  The relation between long gamma-ray bursts (LGRBs) and low-luminosity GRBs
(llgrbs) is a long standing puzzle -- on the one hand their high energy
emission properties are fundamentally different, implying a different gamma-ray
source, yet both are associated with similar supernovae of the same peculiar
type (broad-line Ic), pointing at a similar progenitor and a similar explosion
mechanism. Here we analyze the multi-wavelength data of the particularly
well-observed SN 2006aj, associated with llgrb 060218, finding that its
progenitor star is sheathed in an extended ($>100R_\odot$), low-mass ($\sim
0.01M_\odot$) envelope. This progenitor structure implies that the gamma-ray
emission in this llgrb is generated by a mildly relativistic shock breakout. It
also suggests a unified picture for llgrbs and LGRBs, where the key difference
is the existence of an extended low-mass envelope in llgrbs and its absence in
LGRBs. The same engine, which launches a relativistic jet, can drive the two
explosions, but, while in LGRBs the ultra-relativistic jet emerges from the
bare progenitor star and produces the observed gamma-rays, in llgrbs the
extended envelope smothers the jet and prevents the generation of a large
gamma-ray luminosity. Instead, the jet deposits all its energy in the envelope,
driving a mildly relativistic shock that upon breakout produces a llgrb. In
addition for giving a unified view of the two phenomena, this model provides a
natural explanation to many observed properties of llgrbs. It also implies that
llgrbs are a viable source of the observed extra-galactic diffuse neutrino flux
and that they are promising sources for future gravitational wave detectors.
