A Physical Model of Delayed Rebrightenings in Shock-interacting Supernovae without Narrow-line Emission

Core-collapse supernovae can display evidence of interaction with preexisting, circumstellar shells of material by rebrightening and forming spectral lines, and can even change types as hydrogen appears in previously hydrogen-poor spectra. However, a recently observed core-collapse supernova - SN 2019tsf - was found to brighten after roughly 100 days after it was first observed, suggesting that the supernova ejecta was interacting with surrounding material, but it lacked any observable emission lines and thereby challenged the standard supernova-interaction picture. We show through linear perturbation theory that delayed rebrightenings without the formation of spectral lines are generated as a consequence of the finite sound-crossing time of the postshock gas left in the wake of a supernova explosion. In particular, we demonstrate that sound waves - generated in the postshock flow as a consequence of the interaction between a shock and a density enhancement - traverse the shocked ejecta and impinge upon the shock from behind in a finite time, generating sudden changes in the shock properties in the absence of ambient density enhancements. We also show that a blast wave dominated by gas pressure and propagating in a wind-fed medium is unstable from the standpoint that small perturbations lead to the formation of reverse shocks within the postshock flow, implying that the gas within the inner regions of these blast waves should be highly turbulent.