  Helium star--compact object binaries, and helium star--neutron star binaries
in particular, are widely believed to be the progenitors of the observed double
neutron star systems. In these, the second neutron star is presumed to be the
compact remnant of the helium star supernova. Here, the observational
implications of such a supernova are discussed, and in particular are explored
as a candidate gamma-ray burst mechanism. In this scenario the supernova
results in a transient period of rapid accretion onto the compact object,
extracting via magnetic torques its rotational energy at highly super-Eddington
luminosities in the form of a narrowly beamed, strongly electromagnetically
dominated jet. Compton scattering of supernova photons advected within the
ejecta, and photons originating at shocks driven into the ejecta by the jet,
will cool the jet and can produce the observed prompt emission characteristics,
including the peak--inferred isotropic energy relation, X-ray flash
characteristics, subpulse light curves, energy dependent time lags and subpulse
broadening, and late time spectral softening. The duration of the burst is
limited by the rate of Compton cooling of the jet, eventually creating an
optically thick, moderately relativistically expanding fireball which can
produce the afterglow emission. If the black hole or neutron star stays bound
to a compact remnant, late term light curve variability may be observed as in
SN 2003dh.
