TITLE: GCN CIRCULAR NUMBER: 29490 SUBJECT: GRB 210204A: observations with the 3.6m Devasthal Optical Telescope and possible jet break DATE: 21/02/12 12:06:31 GMT FROM: Rahul Gupta at ARIES, India R. Gupta, S. B. Pandey, A. Kumar, Dimple, A. Ghosh, A. Aryan, K. Misra (ARIES), D. Bhattacharya (IUCAA), V. Bhalerao (IITB), A. Pozanenko (IKI), and D. Buckley (SAAO) as a part of larger BRICS collaboration: GRB 210204A was triggered by the Fermi-GBM at 06:29:25 UT on 4 Feb 2021 (GCNs 29390, and 29393) and the prompt emission was also detected by other space-based facilities like GECAM (29392), AstroSat (GCN 29410), and Konus-Wind (GCNC 29415) up to MeV energies. So, we searched for extended GeV emission for a temporal window of 50 ks since GBM trigger-based on available LAT data, however, we do not find detection of any GeV photons with a probability greater than 50 % to be associated with the source. The optical counterpart associated with this burst was independently discovered by Kool et al., 2021 using the ZTF telescope (GCN 29405) and later follow-up observations were continued by several ground-based facilities (GCNs 29411, 29414, 29417, 29432, 29433, and 29438). Xu et al. 2021 report a redshift value of 0.876 for this burst using VLT/X-shooter observations (GCN 29411). We performed late time follow-up observations of the optical counterpart using the 4Kx4K CCD Imager (Pandey et al. 2017, arXiv:1711.05422v1) mounted at the axial port of the 3.6m Devasthal Optical Telescope of ARIES Nainital starting ~ 58 hours after the burst. Multiple frames were taken in various broad-band filters including B, V, R, and I bands. A fading afterglow candidate was clearly seen in individual frames decayed around ~ 0.6 mags in R Band during our observing run. We report the preliminary brightness of the afterglow to be R= 20.19 +/- 0.03 mag ~ 58 hours after the GBM trigger. The magnitude value reported is calibrated against UNSO B1 nearby stars. Our R-band light curve along with early data published by Kool et al., 2021 (GCN 29405) is well-described with a broken power-law model. We found that the temporal decay index before break time (~ 36 hours post burst) is 0.55 +/- 0.03 steepen to 1.43 +/- 0.09 at later epochs. Considering this break as a jet break (as reported in GCN 29433), we calculated the optical spectral index (using BVRI data at ~ 58 hours) \beta_O to be 1.08 +/- 0.10. Considering the closure relations after the jet break (both for ISM and WIND like a medium, without energy injection), the optical emission could be better described with spectral regime \nu > \nu_c for WIND medium demanding electron energy index value $p$ equal to 2.16 +/- 0.20 for the available data-set so far. This circular maybe cited. 3.6m Devasthal Optical Telescope (DOT) is a recently commissioned facility in the Northern Himalayan region of India (long:79 41 04E, lat:29 21 40N, alt:2540m) owned and operated by the Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital ( https://www.aries.res.in). Authors of this GCN circular thankfully acknowledge consistent support from the staff members to run and maintain the 3.6m DOT.