A Unified Picture for Low-luminosity and Long Gamma-Ray Bursts Based on the Extended Progenitor of llGRB 060218/SN 2006aj

The relation between long gamma ray bursts (LGRBs) and low-luminosity GRBs (llGRBs) is a long standing puzzle—on 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 (\gt 100{R}_⊙ ), low-mass (∼ 0.01{M}_⊙ ) 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.