The spectral lag of GRB 060505: A likely member of the long-duration class

Two long gamma-ray bursts, GRB 060505 and GRB 060614, occurred in nearby galaxies at redshifts of 0.089 and 0.125, respectively. Due to their proximity and durations, deep follow-up campaigns to search for supernovae (SNe) were initiated. However, none were found in either case, to limits more than 2 orders of magnitude fainter than the prototypical GRB-associated SN, 1998bw. It was suggested that the bursts, in spite of their durations (similar to 4 and 102s), belonged to the population of short GRBs which has been shown to be unrelated to SNe. In the case of GRB 060614 this argument was based on a number of indicators, including the negligible spectral lag, which is consistent with that of short bursts. GRB 060505 has a shorter duration, but no spectral lag was measured. We present the spectral lag measurements of GRB 060505 using Suzaku's Wide Area Monitor and the Swift Burst Alert Telescope. We find that the lag is 0.36 +/- 0.05 s, inconsistent with the lags of short bursts and consistent with the properties of long bursts and SN GRBs. These results support the association of GRB 060505 with other low-luminosity GRBs also found in star-forming galaxies and indicate that at least some massive stars may die without bright SNe.