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

Abstract

We present detailed observations of ZTF18abukavn (SN2018gep), discovered in high-cadence data from the Zwicky Transient Facility as a rapidly rising ($1.4\pm 0.1$ 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 ${\sec }^{-1}$), 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 ($T_{\mathrm{eff}}\gtrsim 40,000$ 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 $E_{{\hat{\mathrm{I}}}^3,\mathrm{iso}}<4.9\times 10^{48}$ erg, a limit on X-ray emission $L_{\mathrm{X}}<10^{40}$erg sec${}^{-1}$, and a limit on radio emission $\hat{I}½L_{\hat{I}}½\lesssim 10^{37}$erg sec${}^{-1}$. Taken together, we find that the early ($<10$days) data are best explained by shock breakout in a massive shell of dense circumstellar material ($0.02M_{\odot }$) at large radii ($3\times 10^{14}$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.

Accepted to ApJ on 1 Oct 2019. In this version, we made minor corrections and removed extraneous references