TITLE:   GCN CIRCULAR
NUMBER:  29257
SUBJECT: GRB 210104A: 1.3m DFOT Optical Afterglow Detection
DATE:    21/01/05 11:38:59 GMT
FROM:    Amit Kumar at ARIES, India  <amitkundu515@gmail.com>

Amit Kumar (ARIES), Rahul Gupta (ARIES), Dimple (ARIES), Ankur Ghosh
(ARIES), Nikita Rawat (ARIES), Brajesh Kumar (ARIES), Vibhore Negi (ARIES),
Amar Aryan (ARIES), Shashi B. Pandey (ARIES), and Kuntal Misra (ARIES)
report:

We observed the optical afterglow of the Fermi (Fermi GBM team GCNC 29232,
Biltzinger et al. GCNC 29234) and Swift detected GRB 210104A (Troja et al.
GCNC 29233) with the 1.3m Devsthal Fast Optical Telescope (DFOT) at
Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital
(India), from 2021-01-04 17:50:11 to 2021-01-04 18:35:28 UTC (from ~6.386
to 7.141 hours after the burst). We observed 10 frames each of 240 seconds
in the Bessell R filter. We clearly detected the optical afterglow of GRB
210104A in all individual frames within the Swift XRT enhanced error circle
(Beardmore et al. 29245) as reported by Xin et al. GCN 29235 and later by
Hu et al. GCN 29236, Hosokawa et al. GCN 29237, Kim et al. GCN 29238,
Takashi et al. GCN 29241, Lipunov et al. GCN 29248, Zhu et al. GCN 29252
and Gregory et al. GCN 29254.

The estimated preliminary magnitude for the first R-band image is as
follows:

T_start-T0 (hours) Start Date (UTC) Filter Magnitude (mag)

-----------------------------------------------------------
-----------------------------------------------------------
6.386 2021-01-04T17:50:11 R 18.95 +- 0.07


Photometry is done based on the USNO-B1.0 catalog. The quoted magnitude is
not corrected for the Galactic extinction in the direction of GRB 210104A.

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