​GRB 221009A: Multi-color detection of the optical

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.