TITLE:   GCN CIRCULAR
NUMBER:  29845
SUBJECT: GRB 210420B: MASTER optical counterpart detection
DATE:    21/04/20 20:36:01 GMT
FROM:    Vladimir Lipunov at Moscow State U/Krylov Obs  <lipunov@xray.sai.msu.ru>

V. Lipunov, V.Kornilov, E.Gorbovskoy, A.Chasovnikov, P.Balanutsa,V.Vladimirov, A.Kuznetsov,
D. Vlasenko, N.Tiurina, I.Gorbunov,  D.Zimnukhov, V.Senik,V.Topolev,
F.Balakin, D.Cheryasov (Lomonosov Moscow State University,SAI, PhysicsDepartment),
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 Felix Aguilar 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 etal.,2010,Advances in Astronomy,2010,30L)
started GRB 210420B (Swift trigger 1044382 Moss  et al. GCN 29844,Ttrigger=2021-04-20 18:20-50UT)
at MASTER-Kislovodsk at 2021-04-20 20:01:24UT

There is optical counterpart (changing)
at RA,Dec(2000)= 16h 57m 18.23s ,+42d 34m 11s.8 +-1"
MASTER OT J165718.23+423411.8
found by MASTER auto-detection system 
with m_OT~19 at 2021-04-20 20:05:23TU

Observation started at alert altitude 45deg.