  Supernova LSQ13abf was discovered soon after explosion by the La Silla-QUEST
Survey and followed by the CSP II at optical and near-IR wavelengths. Our
analysis indicates LSQ13abf was discovered within two days of explosion and its
first 10 days of evolution reveal a B-band light curve with an abrupt drop in
luminosity. Contemporaneously, the V-band light curve exhibits a rise towards a
first peak and the r- and i-band light curves show no early peak. The early
light-curve evolution of LSQ13abf is reminiscent of the post explosion cooling
phase observed in the Type Ib SN 2008D, and the similarity between the two
objects extends over weeks. Spectroscopically, LSQ13abf resembles SN 2008D with
P Cygni He I features that strengthen over time. Spectral energy distributions
are constructed from broad-band photometry, and by fitting black-body (BB)
functions a UVOIR light curve is constructed, and the underlying BB-temperature
and BB-radius profiles are estimated. Explosion parameters are estimated by
simultaneously fitting an Arnett model to the UVOIR light curve and the
velocity evolution derived from spectral features, and a post-shock breakout
cooling model to the first two epochs of the bolometric evolution. This
combined model suggests an explosion energy of 1.3x10$^{51}$ ergs, a relatively
high ejecta mass of 5.94 M$_{\odot}$, a Ni mass of 0.16 M$_{\odot}$, and a
progenitor-star radius of 28.0 R$_{\odot}$. The ejecta mass suggests the
origins of LSQ13abf lie with a >25 M$_{\odot}$ ZAMS progenitor and its radius
is three and nine times larger than values estimated from the same analysis
applied to observations of SNe 2008D and 1999ex, respectively. Alternatively,
comparison of hydrodynamical simulations of >20-25 M$_{\odot}$ ZAMS progenitors
that evolve to pre-SN envelope masses around 10 M$_{\odot}$ and extended (~100
R$_{\odot}$) envelopes also match the observations of LSQ13abf.

High Energy Astrophysical Phenomena