  We present extensive multi-wavelength observations of the extremely rapidly
declining Type Ic supernova, SN 2005ek. Reaching a peak magnitude of M_R =
-17.3 and decaying by ~3 mag in the first 15 days post-maximum, SN 2005ek is
among the fastest Type I supernovae observed to date. The spectra of SN 2005ek
closely resemble those of normal SN Ic, but with an accelerated evolution.
There is evidence for the onset of nebular features at only nine days
post-maximum. Spectroscopic modeling reveals an ejecta mass of ~0.3 Msun that
is dominated by oxygen (~80%), while the pseudo-bolometric light curve is
consistent with an explosion powered by ~0.03 Msun of radioactive Ni-56.
Although previous rapidly evolving events (e.g., SN 1885A, SN 1939B, SN 2002bj,
SN 2010X) were hypothesized to be produced by the detonation of a helium shell
on a white dwarf, oxygen-dominated ejecta are difficult to reconcile with this
proposed mechanism. We find that the properties of SN 2005ek are consistent
with either the edge-lit double detonation of a low-mass white dwarf or the
iron-core collapse of a massive star, stripped by binary interaction. However,
if we assume that the strong spectroscopic similarity of SN 2005ek to other SN
Ic is an indication of a similar progenitor channel, then a white-dwarf
progenitor becomes very improbable. SN 2005ek may be one of the lowest mass
stripped-envelope core-collapse explosions ever observed. We find that the rate
of such rapidly declining Type I events is at least 1-3% of the normal SN Ia
rate.
