TITLE:   GCN CIRCULAR
NUMBER:  29274
SUBJECT: GRB210104A: VIRT optical transient detection.
DATE:    21/01/07 05:51:35 GMT
FROM:    Priyadarshini Gokuldass at U. of the Virgin Islands  <priyadass.94@gmail.com>

We observed the field of GRB210104A (The Fermi GBM team, GCN 29232, Troja
et al., GCN 29233) with the 0.5m Virgin Island Robotic Telescope (VIRT) at
the University of the Virgin Islands' Etelman Observatory on 01-05-2021
starting at 23:44:00 UT with a midpoint of observation of T+40 hrs. We
performed a series of exposures in R filter with a total exposure of ~10ks.
The weather conditions were clear during the hours of observation with an
average airmass of 1.7.

We detect a source consistent with the UVOT position (Troja et al., GCN
29233) and optical transient identified by others (e.g., Bilitzinger et
al., GCN 29234, Xin et al., GCN 29235, Hu et al., GCN 29236) with magnitude:

R= 20.8 +/- 0.3

This magnitude is consistent with the powerlaw decay rate of alpha = 0.73
reported by Zhu et al, GCN 29252.

The magnitude is estimated from comparison to nearby USNO B1 stars and is
not corrected for Galactic extinction. Further analysis is in progress. The
VIRT is still in the commissioning phase.

This work is supported by NASA-MUREP-MIRO grant NNX15AP95A, NSF EiR AST
Award 1901296, and NSF HBCU-UP AST Award 1831682. This message can be cited.