Detection of the optical afterglow of GRB 000630: Implications for dark bursts

We present the discovery of the optical transient of the long-duration gamm a-ray burst GRB 000630. The optical transient was detected with the Nordic Optical Telescope 21.1 hours after the burst. At the time of discovery the magnitude of the transient was R = 23.04 +/- 0.08. The transient displayed a power-law decline characterized by a decay slope of alpha = -1.035 +/- 0. 097. A deep image obtained 25 days after the burst shows no indication of a contribution from a supernova or a host galaxy at the position of the tran sient. The closest detected galaxy is a R = 324.68 +/- 0.15 galaxy 2.0 arcs ec north of the transient. The magnitudes of the optical afterglows of GRB 980329, GRB 980613 and GRB 000630 were all R greater than or similar to 23 less than 24 hours from the burst epoch. We discuss the implications of thi s for our understanding of GRBs without detected optical transients. We con clude that i) based on the gamma-ray: properties of the current sample we c annot conclude that GRBs with no detected OTs belong to another class of GR Bs than GRBs with detected OTs and ii) the majority (greater than or simila r to 75%) of GRBs for which searches for optical afterglow have been unsucc essful are consistent with no detection if they were similar to bursts like GRP, 000630 at optical wavelengths.