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 ± 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× {10}⁴⁴ {erg} {{{s}}}^-1), the short rise time ({t}_rise}=3 {days} 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}≳ {{40,000}} {{K}}) 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× {10}⁴⁸ {erg}, a limit on X-ray emission {L}_{{X}}< {10}⁴⁰ {erg} {{{s}}}^-1, and a limit on radio emission ν {L}_ν ≲ {10}³⁷ {erg} {{{s}}}^-1. Taken together, we find that the early (< 10 {days}) data are best explained by shock breakout in a massive shell of dense circumstellar material (0.02 {M}_⊙ ) at large radii (3× {10}¹⁴ {cm}) that was ejected in eruptive pre-explosion mass-loss episodes. The late-time (> 10 {days}) light curve requires an additional energy source, which could be the radioactive decay of Ni-56.