Fast Blue Optical Transients Due to Circumstellar Interaction and the Mysterious Supernova SN 2018gep

Abstract

The discovery of SN 2018gep (ZTF 18abukavn) challenged our understanding of the late-phase evolution of massive stars and their supernovae (SNe). The fast rise in luminosity of this SN (spectroscopically classified as a broad-lined Type Ic SN) indicates that the ejecta interacts with a dense circumstellar medium (CSM), while an additional energy source such as ⁵⁶Ni decay is required to explain the late-time light curve. These features hint at the explosion of a massive star with pre-SN mass loss. In this work, we examine the physical origins of rapidly evolving astrophysical transients like SN 2018gep. We investigate the wave-driven mass-loss mechanism and how it depends on model parameters such as progenitor mass and deposition energy, searching for stellar progenitor models that can reproduce the observational data. A model with an ejecta mass ~2 M⊙, explosion energy ~10⁵² erg, a CSM of mass ~0.3 M⊙ and radius ~1000 R⊙, and a ⁵⁶Ni mass ~0.3 M⊙ provides a good fit to the bolometric light curve. We also examine how interaction-powered light curves depend more generally on these parameters and how ejecta velocities can help break degeneracies. We find both wave-driven mass loss and mass ejection via pulsational pair instability can plausibly create the dense CSM in SN 2018gep, but we favor the latter possibility.