  The gamma-ray burst (GRB) 060218 has ~ 10^5 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. 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 10^6s. This behavior contrasts with the long
intermittent activities with 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 ~ 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 P_0
\~ 10ms and the magnetic field B ~ 10^16G. A giant flare from the magnetar
might be observed in future.
