2019ApJ...887..169H

We present detailed observations of ZTF18abukavn (SN2018gep), discovered in high-cadence data from the Zwicky Transient Facility as a rapidly rising (1.4 ± 0.1 mag hr^–1) and luminous (M_g,peak=-20 mag) transient. It is spectroscopically classified as a broad-lined stripped-envelope supernova (Ic-BL SN). The high peak luminosity (L_bol≳3×10⁴⁴ergs^–1), the short rise time (t_rise=3days in g band), and the blue colors at peak (g–r∼-0.4) all resemble the high-redshift Ic-BL iPTF16asu, as well as several other unclassified fast transients. The early discovery of SN2018gep (within an hour of shock breakout) enabled an intensive spectroscopic campaign, including the highest-temperature (T_eff≳40,000K) spectra of a stripped-envelope SN. A retrospective search revealed luminous (M_g∼M_r~-14 mag) emission in the days to weeks before explosion, the first definitive detection of precursor emission for a Ic-BL. We find a limit on the isotropic gamma-ray energy release E_γ,iso_< 4.9×10⁴⁸erg, a limit on X-ray emission L_X< 10⁴⁰ergs^–1, and a limit on radio emission νL_ν≲10³⁷ergs^–1. Taken together, we find that the early (< 10days) data are best explained by shock breakout in a massive shell of dense circumstellar material (0.02 M_☉) at large radii (3×10¹⁴cm) that was ejected in eruptive pre-explosion mass-loss episodes. The late-time (> 10days) light curve requires an additional energy source, which could be the radioactive decay of Ni-56.