Evidence for Late-stage Eruptive Mass-loss in the Progenitor to SN2018gep, a Broad-lined Ic Supernova: Pre-explosion Emission and a Rapidly Rising Luminous Transient

We present detailed observations of ZTF18abukavn (SN2018gep), discovered in high-cadence data from the Zwicky Transient Facility as a rapidly rising (\(1.4\pm0.1\) mag/hr) and luminous (\(M_{g,\mathrm{peak}}=-20\) mag) transient. It is spectroscopically classified as a broad-lined stripped-envelope supernova (Ic-BL SN). The high peak luminosity (\(L_{\mathrm{bol}} \gtrsim 3 \times 10^{44}\) erg \(\mathrm{sec}^{-1}\)), the short rise time (\(t_{\mathrm{rise}}= 3\) days in \(g\)-band), and the blue colors at peak (\(g-r\sim-0.4\)) 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 (\(T_{\mathrm{eff}}\gtrsim40,000\) K) spectra of a stripped-envelope SN. A retrospective search revealed luminous (\(M_g \sim M_r \approx -14\,\)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_\mathrm{\gamma,iso}