2019/04/30 by Anna Y. Q. Ho, Daniel A. Goldstein, Steve Schulze +47 · 1 citation
Physics and Astronomy · #astro-ph.HE
paper · pdf · doi:10.3847/1538-4357/ab55ec
Accepted to ApJ on 1 Oct 2019. In this version, we made minor corrections and removed extraneous references
arxiv created 2019/12/01 · arxiv updated 2020/01/08
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) and luminous (Mg,peak=-20 mag) transient. It is spectroscopically classified as a broad-lined stripped-envelope supernova (Ic-BL SN). The high peak luminosity (Lbol \gtrsim 3 × 1044 erg sec-1), the short rise time (trise= 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 (Teff\gtrsim40,000 K) spectra of a stripped-envelope SN. A retrospective search revealed luminous (Mg ∼ Mr ≈ -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 Eγ,iso<4.9 × 1048 erg, a limit on X-ray emission LX < 1040 erg sec-1, and a limit on radio emission νLν\lesssim 1037 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 (0.02 M_\odot) 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.