2023MNRAS.524.5309M

Some interaction-powered supernovae have long rise times of more than 100 d. We show that such long rise times are naturally expected if cirumstellar matters (CSM) have a flat density structure (s ≲ 1.5, where ρ_CSM∝r^-s^). In such cases, bolometric luminosities from the CSM interaction keep increasing as long as the CSM interacts with the outer layers of the SN ejecta. Thus, the rise time is determined by the dynamical time-scale in which the reverse shock propagates the outer layers of the SN ejecta, not by the time-scales in which photons diffuse in the CSM as often considered. Interaction-powered supernovae with very long rise times can be an important probe of extensive non-steady mass-loss in massive stars.