TITLE: GCN CIRCULAR NUMBER: 10052 SUBJECT: GRB 091020: MASTER optical transient polarimetry DATE: 09/10/21 05:15:37 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs E. Gorbovskoy,D.Kuvshinov, V. Lipunov, V.Kornilov, D.Kuvshinov, A.Belinski, A. Krylov, N.Shatskiy, N.Tyurina, P.Balanutsa, V.V.Chazov, Sternberg Astronomical Institute, Moscow State University A. Tlatov, A.V. Parkhomenko, D. Dormidontov Kislovodsk Solar Station of the Pulkovo Observatory V.Krushinski, I.Zalognikh, T.Kopytova, A. Popov Ural State University, Kourovka S.Yazev, K.Ivanov, N.M.Budnev Irkutsk State University V.Yurkov Blagoveschensk Educational State University, Blagoveschensk MASTER II robotic telescope (MASTER-Net: http://observ.pereplet.ru, 2x400 mm, 8 square degrees, 2x16Mpx Apogee CCD) located at Kislovodsk was pointed to the GRB 091020 (Racusin et al., GCN 10048) by internet 3422 s after the GRB time. The large delay was caused with technical problems on site. We see bright optical counterpart (~17.5 mag) on the firrst images at UVOT position. We have 130 images with 180 sec synchronous exposition each and 19.5 magnitude limit during ~4 hours in two polarization. The power low decay with alpha ~= 1.3 during 2.5 hours is observed. This is the preliminary result. The reduction is continued. The message may be cited. mailto: lipunov@sai.msu.ru