TITLE: GCN GRB OBSERVATION REPORT NUMBER: 4988 SUBJECT: GRB(?)060421:MASTER optical limit DATE: 06/04/21 11:50:19 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs V. Lipunov, V.Kornilov, N.Tyurina, A.Belinski, D.Kuvshinov, E.Gorbovskoy, A.Krylov, G.Borisov, A.Sankovich, V.Vladimirov, P.Gritsyk, S.Korobkin Sternberg Astronomical Institute, Moscow Union 'Optic' MASTER robotic system (http://observ.pereplet.ru) responded to GRB060421 (GRB_TIME is 2006-04-21 00:39:23.33, M. R. Goad et al. GCN4985). Unfortunaly 2 minutes before notice time roof closed roboticaly due to meteo alert "cloudy". The first image was at 2006-04-21 00:48:45 UT, 562 s after the GRB time. The unfiltered image is calibrated relative to USNO A2.0 (0.8 R + 0.2 B). The robot not find OT-candidate in error box brighter then 16.8 (s/n=3) at Swift XRT position (Mike Goad, Kim Page GCN4987). This work is supported by RFFI 04-02-16411 grant. This message can be cited. Mailto: lipunov@sai.msu.ru