SN 1993J VLBI. II. Related Changes of the Deceleration, Flux Density Decay, and Spectrum

Phase-referenced multifrequency VLBI observations of supernova 1993J in the nearby galaxy M81 at 34 epochs from t = 30 to ~3000 days after shock breakout show the detailed characteristics of the non-self-similar expansion. For the first year, the supernova expanded almost freely, with the outer radius of the radio shell ro ∝ tm(t) and m(t) = 0.919 ± 0.019. During the following 4 years the deceleration grew to m(t) = 0.781 ± 0.009, the expansion velocity slowed down by about half from originally v0 ~ 17,200 km s-1 to just ~8900 km s-1, and the supernova swept up a mass of ~0.3 M☉. Momentum conservation suggests that this mass is about equal to the mass of the shocked ejecta, likely composed of all the low-mass envelope of the progenitor left over after mass transfer to a purported binary companion, therefore supporting the binary scenario. Subsequently the deceleration changed again, this time decreasing to m(t) = 0.860 ± 0.011 from t = 5 to 8 yr, consistent with hydrodynamic simulations. This late upturn of m(t) may be caused by the massive inner ejecta, which have reached the increasingly decelerated shocked ejecta, passing now through the reverse shock and pushing the low-mass envelope. This upturn of m(t) is mirrored by changes in the slope of the radio light curves and a flattening of the radio spectra, all consistent with power laws, with spectral indices changing from α = -0.85 ± 0.03 to -0.64 ± 0.03. The magnetic field in the radio shell region is inferred to decrease from originally ∝ r to ∝ r toward the end of our observations. After 7 years the radio light curves start to drop rapidly, suggesting that the supernova is now expanding into a zone of the circumstellar medium with a steep density profile.