SN 2006aj 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 underluminous in gamma rays compared with 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. The existence of a range of intrinsic energies of GRBs may present challenges to using GRBs as standard candles.