TITLE:   GCN CIRCULAR
NUMBER:  40490
SUBJECT: Swift GRB 250520A: Global MASTER-Net observations report
DATE:    25/05/20 02:56:51 GMT
FROM:    Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>

V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, 
G.Antipov,  A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),
O.Gress, N.Budnev (ISU),
C.Francile,  F. Podesta, R.Podesta, E. Gonzalez  (Observatorio Astronomico Felix Aguilar (OAFA),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A.Sosnovskij (CrAO),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity),
D.Buckley (SAAO),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) 

MASTER-SAAO robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in South Africa (South African Astronomical Observatory) was pointed to the Swift GRB250520.11 (trigger No 1315630,18h 49m 04.32s , -11d 50m 49.2s, R=0.05) errorbox  23 sec after notice time and 46 sec after trigger time at 2025-05-20 02:43:03 UT, with upper limit up to  17.0 mag. The observations began at zenith distance = 25 deg. The sun  altitude  is -33.7 deg. 

The galactic latitude b = -5 deg., longitude l = 22 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2875870

We obtain a following upper limits.  

Tmid-T0  |          Site       |Filt.| Expt. | Limit| Comment
_________|_____________________|_____|_______|______|________

      52 |         MASTER-SAAO |   C |    10 | 16.3 |        
      80 |         MASTER-SAAO |   C |    10 | 16.3 |        
     115 |         MASTER-SAAO |   C |    20 | 16.4 |        
     159 |         MASTER-SAAO |   C |    30 | 16.7 |        
     213 |         MASTER-SAAO |   C |    40 | 16.7 |        
     277 |         MASTER-SAAO |   C |    50 | 16.7 |        
     351 |         MASTER-SAAO |   C |    60 | 17.0 |        
     430 |         MASTER-SAAO |   C |    60 | 16.9 |        
Filter C is a clear (unfiltred) band. 


The observation and reduction will continue. 
The message may be cited.