TITLE: GCN CIRCULAR NUMBER: 33627 SUBJECT: GRB 230409B: 3.6m DOT near-infrared detection, afterglow confirmation DATE: 23/04/15 08:39:33 GMT FROM: 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 ( 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. We also thank Director ARIES for approving the DDT slot for the ToO observations.