eMERLIN.