GCN Circular 29526

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
Not found
 
 
). 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
Not found
 
 
). We compare the
reported value of BAT fluence and peak photon flux for this GRB (GCN 29409
Not found
 
 
)
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
Not found
 
 
). 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
Not found
 
 
, 29398
Not found
 
 
, 29400
Not found
 
 
, 29401
Not found
 
 
, 29402
Not found
 
 
, 29403
Not found
 
 
, and 29406
Not found
 
 
). So,
we performed the search for the optical counterpart of this XRT localized
GRB 210205A (GCN 29399
Not found
 
 
) using the 4Kx4K CCD Imager (Pandey et al. 2017,
arXiv:1711.05422v1
Not found
 
 
) 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.