Anna Ho

With increasing cadence and areal coverage, optical transient surveys can probe fast and rare regions of transient parameter space. In particular, ~100 transients have been discovered whose properties (a fast rise to peak, and high peak luminosity) challenge our understanding of how energy is deposited and transported in stellar explosions. Here we present ZTF18abukavn (SN2018gep), discovered as a rapidly rising transient (1.3 mag/day) and spectroscopically classified as a high-velocity stripped-envelope supernova (Ic-BL SN). SN2018gep was discovered 20 minutes after first light, and within hours we commenced an intensive multi-wavelength monitoring campaign, including the earliest-ever spectra of a stripped-envelope SN. A retrospective search revealed luminous (M = -14 mag) emission in the days to months before explosion, the first definitive detection of precursor emission for a Ic-BL. The rapid rise (~1 day), blue colors at peak (g-r = -0.3) and proximity (z=0.0322) establish SN2018gep as the second fast-luminous optical transient (after AT2018cow) studied up close in real time. Taken together, we find that the data are best explained by shock breakout in a massive shell of dense circumstellar material (0.02 Msol) at large radii (3 x 10^14 cm) that was ejected in eruptive pre-explosion mass-loss episodes.