TITLE:   GCN CIRCULAR
NUMBER:  29765
SUBJECT: GRB 210402A: MASTER prompt and follow up optical observation 
DATE:    21/04/06 08:33:46 GMT
FROM:    Vladimir Lipunov at Moscow State U/Krylov Obs  <lipunov@xray.sai.msu.ru>

K.Zhirkov, V. Lipunov, V.Kornilov, E.Gorbovskoy,P.Balanutsa,V.Vladimirov, A.Kuznetsov,
D. Vlasenko, N.Tiurina, I.Gorbunov, A.Chasovnikov, D.Zimnukhov, V.Senik, V.Topolev,
F.Balakin, 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 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 et al.,2010,Advances in Astronomy,2010,30L)
observed GRB 210402A (Swift trigger 1040123 Page et al. GCN 29743,
Evans et al. GCN 29750, Laha et al. 29761, Ttrigger=2021-04-02T22:03:17.22, 
also Fermi trigger #639093861   T_trigger(Fermi) = 22 04 17 Roberts et al. GCN 29763)
at MASTER-IAC, -Tavrida, -SAAO, -Kislovodsk, -OAFA, -Tunka (Lipunov et al. GCN 29741)
by MASTER-VWF cameras (two 24.3x16.2sq.deg.) with mlim=13
and by MASTER-II (two 4sq.deg.)  with mlims at MASTER cover map 
https://master.sai.msu.ru/site/master2/observ.php?id=1582249

We observed this field with the MASTER-VWFC of MASTER-IAC, covering the Swift/XRT position from 
2021-04-02 22:04:21 to 2021-04-02 22:07:17, i.e. during the prompt emission.
Frames were taken with the exposure time of 5s and with no filter .

Calibrated with NOMAD V-filter, we  marginally see optical source inside 
T90 at 2 images:
started at 2021-04-02 22:05:36(79 from Fermi trigger time) and
at 2021-04-02 22:05:46(89 from Fermi trigger)
with m_OT~11
at XRT position, but the pixel of this camera is 21".

Comparing to the GAIA EDR3, we see no optical counterpart down to the
magnitude of 10.5 at other images. It puts the upper limit on the flux of 
the prompt
optical emission at ~5e-10 erg cm^-2 s^-1. Considering that the x-ray flux
was ~1e-08 erg cm^-2 s^-1, we constrain the relation between optical flux
and x-ray flux at ~10^-2.