{
  "bibcode": "2021GCN.29490....1G",
  "body": "R. Gupta, S. B. Pandey, A. Kumar, Dimple, A. Ghosh, A. Aryan, K. Misra\n(ARIES), D. Bhattacharya (IUCAA), V. Bhalerao (IITB), A. Pozanenko (IKI),\nand D. Buckley (SAAO) as a part of larger BRICS collaboration:\n\nGRB 210204A was triggered by the Fermi-GBM at 06:29:25 UT on 4 Feb 2021\n(GCNs 29390, and 29393) and the prompt emission was also detected by other\nspace-based facilities like GECAM (29392), AstroSat (GCN 29410), and\nKonus-Wind (GCNC 29415) up to MeV energies.  So, we searched for extended\nGeV emission for a temporal window of 50 ks since GBM trigger-based on\navailable LAT data, however,  we do not find detection of any GeV photons\nwith a probability greater than 50 % to be associated with the source.\n\nThe optical counterpart associated with this burst was independently\ndiscovered by Kool et al., 2021 using the ZTF telescope (GCN 29405) and\nlater follow-up observations were continued by several ground-based\nfacilities (GCNs 29411, 29414, 29417, 29432, 29433, and 29438). Xu et al.\n2021 report a redshift value of 0.876 for this burst using\nVLT/X-shooter observations\n(GCN 29411).\n\nWe performed late time follow-up observations of the optical counterpart\nusing the 4Kx4K CCD Imager (Pandey et al. 2017, arXiv:1711.05422v1) mounted\nat the axial port of the 3.6m Devasthal Optical Telescope of ARIES Nainital\nstarting ~ 58 hours after the burst. Multiple frames were taken in various\nbroad-band filters including B, V, R, and I bands. A fading afterglow\ncandidate was clearly seen in individual frames decayed around ~ 0.6 mags\nin R Band during our observing run. We report the preliminary brightness of\nthe afterglow to be R= 20.19 +/- 0.03 mag ~ 58 hours after the GBM trigger.\nThe magnitude value reported is calibrated against UNSO B1 nearby stars.\nOur R-band light curve along with early data published by Kool et al., 2021\n(GCN 29405) is well-described with a broken power-law model. We found that\nthe temporal decay index before break time (~ 36 hours post burst) is 0.55\n+/- 0.03 steepen to 1.43 +/- 0.09 at later epochs. Considering this break\nas a jet break (as reported in GCN 29433), we calculated the optical\nspectral index (using BVRI data at ~ 58 hours) \\beta_O to be 1.08 +/- 0.10.\nConsidering the closure relations after the jet break (both for ISM and\nWIND like a medium, without energy injection), the optical emission could\nbe better described with spectral regime \\nu > \\nu_c for WIND medium\ndemanding electron energy index value $p$ equal to 2.16 +/- 0.20 for the\navailable data-set so far.\n\nThis circular maybe cited. 3.6m Devasthal Optical Telescope (DOT) is a\nrecently commissioned facility in the Northern Himalayan region of India\n(long:79 41 04E, lat:29 21 40N, alt:2540m) owned and operated by the\nAryabhatta Research Institute of Observational Sciences (ARIES), Nainital (\nhttps://www.aries.res.in). Authors of this GCN circular thankfully\nacknowledge consistent support from the staff members to run and maintain\nthe 3.6m DOT.",
  "circularId": 29490,
  "createdOn": 1613131591000,
  "email": "rahulbhu.c157@gmail.com",
  "subject": "GRB 210204A: observations with the 3.6m Devasthal Optical Telescope and possible jet break",
  "submitter": "Rahul Gupta at ARIES, India  <rahulbhu.c157@gmail.com>",
  "eventId": "GRB 210204A"
}