SN 2006aj and the Nature of Low-Luminosity Gamma-Ray Bursts

We present SMARTS consortium optical/IR light curves of SN 2006aj associatedwith GRB 060218. We find that this event is broadly similar to two previouslyobserved events SN 1998bw/GRB 980425 and SN 2003lw/GRB 031203. In particular,all of these events are greatly under-luminous in gamma-rays compared totypical long-duration GRBs. We find that the observation by Swift of even onesuch event implies a large enough true event rate to create difficulties ininterpreting these events as typical GRBs observed off-axis. Thus these eventsappear to be intrinsically different from and much more common thanhigh-luminosity GRBs, which have been observed in large numbers out to aredshift of at least 6.3. The existence of a range of intrinsic energies ofGRBs may present challenges to using GRBs as standard candles.Comment: ApJ Letters - accepte