Program: GS-2021A-Q-124

We propose a joint Gemini-Zwicky Transient Facility (ZTF) ToO program to investigate young, relativistic stellar explosions and supernovae (SNe) on timescales of hours to days. High-cadence ZTF observations over large areas will regularly uncover very young transients. For the most common stellar explosions, supernovae (SNe), Gemini observations will prove critical in solving the long-standing problem of mapping between SN types and progenitor systems. For SNe Ia, early spectroscopy is sensitive to the surface layer composition and immediate circumstellar medium (CSM). Gemini will play a key role in triggering our approved HST program for UV spectroscopy of the signal expected in the single degenerate model. For core collapse SNe, Gemini will help map SN types to progenitor properties (radius, mass-loss history), while also probing the unique CSM seen around some massive stars. Finally, there are classes of relativistic explosions predicted to exist but never found: dirty fireballs and orphan afterglows. This Gemini-ZTF program has the capability to discover these events for the first time, while also setting the energy scale and rates for these transients.