TITLE:   GCN CIRCULAR
NUMBER:  22668
SUBJECT: GRB 180418A : SQUEAN observations
DATE:    18/04/21 15:57:47 GMT
FROM:    Changsu Choi at Seoul Nat U  <changsu@astro.snu.ac.kr>

Changsu Choi (CEOU/SNU), Yongjung Kim (CEOU/SNU), Woojin Park (KHU), Suhyun
Shin (CEOU/SNU) and Myungshin Im (CEOU/SNU),  on behalf of a larger
collaboration

We observed the field of GRB 180418A (Swift BAT trigger 826428; D'Elia et
al., GCN #22646) using SQUEAN installed at 2.1m Otto Struve telescope of
McDonald Observatory in US, on 18th and 19th April (UT).

The optical counter part of GRB180418A was clearly detected by observation
on 18th April, starting from 2018-04-18T08:18:27 to 08:31:38 UT which is
0.065 days to 0.0742 after BAT trigger.
No obvious source is found in the images taken on 2018-04-19 (UT).

The preliminary photometry results are given below.

All magnitudes are reported in AB magnitudes. The photometric calibration
is based on the stars in SDSS catalog in the observed field. No galactic
extinction correction is applied.

Date           t-T0      Filter Exp.      Magnitude Error
(UT, start)  (mid, days)            (s)     (3" aperture)
2018-04-18T08:18:27 0.065 r       60*3    20.44 0.07
2018-04-18T08:22:53 0.068 i 60*3 20.34 0.04
2018-04-18T08:25:02 0.070 r 60*6 20.47 0.06
2018-04-18T08:27:48 0.072 z 60*3 20.61 0.12
2018-04-18T08:31:38 0.075 r 60*3 20.57 0.08