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

Author(s): Ho, Anna YQ; Goldstein, Daniel A; Schulze, Steve; Khatami, David K; Perley, Daniel A; Ergon, Mattias; Gal-Yam, Avishay; Corsi, Alessandra; Andreoni, Igor; Barbarino, Cristina; Bellm, Eric C; Blagorodnova, Nadia; Bright, Joe S; Burns, Eric; Cenko, S Bradley; Cunningham, Virginia; De, Kishalay; Dekany, Richard; Dugas, Alison; Fender, Rob P; Fransson, Claes; Fremling, Christoffer; Goldstein, Adam; Graham, Matthew J; Hale, David; Horesh, Assaf; Hung, Tiara; Kasliwal, Mansi M; Kuin, N Paul M; Kulkarni, Shri R; Kupfer, Thomas; Lunnan, Ragnhild; Masci, Frank J; Ngeow, Chow-Choong; Nugent, Peter E; Ofek, Eran O; Patterson, Maria T; Petitpas, Glen; Rusholme, Ben; Sai, Hanna; Sfaradi, Itai; Shupe, David L; Sollerman, Jesper; Soumagnac, Maayane T; Tachibana, Yutaro; Taddia, Francesco; Walters, Richard; Wang, Xiaofeng; Yao, Yuhan; Zhang, Xinhan | Abstract: 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 $E_\mathrm{\gamma,iso}l4.9 \times 10^{48}$ erg, a limit on X-ray emission $L_{\mathrm{X}} l 10^{40}\,$erg sec$^{-1}$, and a limit on radio emission $\nu L_\nu \lesssim 10^{37}\,$erg sec$^{-1}$. Taken together, we find that the early ($l10\,$days) data are best explained by shock breakout in a massive shell of dense circumstellar material ($0.02\,M_\odot$) at large radii ($3 \times 10^{14}\,$cm) that was ejected in eruptive pre-explosion mass-loss episodes. The late-time ($g10$ days) light curve requires an additional energy source, which could be the radioactive decay of Ni-56.