TITLE:   GCN CIRCULAR
NUMBER:  29825
SUBJECT: GRB 210413B: LOAO optical upper limit
DATE:    21/04/17 06:45:28 GMT
FROM:    Gregory SungHak Paek at SNU  <shpaek@astro.snu.ac.kr>

Gregory S.H. Paek (SNU ARC/SNU), Myungshin Im (SNU ARC/SNU), Chung-Uk Lee
(KASI), Seung-Lee Kim (KASI), Hyun-Il Sung (KASI) on behalf of a larger
collaboration

We searched for the optical counterpart of GRB 210413B (A. Tohuvavohu et
al. GCN #29819, and P. A. Evans et al., GCN #29821) with the 1-m class
telescope in Lemonsan Optical Astronomy Observatory (LOAO), one of
facilities of the GW EM-Counterpart Korean Observatory (GECKO).

We observed the center of Swift/BAT localization (RA, Dec = 182.558 +55.965),
but could not find any transient within the field of view of LOAO
(28.1'x28.1'). We calibrated flux with the PS1 catalog and used an AB
magnitude system.

------
Filter Date-obs[UT] Exp.time[s] Depth(5sigma)
B 2021-04-16T03:17:51.667 180*3 20.780
V 2021-04-16T03:27:54.933 180*3 20.347
R 2021-04-16T03:37:58.867 180*3 19.594
I 2021-04-16T03:47:55.800 180*3 19.293

We thank the LOAO opertator for performing the observation.
Gravitational-wave EM Counterpart Korean Observatory (GECKO) is a network
of 10+ 0.5m to 1m class telescopes over the world.