TITLE:   GCN CIRCULAR
NUMBER:  30237
SUBJECT: Swift-BAT Trigger 1055000/GRB210612.54: MASTER optical source observation 
DATE:    21/06/15 15:05:18 GMT
FROM:    Vladimir Lipunov at Moscow State U/Krylov Obs  <lipunov@xray.sai.msu.ru>

V. Lipunov, A.Chasovnikov, K.Zhirkov, V.Kornilov, N.Tiurina, I.Gorbunov,P.Balanutsa,V.Vladimirov,A.Kuznetsov,E.Gorbovskoy,
D. Vlasenko, D.Zimnukhov, V.Senik, V.Topolev, A.Chasovnikov,D.Cheryasov (Lomonosov MSU,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 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)
started Swift-BAT Trigger 1055000 (GRB 210612.54, Evans et al. GCN 30222;Ttrigger= 13:01:32UT, source is 4U 1543-47)
optical observations at 2021-06-12 16:23:38 UT at MASTER-SAAO and 
continued at MASTER-IAC and MASTER-OAFA (Lipunov et al. GCN 30224) at 12, 13, 14, 15 of June.

This source activity was also detected by MAXI (Negoro et al. ATel 14701) at at 12:00 on June 11, 2021.

We detected variable bright source MASTER OT J154708.10-474008.7 +-1.5arcsec (very bright neibour star),
publishied by Swift in Atel 14715 Raman et al.,
for ex.,  with automatically detected m_OT~14.0 at 2021-06-12 23:16:21UT

Observation and reduction will be continued.