Low-luminosity Grb 060218: a Collapsar Jet from a Neutron Star, Leaving a Magnetar as a Remnant?

The gamma-ray burst (GRB) 060218 has ∼ 105 times lower luminosity than typical long GRBs, and is associated with a supernova (SN). The radio afterglow displays no jet break, so that this burst might arise from a mildly-relativistic spherical outflow produced by the SN shock sweeping the stellar surface. Since this model is energetically difficult, we propose that the radio afterglow is produced by a non-relativistic phase of an initially collimated outflow (jet). Our jet model is supported by the detection of optical linear polarization in the SN component. We also show analytically that the jet can penetrate a progenitor star. Furthermore, we analyzed the observational data of the prompt emission of this burst and obtained the implications that it may last longer than 106 s, which prefers a neutron star engine to a black hole engine. The collimation-corrected event rate of such low-luminosity GRBs is ∼ 10 times higher than that of typical long GRBs, and they might form a different GRB population: low-luminosity GRBs are produced by mildly-relativistic jets from neutron stars at the collapses of massive stars, while typical long GRBs by highly-relativistic jets from black holes. We suggest that the central engine of GRB 060218 is a pulsar (or a magnetar) with the initial rotation period P0 ∼ 10 ms and the magnetic field B ∼ 1016 G. A giant flare from the magnetar might be observed in future. Subject headings: gamma rays: bursts — gamma rays: theory — supernovae: general