[vsnet-grb-info 27387] GRB 210204A: observations with the 3.6m Devasthal Optical Telescope and possible jet break

[GCN Circulars]

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  <rahulbhu.c157 at gmail.com>

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.