The Bright Supernova 1996cr in the Circinus Galaxy Imaged with VLBI: Shell Structure with Complex Evolution

We present broadband radio flux-density measurements supernova (SN) 1996cr, made with MeerKAT, ATCA and ALMA, and images made from very long baseline interferometry (VLBI) observations with the Australian Long Baseline Array. The spectral energy distribution of SN 1996cr in 2020, at age, $t\sim$8700 d, is a power-law, with flux density, $S\propto {\nu }^{-0.588\pm 0.011}$ between 1 and 34 GHz, but may steepen at $>35$ GHz. The spectrum has flattened since $t=5370$ d (2010). Also since $t=5370$ d, the flux density has declined rapidly, with $S_{9\mathrm{G}\mathrm{H}\mathrm{z}}\propto t^{-2.9}$. The VLBI image at $t=8859$ d shows an approximately circular structure, with a central minimum reminiscent of an optically-thin spherical shell of emission. For a distance of 3.7 Mpc, the average outer radius of the radio emission at $t=8859$ d was $(5.1\pm 0.3)\times {10}^{17}$ cm, and SN 1996cr has been expanding with a velocity of $4650\pm 1060$ km s${}^{-1}$ between $t=4307$ and 8859 d. It must have undergone considerable deceleration before $t=4307$ d. Deviations from a circular shell structure in the image suggest a range of velocities up to $\sim$7000 km s${}^{-1}$, and hint at the presence of a ring- or equatorial-belt-like structure rather than a complete spherical shell.