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 r o ∝ t m ( 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 v 0 ~ 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.