GCN Circular 32659

Gregory S.H. Paek (SNU ARC/SNU), Myungshin Im (SNU ARC/SNU), Yuji Urata
(NCU), Hyun-Il Sung (KASI) on behalf of a larger collaboration

 We detected the optical counterpart/NIR afterglow of extremely bright GRB
221009A (Dichiara et al., GCN #32632) with the 1-m class telescopes in
Lemonsan Optical Astronomy Observatory (LOAO) facilities of the GW
EM-Counterpart Korean Observatory (GECKO).

 We observed the center of UVOT localization (RA, Dec = 288.265, +19.774)
+15 hours after the report in B, V, R, I, z, and Y-bands. We clearly
detected the afterglow in all V, R, I, z, and Y-bands except for the
B-band. We calibrated flux with the PANSTARRS catalog and used an AB
magnitude system. Depth means 5 sigma upper limit for a point source
detection. The magnitudes are not corrected for galactic extinction.

------------------- ----------- ----------- ------ ----------- ----------
----- ------ -----
DATE-OBS[UTC]       JD          t-t0[days]  FILTER Observatory EXPTIME[s]
MAG   MAGERR DEPTH
------------------- ----------- ----------- ------ ----------- ----------
----- ------ -----
2022-10-10T04:18:54 2459862.680 0.626       B      LOAO        60s*5
 None   None  19.66
2022-10-10T04:20:38 2459862.681 0.628       V      LOAO        60s*5
 18.74  0.13  19.48
2022-10-10T04:22:04 2459862.682 0.629       R      LOAO        60s*5
 17.55  0.06  19.75
2022-10-10T04:23:31 2459862.683 0.630       I      LOAO        60s*5
 16.41  0.05  19.56
2022-10-10T04:24:39 2459862.684 0.631       z      LOAO        60s*5
 TBD    TBD   TBD
2022-10-10T04:25:58 2459862.685 0.631       Y      LOAO        60s*5
 TBD    TBD   TBD

 We will continue the follow-up observation for this target with the LOAO,
and other GECKO facilities in Australia, and Chile. Gravitational-wave EM
Counterpart Korean Observatory (GECKO) is a network of 10+ 0.5m to 1m class
telescopes worldwide.