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 ± 0.1 mag/hr) 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 day 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, 000K) arXiv:1904.11009v4 [astro-ph.HE] 1 Dec 2019 2 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 νL ν 10 37 erg s −1 . Taken together, we find that the early (< 10 day) 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 day) light curve requires an additional energy source, which could be the radioactive decay of Ni-56.

SN 2020bvc: A Broad-line Type Ic Supernova with a Double-peaked Optical Light Curve and a Luminous X-Ray and Radio Counterpart

“…As we discussed in our analysis of another fast-rising luminous Ic-BL SN, SN 2018gep (Ho et al 2019a), it can be difficult to know when it is appropriate to model such emission as arising from shock breakout in low-mass, large-radius material and when it is appropriate to model such emission as arising from postshock cooling in higher-mass, smaller-radius material (e.g., Nakar & Piro 2014;Piro 2015). In short, it is extremely difficult at present to explain why we see such diversity in the early light curves of Ic-BL SNe.…”

The Zwicky Transient Facility Bright Transient Survey. II. A Public Statistical Sample for Exploring Supernova Demographics*

“…Most of these events are quite luminous (M<−18 mag), making them incompatible with radioactive heating as the primary energy source (Arnett et al 1989) for the main peak, although a few show subsequent second peaks or plateaus on a classical SN timescale. Shockbreakout into an extended envelope or circumstellar medium (CSM) has been suggested as the likely explanation for previous fast-peaking, luminous events (both for AT2018cow and for less-extreme RETs: Ofek et al 2010;Drout et al 2014;Pursiainen et al 2018;Ho et al 2019b;Margutti et al 2019;Perley et al 2019), and it is reasonable to hypothesize that this process is also responsible for most or all of the transients in this portion of the diagram in our sample.…”

SN 2019ehk: A Double-peaked Ca-rich Transient with Luminous X-Ray Emission and Shock-ionized Spectral Features

“…Equations for radiation hydrodynamics are solved using a gray flux-limited nonequilibrium diffusion approximation. The models are similar to those applied to the SNIc-BL, 2018gep (Ho et al 2019) and the fast-evolving luminous transient KSN2015K (Rest et al 2018), but have been adapted to the observables in SN2019ehk.…”