TITLE:   GCN CIRCULAR
NUMBER:  29526
SUBJECT: GRB 210205A: observations with the 3.6m DOT, a potential dark burst?
DATE:    21/02/18 09:07:31 GMT
FROM:    Rahul Gupta at ARIES, India  <rahulbhu.c157@gmail.com>

S. B. Pandey, R. Gupta, A. Kumar, Dimple, A. Ghosh, A. Aryan,  and K. Misra
(ARIES) on behalf of a larger collaboration report:

GRB 210205A was detected by the Swift Burst Alert Telescope (BAT) at
11:11:17 UT on 5th Feb 2021 (GCN 29397). The prompt emission mask-weighted
BAT light curve consists of a multi-peaked structure with a T90 duration of
22.70 +- 4.18 sec in 15-350 keV energy range (GCN 29409). We compare the
reported value of BAT fluence and peak photon flux for this GRB (GCN 29409)
with all the BAT detected samples, we find this burst is positioned nearly
the middle of this distribution. The Swift XRT detected an X-ray afterglow
~ 134.7 sec after the BAT trigger (GCN 29397). The X-ray light curve could
be best described with a simple power-law model with a temporal index of
1.100 (+0.092, -0.101) and the afterglow is fainter (0.016 x 10^-11
erg/cm2/s, at 11-hour post burst) than typical X-ray afterglows. As no
red-shift has been reported for this source, we modeled the time-averaged
XRT spectrum (T0 + 143 to 39716 s) considering red-shift = 2, roughly
average red-shift value for long GRBs. The spectrum could be modeled using
an absorption power-law with following spectral parameters: NH_host= 5.34
(-4.58,+6.18) * 10^{22} cm^{-2} and \beta_x= 1.11 (-0.37,+0.40).
Considering the adiabatic deceleration without energy injection, closure
relations indicate that the X-ray afterglow could be best described with
\nu > \nu_c spectral regime for ISM as well as WIND medium for the electron
energy index p ~ 2.22.

Many optical telescopes searched for the optical afterglow but no
counterpart associated with this burst was detected to deeper limits at
early epochs(GCN  29397, 29398, 29400, 29401, 29402, 29403, and 29406). So,
we performed the search for the optical counterpart of this XRT localized
GRB 210205A (GCN 29399) using the 4Kx4K CCD Imager (Pandey et al. 2017,
arXiv:1711.05422v1) mounted at the axial port of the 3.6m Devasthal Optical
Telescope (DOT) of ARIES Nainital. Multiple frames having exposure times of
300s each were taken in R and I filters.  We do not find any evidence of an
afterglow candidate inside the XRT error circle, consistent with other
non-detections. We constrain the following 3-sigma upper limits.

T-T0 (days) Exp. (s) Filter  OT   UL   Telescope

1.0921       2*300   R       NA   22.8     3.6m DOT
1.1033       2*300    I        NA   22.6     3.6m DOT

The limiting magnitudes quoted are not corrected for the Galactic and Host
extinction in the direction of the burst. Considering no spectral break
between X-ray and optical frequencies, we extrapolated the X-ray spectral
index towards optical frequencies. We found that using reported limiting
values at optical (Galactic extinction corrected) roughly lie below the
extrapolated X-ray power-law slope, suggesting that this burst could be a
potential dark GRB candidate either highly extinguished or as a high
redshift event.

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. This circular may be
cited.