TITLE: GCN CIRCULAR NUMBER: 29168 SUBJECT: GRB 201223A: GROWTH-India Telescope (GIT) photometric follow-up. DATE: 20/12/24 14:21:13 GMT FROM: Harsh Kumar at Indian Inst of Tech,Bombay H. Kumar (IITB), U. Stanzin (IAO), V. Bhalerao (IITB), G. C. Anupama (IIA), S. Barway (IIA), report on behalf of the GROWTH-India collaboration: We observed GRB 201223A detected by Swift-BAT (J.D. Gropp et al., GCN #29158); optical counterpart first reported by V. Lipunov et al., (GCN #29157) with 0.7m GROWTH-India Telescope, starting at 2020-12-23T18:30:33.51 UT. We obtained multiple images of 300 sec each in the r' filter. We clearly detected the afterglow at a position consistent with Swift-UVOT (GCN #29158) and MASTER-Net OT (GCN #29157) positions. We obtained the following photometric results: ------------------------------------------------------------------ JD (start) | T_start-T0 (hrs) | Filter | Mag (AB)| ------------------------------------------------------------------ 2459207.27122 | 0.5 | r' | 18.52 +/- 0.03 2459207.28623 | 0.9 | r' | 19.12 +/- 0.04 2459207.32568 | 1.8 | r' | 19.98 +/- 0.09 2459207.34029 | 2.2 | r' | 20.15 +/- 0.09 ------------------------------------------------------------------ Combining our photometric results with r band measurements of Z.P. Zhu et. al., (GCN #29159), we conclude that the source is fading with a power-law index of 1.04 +/- 0.01. The magnitudes are calibrated against PanSTARRs PS1 data release, (Flewelling et al., 2018) and not corrected for galactic extinction. Processing of more data is underway. The GROWTH India Telescope (GIT) is a 70-cm telescope with a 0.7 degree field of view, set up by the Indian Institute of Astrophysics and the Indian Institute of Technology Bombay with support from the Indo-US Science and Technology Forum (IUSSTF) and the Science and Engineering Research Board (SERB) of the Department of Science and Technology (DST), Government of India (https://sites.google.com/view/growthindia/). It is located at the Indian Astronomical Observatory (Hanle), operated by the Indian Institute of Astrophysics (IIA).