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 \sim 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 \sim 0.3 M _ { \odot } that is dominated by oxygen ( \sim 80 % ) , while the pseudo-bolometric light curve is consistent with an explosion powered by \sim 0.03 M _ { \odot } of radioactive ^ { 56 } Ni .
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 .