SN 1993J and M81: a fruitful astrophysical collaboration

SN1993J, a powerful radio supernova high in the northern sky, has been monitored with VLBI though its lifetime. The VLBI observations have been phase-referenced to the core of M81. We will highlight the main results obtained: its extremely circular shell-like radio structure has expanded over 15 years in a rather self-similar way; the expansion is wavelength dependent, a result which can be mainly explained by a combination of a varying free-free opacity in the supernova ejecta and a radial shape of the magnetic field intensity in the emitting region; from day 3100 after explosion onwards, there is an abrupt flux-density decay which can be explained by the supernova shock surpassing beyond the circumstellar/interstellar boundary.  

On the other hand, we have taken advantage of the fact that SN1993J is a perfect astrometric reference to study with great detail the absolute kinematics and the core-shift effect in M81*, the low luminosity AGN of its host galaxy M81. We will interpret our results as a signal of jet precession.