Early-time photometry and spectroscopy of the fast evolving SN 2006aj associated with GRB 060218

We present early photometric and spectroscopic data on the afterglow of GRB 060218 and report the evolution of the underlying supernova SN 2006aj. Our data span a time range of 4-23 days after the GRB and clearly establish that SN 2006aj is a fast-evolving broad-lined Type Ic SN with an extremely short rise time (similar to 10 days) and a large optical peak luminosity (M-V = -18.7 mag). The supernova properties are deduced well since the gamma-ray burst V (GRB) afterglow does not contribute a significant amount to the total light output. The spectra show broad lines indicative of large expansion velocities but are better matched by those of SN 2002ap and SN 1997ef ( not associated with a GRB) than those of the prototypical GRB-related SN 1998bw. We refine the redshift estimate to z = 0.03351 +/- 0.00007. The host galaxy is a low-metallicity dwarf galaxy (with M-V,M-host = -16.0 mag), similar to host galaxies of other GRB- associated SNe.