TITLE:   GCN CIRCULAR
NUMBER:  33301
SUBJECT: GRB 230204B: MASTER early OT detection
DATE:    23/02/08 11:22:06 GMT
FROM:    Vladimir Lipunov at Moscow State U/Krylov Obs  <lipunov@xray.sai.msu.ru>

V.Lipunov (Lomonosov MSU), D.A.H.Buckley (SAAO), A.Chasovnikov, Ya.Kechin, A.Kuznetsov, 
N.Tiurina, O.Gress, E.Gorbovskoy, G.Antipov, P.Balanutsa, K.Zhirkov, D.Vlasenko, V.Senik, D.Kuvshinov,
V.Topolev, Yu.Tselik, D.Cheryasov, I.Gorbunov (Lomonosov Moscow State University, SAI, Physics Department),
C.Francile, R. Podesta, F. Podesta (Observatorio Astronomico Felix Aguilar of San Juan National University of Argentina),
R. Rebolo, M. Serra (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.Corella,L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory),
N.M.Budnev (ISU,API),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A. Gabovich, V.Yurkov (Blagoveschensk Educational State University)

MASTER OT J131034.94-214304.8 early detection of MAXI GRB 230204B optical 
counterpart (Swain et al. GCN 33269, Smartt et al. GCN 33278, Saccardi et al. GCN 33281, Ror et al. GCN 33284, Siegel et al. GCN 33292, Strausbaugh et al. GCN 33293)

MASTER-SAAO robotic telescope  (Global MASTER-Net http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, v.2010, 30L)
located in South African Astronomical Observatory,
started inspect of the MAXI GRB230204.91 (trigger 979708795, 13h10m14.88s, -22d02m13.2s, R=1deg, Serino et al. GCN 33265; 
Kennea et al. GCN 33267, Waratkar et al. GCN 33268, Casentini et al. GCN 33272, Dafcikova et al. GCN 33273, D'Elia et al. GCN 33285, Poolakkil et al. GCN 33288)
errorbox 85 sec after notice time (1075 sec after trigger time) at 2023-02-04 22:06:06 UT
with upper limit  to  19.4m ( unfiltered ) .

Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2196693

The observations began at zenith distance = 63 deg. The sun  altitude: -40.5 deg., the distance to Moon (alt=12 deg.) was 89 deg., the Moon phase 0.99). The galactic latitude b = 40 deg., longitude l = 309 deg.

MASTER-SAAO auto-detection system ( Lipunov et al., 2010 )
detected OT source at (RA, Dec) = 13h 10m 34.94s -21d 43m 04.8s at 2023-02-04.92090 UT.
The OT unfiltered magnitude at first image is 12.9m (mlim=17.6). Automatic light curve decay has pecularity.
We have reference image on 2020-04-24.82001 UT with  unfiltered mlim=20.6m.

We observed this field also in very wide field cameras (MASTER-VWFC).
The reduction will be continued.