TITLE: GCN CIRCULAR NUMBER: 29724 SUBJECT: GRB 210323A: MASTER optical observation DATE: 21/03/24 23:16:11 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs N.Tiurina, V. Lipunov, V.Kornilov, E.Gorbovskoy,P.Balanutsa,F.Balakin, V.Vladimirov, A.Kuznetsov,K.Zhirkov,D. Vlasenko, I.Gorbunov, D.Zimnukhov, V.Senik, A.Chasovnikov,A.Pozdnyakov,V.Topolev, D.Cheryasov(Lomonosov Moscow State University, SAI, Physics Department), D. Buckley (South African Astronomical Observatory), O.A. Gres, N.M. Budnev (Irkutsk State University, API), R. Podesta, C.Lopez, F. Podesta, C.Francile (Observatorio Astronomico FelixAguilar OAFA), R. Rebolo, M. Serra(The Instituto de Astrofisica de Canarias), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity) MASTER Global robotic net (MASTER-Net:http://observ.pereplet.ru Lipunov et al.,2010,Advances in Astronomy,2010,30L) automatically started Swift GRB 210323A (Gropp et al. GCN 29699, Ttrigger=22:02:18UT, https://gcn.gsfc.nasa.gov/other/210323A.gcn3) optical observations at MASTER-Tunka (in ~100km of Mondy , Pozanenko et al. GCN 29708) at 2021-03-23 22:03:12UT with 10s expositions by MASTER-600 twin robotic telescope and by MASTER-VWFC (very wide field cameras)at sunrise. The Sun altitude at MASTER-Tunka and Mondy was -9.6, see alert location and conditions of observation at http://observ.pereplet.ru/images/tunka2021-03-23-22-03-13.jpg (webcamera). The distance to Sun of GRB coordinates was 49.6deg., that made 18.4m OT detection little likelihood for the telescopes at this location and at this time. MASTER observed this alert's error-box also at MASTER-Tavrida (2021-03-24 02:07:41 UT, with upper limit 18.2), MASTER-IAC (2021-03-24 03:40:27 UT, Lipunov et al. 29702), and MASTER-OAFA (started at 2021-03-24 09:40:18UT), see cover map, limits and altitudes at https://master.sai.msu.ru/site/master2/observ.php?id=1575256 Reduction and analyses will continue. This message may be cited.