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 × 10⁴⁴ 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 ≈ 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 < 10⁴⁰ erg s^-1, and a limit on radio emission ν L_ν ≲ 10³⁷ 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 × 10¹⁴ 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.