2009A&A...505..927M

We studied the growth of the shell-like radio structure of supernova SN1993J in M81 from September 1993 to October 2003 with very-long-baseline interferometry (VLBI) observations at the wavelengths of 3.6, 6, and 18cm. We developed a method to accurately determine the outer radius (R) of any circularly symmetric compact radio structure such as SN1993J. The source structure of SN1993J remains circularly symmetric (with deviations from circularity under 2%) over almost 4000 days. We characterize the decelerated expansion of SN1993J until approximately day 1500 after explosion with an expansion parameter m=0.845±0.005 (R ∝t^m). However, from that day onwards the expansion differs when observed at 6 and 18cm. Indeed, at 18cm, the expansion can be well characterized by the same m as before day 1500, while at 6cm the expansion appears more decelerated, and is characterized by another expansion parameter, m₆=0.788±0.015. Therefore, since about day 1500 onwards, the radio source size has been progressively smaller at 6cm than at 18cm. These findings differ significantly from those of other authors in the details of the expansion. In our interpretation, the supernova expands with a single expansion parameter, m=0.845±0.005, and the 6cm results beyond day 1500 are caused by physical effects, perhaps also coupled to instrumental limitations. Two physical effects may be involved: (a) a changing opacity of the ejecta to the 6cm radiation; and (b) a radial decrease of the magnetic field in the emitting region. We also found that at 6cm about 80% of the radio emission from the backside of the shell behind the ejecta is absorbed (our average estimate, since we cannot determine any possible evolution of the opacity), and the width of the radio shell is (31±2)% of the outer radius. The shell width at 18cm depends on the degree of assumed absorption. For 80% absorption, the width is (33.5±1.7)%, and for 100% absorption, it is (37.8±1.3)%. A comparison of our VLBI results with optical spectral line velocities shows that the deceleration is more pronounced in the radio than in the optical. This difference might be due to a progressive penetration of ejecta instabilities into the shocked circumstellar medium, as also suggested by other authors.