TITLE: GCN CIRCULAR NUMBER: 6338 SUBJECT: GRB 070420: optical rebrightening observed at Mt. John DATE: 07/04/20 18:43:44 GMT FROM: Martin Jelinek at Inst.Astrophys.Andalucia,Granada M. Jelínek (IAA-CSIC Granada), P. Tristram (MOA, Univ. of Canterbury), A. de Ugarte Postigo, J. Gorosabel, (IAA-CSIC), F. Hale (Earth & Sky, Tekapo), A. Gilmore, P. Kilmartin (Univ. of Canterbury) and A. J. Castro-Tirado (IAA-CSIC), on behalf of a larger collaboration, report: "Following the detection by Swift of the GRB070420 (Stamatikos et al. GCNC 6330), we obtained V(26), R (14) and I (1) -band images at the 0.6m B&C telescope at the Mt. John Observatory on Lake Tekapo. They were taken starting on Apr 20.33 UT (i.e. 1.6 hr post-burst) and continued until Apr 20.51 UT (~6 hours after the burst). Our preliminary V-band lightcurve shows a simple powerlaw decay between 1.6 and 3.5 hours after the burst with alpha~-1.2+-0.2 which is somewhat stee- per than the value reported from UVOT (Immler et al. GCNC 6336). At 4 hours after the GRB we see a sudden rebrigh- tening of the order of 1 magnitude in 40min above the value extrapolated from the previous decay. Further analysis is in progress." This message can be quoted. [GCN OPS NOTE(22apr07): Per author's request, the "(Stamatikos et al. GCNC 6330)" was changed to "(Immler et al GCNC 6336)".]