Low-luminosity GRB 060218: A collapsar jet from a neutron star, leaving a magnetar as a remnant?

The gamma-ray burst (GRB) 060218 has a luminosity similar to 10(5) times lower than that of typical long GRBs and is associated with a supernova (SN). The radio afterglow displays no jet break, so 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 nonrelativistic 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. We analyzed the observational data of the prompt emission of this burst and obtained a smooth power-law light curve which might last longer than 10(6) s. This behavior contrasts with the long intermittent activities of the X-ray flares of typical GRBs, implying that the central engine of this burst is different from those of typical GRBs. This argument is consistent with the analysis of the SN component of this burst, which suggests that the progenitor star was less massive and collapsed to a neutron star instead of a black hole. The collimation-corrected event rate of such low-luminosity GRBs is estimated to be similar to 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 are produced by highly relativistic jets from black holes. We suggest that the central engine of GRB 060218 is a pulsar (or a magnetar) with initial rotation period P-0 similar to 10 ms and magnetic field B similar to 10(16) G. A giant flare from the magnetar might be observed in future.