TITLE:   GCN CIRCULAR
NUMBER:  30578
SUBJECT: GRB 210731A: MASTER optical afterglow  detection 
DATE:    21/08/01 18:45:53 GMT
FROM:    Vladimir Lipunov at Moscow State U/Krylov Obs  <lipunov@xray.sai.msu.ru>

V. Lipunov,  E. Gorbovskoy, N.Tyurina, K.Minkina, I.Gorbunov,
P.Balanutsa,V.Vladimirov, A.Kuznetsov, D.Cheryasov (Lomonosov Moscow State 
University,SAI, Physics Department),
O.A. Gres, N.M. Budnev (Irkutsk State University, API),
R. Podesta, F. Podesta, C.Francile (Observatorio Astronomico Felix Aguilar
OAFA),

R. Rebolo, M. Serra, N. Lodiu (The Instituto de Astrofisica de Canarias),

D. Buckley (South African Astronomical Observatory),

R. Podesta, F. Podesta, C.Francile (Observatorio Astronomico Felix Aguilar
OAFA),

A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),

A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity)

Three MASTER robotic telescopes  (Global MASTER-Net: 
http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, 
vol. 2010, 30L)  located in Argentina (OAFA observatory of San Juan 
National University) in Spain (IAC, Canary) and South Africa (SAAO observatory)   automatically 
observed Swift GRB 210801A (Troja et al., GCN 30568) started 60-70s after 
trigger (Lipunov ) up to 2021-08-01 09:16:46 (Lipunov et al., GCN 30567).


MASTER-OAFA auto-detection system ( Lipunov et al., "MASTER Global Robotic 
Net", Advances in Astronomy, 2010, 30L )
  discovered OT source at (RA, Dec) = 20h 01m 13.07s , -28d 03m 39s.6 
(err = +-0".9).

The OT magnitude in 'clear' filter is 18.3m (mlim = 19.8) at plato ( see 
detection de  Wet et al., GCN 30570).

The OT is seen in dozeens images. There is no minor planet at this place.

We have reference image on 2017-08-22.12663 UT with 'clear' filter 19.7m.