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GCN Circular 33627

GRB 230409B: 3.6m DOT near-infrared detection, afterglow confirmation
2023-04-15T08:39:33Z (a year ago)
Rahul Gupta at ARIES, India <>
Rahul Gupta, Amit K. Ror, S. B. Pandey, A. Aryan, K. Misra (ARIES), A. J.
Castro-Tirado (IAA-CSIC), D. Bhattacharya (Ashoka University/IUCAA), and V.
Bhalerao (IITB) report on behalf of a larger collaboration:

GRB 230409B was detected by the Swift Burst Alert Telescope (BAT) at
04:56:57 UT on 9th April 2023 (Beardmore et al., GCN 33592). The prompt
emission mask-weighted BAT light curve consists of a single-peaked
structure with a T90 duration of 9.79 +- 2.59 sec in 15-350 keV energy
range (Palmer et al., GCN 33598). We compare the reported value of BAT
fluence and peak photon flux for this GRB (Palmer et al., GCN 33598) with
all the BAT-detected GRBs sample; this burst is positioned at the
middle-near top of this distribution. We also determine the peak energy of
the burst using BAT energy fluence and peak energy correlation. We
calculated the peak energy of the burst is 61.49 (+23.39, -16.96) KeV; the
softer value of peak energy is consistent with those of long GRBs (Type II).

The Swift XRT detected an X-ray afterglow ~ 101.2 sec after the BAT trigger
(Beardmore et al., GCN 33592). The XRT count-rate light curve could be best
described with a broken power-law model with temporal indices of 0.62
(+0.12, -0.12) and 1.15 (+0.19, -0.18) before and after break time
(~4600-sec post-BAT detection), respectively. As no redshift has been
reported for this source, we modeled the late time time-averaged XRT
spectrum (T0 + 18528 to 113195 sec) considering redshift = 2, roughly
average redshift value for long GRBs (Type II). The spectrum could be
modeled using an absorption power-law with the following spectral
parameters: NH_host= 4.85 (-4.85,+8.76) X 10^{22} cm^{-2} and \beta_x= 0.73
(-0.36,+0.40). Considering the adiabatic deceleration without energy
injection, closure relations indicate that the late time X-ray afterglow
could be best described with \nu_m < \nu < \nu_c spectral regime for ISM
medium for the electron energy index p ~ 2.46.

The Swift UVOT detected a weak fading source within the XRT error circle
(Kuin et al., GCN 33595). We performed the follow-up observations of this
fading optical source using the TANSPEC mounted at the axial port of the
3.6m Devasthal Optical Telescope (DOT) of ARIES Nainital at multiple epochs
in J filter (near-infrared). We report the preliminary brightness of the
afterglow to be J = 20.1 +/- 0.3 mag ~ 0.75 days after the BAT trigger. We
obtained the limiting mag of 20.5 mag ~ 2.7 days post-burst at successive
epochs. Our observations confirm this fading source to be the afterglow of
GRB 230409B.

The magnitude values reported are calibrated against 2MASS nearby stars.

This circular may be cited. 3.6m Devasthal Optical Telescope (DOT) is the
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 ( Authors of this GCN circular thankfully
acknowledge consistent support from the staff members to run and maintain
the 3.6m DOT. We also thank Director ARIES for approving the DDT slot for
the ToO observations.
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