TITLE:   GCN CIRCULAR
NUMBER:  31867
SUBJECT: GRB 220408A: LOAO, DOAO, and, SAO optical upper limit
DATE:    22/04/10 03:03:22 GMT
FROM:    Gregory SungHak Paek at SNU  <shpaek@astro.snu.ac.kr>

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

 We searched for the optical counterpart of GRB 220408A possibly associated
with M51 (Caputo et al., and GCN #31848; Fermi/GBM detection, GCN #31847)
with the 1-m class telescopes in Lemonsan Optical Astronomy Observatory
(LOAO), Deokheung Optical Astronomy Observatory (DOAO), and Seoul National
University Astronomy Observatory (SAO), facilities of the GW EM-Counterpart
Korean Observatory (GECKO).

 We observed the center of UVOT localization (RA, Dec = 202.41548,
+47.06931) +2.94 hours after the report, but could not find any detection.
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] t-t0[hours] FILTER Observatory
EXPTIME[s] DEPTH
------------------- ----------- ---------- ----=------ ------ -----------
---------- -----
2022-04-08T11:29:38 2459677.979 0.239      5.7         R      LOAO
 180s*12    21.42
2022-04-08T15:44:59 2459678.156 0.416      10.0        V      SAO
 180s*21    19.75
2022-04-08T15:57:20 2459678.165 0.424      10.2        R      DOAO
 300s*37    21.90
2022-04-08T16:58:50 2459678.208 0.467      11.2        R      SAO
180s*21    19.77

Gravitational-wave EM Counterpart Korean Observatory (GECKO) is a network
of 10+ 0.5m to 1m class telescopes over the world.