2 v 2 6 A pr 2 00 6 SN 2006 aj and the nature of low - luminosity gamma - ray bursts

We present SMARTS consortium optical/IR light curves of SN 2006aj associated with GRB 060218. We find that this event is broadly similar to two previously observed events SN 1998bw/GRB 980425 and SN 2003lw/GRB 031203. In particular, all of these events are greatly under-luminous in gamma-rays compared to typical long-duration GRBs. We find that the observation by Swift of even one such event implies a large enough true event rate to create difficulties in interpreting these events as typical GRBs observed off-axis. Thus these events appear to be intrinsically different from and much more common than high-luminosity GRBs, which have been observed in large numbers out to a redshift of at least 6.3. We conclude that these events do not share a common origin with high-luminosity GRBs. The existence of a range of intrinsic energies of GRBs may present challenges to using GRBs as standard candles.