GRB 210204A: observations with the 3.6m Devasthal Optical Telescope and possible jet break

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.