A UNIFIED PICTURE FOR LOW-LUMINOSITY AND LONG GAMMA-RAY BURSTS BASED ON THE EXTENDED PROGENITOR OFllGRB 060218/SN 2006AJ

Abstract

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.