TITLE:   GCN CIRCULAR
NUMBER:  29291
SUBJECT: GRB 210104A: AROMA-N Optical Afterglow Detection
DATE:    21/01/12 06:36:42 GMT
FROM:    Takanori Sakamoto at AGU  <tsakamoto@phys.aoyama.ac.jp>

M. Nakamura, K. Hasuda, T. Sakamoto (AGU)

We observed the field of GRB 210104A (Troja et al., GCN Circ. 29233;
Malacaria et al., GCN Circ. 29246; Frederiks et al., GCN Circ. 29258;
Cherry et al., GCN Circ. 29268) with the 14-inch AGU Robotic Optical
Monitor for Astrophysical object - Narrow (AROMA-N) located at
the Sagamihara campus of Aoyama Gakuin University.

60 images of 60 sec exposures were taken in the R filter starting
from January 4 11:29:50 (UT) about 171 seconds after the trigger and
stopped on January 4 12:45:20 (UT).  We detected the optical afterglow
at the consistent position previously reported (Troja et al., GCN Circ. 29233;
Xin et al., GCN Circ. 29235; Hu et al., GCN Circ. 29236; Hosokawa et al.,
GCN Circ. 29237; Kim et al., GCN Circ. 29238, 29265; Lipunov et al.,
GCN Circ. 29239; Horiuchi et al., GCN Circ. 29241; Breeveld et al.,
GCN Circ. 29247; Zhu et al., GCN Circ. 29252; Paek et al., GCN Circ. 29254;
Kumar et al., GCN Circ. 29257; Smartt et al., GCN Circ. 29262; Romanov,
GCN Circ. 29269; Anandagoda et al., GCN Circ. 29273; Gokuldass, GCN Circ. 29274;
Mao et al., GCN Circ. 29275; Moskvitin et al., GCN Circ. 29277;
Belkin et al., GCN Circ. 29286).  The clear decay signature was visible in our
images.  The magnitudes of the initial 60 s image and the combined
image from T+2915 sec to T+3577 sec images (total exposure of 540 sec)
were 14.3 mag and 17.1 mag.  The afterglow light curve of our data showed
the initial temporal decay of -1.4 following by the shallow decay of -0.6
at the break around T+650 sec.  Our reported magnitudes are calibrated using
the USNO-B1 catalog.