GCN Circular 4812

Paolo A. Mazzali (MPA, INAF-Ts) and Elena Pian (INAF-Ts) report:

inspection of the VLT-FORS2 spectrum of GRB060218/SN2006aj taken on 21 Feb 
2006 (Masetti et al. 2006, GCN 4803
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 ), i.e. ~3 days after the GRB event, 
indicates that the observed features are similar to those of the energetic 
type Ic SN 2002ap at a comparable epoch (Mazzali et al. 2002, Ap. J. 572, 
L21, Fig. 2). In particular, broad absorptions near 4800 Ang and 5800 Ang 
can be identified as due to FeII and SiII lines, respectively. This 
suggests that the SN and the GRB occurred within ~1 day of one another.

The observed luminosity of the optical counterpart of GRB060218 is larger 
than that of SN2002ap at the same epoch, and is consistent with the 
luminosity of SN1998bw.

If an afterglow is still present, we estimate that it does not contribute 
for more than 25% of the total flux. If it has a power-law shape, this is 
not bluer than nu^{-2}.

Although spectroscopy of SN1998bw did not begin until day 8 (Galama et al. 
1998, Nature 395, 670), and therefore a direct comparison is not yet 
possible, it is likely that SN2006aj will evolve to resemble SN1998bw.