GCN Circular 29291

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

We observed the field of GRB 210104A (Troja et al., GCN Circ. 29233
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;
Malacaria et al., GCN Circ. 29246
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; Frederiks et al., GCN Circ. 29258
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;
Cherry et al., GCN Circ. 29268
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) 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
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;
Xin et al., GCN Circ. 29235
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; Hu et al., GCN Circ. 29236
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; Hosokawa et al.,
GCN Circ. 29237
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; Kim et al., GCN Circ. 29238
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, 29265
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; Lipunov et al.,
GCN Circ. 29239
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; Horiuchi et al., GCN Circ. 29241
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; Breeveld et al.,
GCN Circ. 29247
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; Zhu et al., GCN Circ. 29252
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; Paek et al., GCN Circ. 29254
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;
Kumar et al., GCN Circ. 29257
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; Smartt et al., GCN Circ. 29262
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; Romanov,
GCN Circ. 29269
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; Anandagoda et al., GCN Circ. 29273
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; Gokuldass, GCN Circ. 29274
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;
Mao et al., GCN Circ. 29275
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; Moskvitin et al., GCN Circ. 29277
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;
Belkin et al., GCN Circ. 29286
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).  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.