2023ApJ...945L...3M

Long-term observations of synchrotron emission from supernovae (SNe), covering more than a year after the explosion, provide a unique opportunity to study the poorly understood evolution of massive stars in the final millennium of their lives via changes in the mass-loss rate. Here we present a result of our long-term monitoring of the peculiar Type IIL SN 2018ivc using the Atacama Large Millimeter/submillimeter Array. Following the initial decay, it showed unprecedented rebrightening starting ∼1 yr after the explosion. This is one of the rare examples showing such rebrightening in the synchrotron emission and the first case at millimeter wavelengths. We find it to be in the optically thin regime, unlike the optically thick centimeter emission. As such, we can robustly reconstruct the distribution of the circumstellar matter and thus the mass-loss history in the final ≳1000 yr. We find that the progenitor of SN 2018ivc had experienced a very high mass-loss rate (≳10^–3M_⊙ yr^–1) ∼1500 yr before the explosion, which was followed by a moderately high mass-loss rate (≳10^–4M_⊙ yr^–1) up until the explosion. From this behavior, we suggest that SN 2018ivc represents an extreme version of a binary evolution toward SNe IIb, which bridges the hydrogen-poor SNe (toward SNe Ib/c, without a hydrogen envelope) and hydrogen-rich SNe (SNe IIP, with a massive envelope).