TITLE:   GCN CIRCULAR
NUMBER:  33038
SUBJECT: GRB 221009A: MuSCAT3 observations
DATE:    22/12/10 03:40:25 GMT
FROM:    Mariko Kimura at RIKEN  <mariko.kimura@riken.jp>

M. Kimura (RIKEN), K. Isogai (Tokyo Univ./Kyoto Univ.), M. Arimoto,
D. Yonetoku (Kanazawa Univ.),
N. Narita, M. Tamura (Tokyo Univ./Astrobiology Center),
A. Fukui  (Tokyo Univ.), M. Ikoma (NAOJ)

We have monitored the afterglow of GRB 221009A since three days
after the Swift and Fermi alerts (GCN 32632; GCN 32636) by MuSCAT3
on the 2-meter telescope at Las Cumbres Observatory.
MuSCAT3 is multi-channel imagers with the filters of SDSS g', r', i', zs,
which is designed for observing transiting exoplanets.
https://lco.global/observatory/instruments/muscat3/

The measurements are here:
BJD                     Mag    Err   Filter
2459864.82404  21.33  0.15  g
2459864.82404  20.01  0.03  r
2459864.82404  19.48  0.03  i
2459864.82404  18.26  0.01  z
2459870.80681  23.55  0.21  g
2459870.80681  21.74  0.05  r
2459870.80681  20.47  0.04  i
2459870.80681  19.90  0.03  z
2459872.82760  23.64  0.26  g
2459872.82760  21.91  0.07  r
2459872.82760  20.82  0.06  i
2459872.82760  20.22  0.05  z
2459876.80690  23.99  0.34  g
2459876.80690  21.96  0.09  r
2459876.80690  21.00  0.08  i
2459876.80690  20.50  0.07  z

By plotting these data with the other optical measurements reported to GCN
(GCNs 32625, 32640, 32644, 32645, 32646, 32652, 32659, 32662, 32666,
32667, 32670, 32678, 32679, 32692, 32693, 32705, 32709, 32729, 32730,
32743, 32750, 32758, 32769, 32771, 32799, 32809, 32818),
we found that the optical afterglow light curve shows a power-law decay, and
only i'-band light curve fluctuates along the decline possibly because of
systematic errors.
We did not find a bump reported by GCN 32818 at least around a week after
the Swift and Fermi trigger.

If you have any queries, please contact M. Kimura at the following address.
mariko.kimura@riken.jp