TITLE: GCN CIRCULAR NUMBER: 43236 SUBJECT: GRB 251222A/EP251222b: Optical detection with Kinder observations DATE: 25/12/25 09:54:37 GMT FROM: Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941@gmail.com> M.-H. Lee, A. Aryan, T.-W. Chen, W.-J. Hou, H.-Y. Hsiao (all NCU), A. K. H. Kong (NTHU), J. Gillanders, S. J. Smartt (both Oxford), Y. J. Yang (NYUAD), Y.-H. Lee, A. Sankar.K, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, C.-S. Lin, H.-C. Lin, J.-K. Guo (all NCU), S. Yang, Z. N. Wang, L. L. Fan, G. H. Sun (all HNAS), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, T. Moore, K. W. Smith, C. Angus, A. Aamer (all QUB), A. Schultz and M. Huber (both IfA, Hawaii) report: We observed the field of the GRB/fast X-ray transient GRB 251222A/EP251222ba (Fermi/GBM team, GCN#43186; Yang et al., GCN#43188; Guo et al., GCN#43201; Shi et al., GCN#43215; Neights et al., GCN#43222) using the 40cm SLT at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2025, ApJ, 983, 86, doi:10.3847/1538-4357/adb428). The first SLT epoch of observations started at 10:39 UTC on the 23rd of December 2025 (MJD 61032.444), 17.55 hr after the Fermi/GBM detection. We utilized the astroalign (Beroiz et al., 2020, A&C, 32, 100384) and astropy (Astropy Collaboration et al., 2022, ApJ, 935, 167) packages to align and stack the individual frames. In the stacked image, we clearly detected the optical counterpart candidate proposed by Palmerio et al. (GCN#43189) at a spectroscopic redshift of z = 3.171 (Saccardi et al., GCN#43204). The optical counterpart candidate was confirmed by several observations, including An et al. (GCN#43194); Calapai et al. (GCN#43195); Durbak et al. (NIR detection, GCN#43197); Li et al. (optical rebrightening, GCN#43203); Gupta et al. (GCN#43207, GCN#43224); Saccardi et al. (GCN#43208); Kumar et al. (GCN#43209); Pérez-Fournon et al. (GCN#43212); Globus et al. (GCN#43225); Pankov et al. (GCN#43226); Freeberg et al. (GCN#43229); Volnova et al. (GCN#43230) and Busmann et al. (GCN#43233). Moreover, we further utilized AutoPhOT to perform the PSF photometry. The details of the observations and the measured magnitude (in the AB system) are as follows: Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude. | avg. Seeing | med. Airmass SLT | i | 61032.444 | 17.55 | 300 * 12 | 19.48 +/- 0.07 | 3".7 | 2.27 The BALROG localization of the transient was reported by Preis et al. (GCN#43190), the Swift/XRT counterpart candidate was reported by Beardmore et al. (GCN#43213), and Frederiks et al. (GCN#43214) reported the Konus-Wind detection. The presented magnitude is calibrated using the field stars from the ATLAS-RefCat2 catalog from MAST (Tonry et al., 2018, ApJ, 867, 105) and is not corrected for the expected Galactic foreground extinction of A_i = 0.17 mag in the direction of the transient (Schlafly & Finkbeiner, 2011). The methodology, details on the Lulin observatory telescopes, and a compilation of our optical follow-up campaign for FXTs discovered within the first year of operation of the Einstein-Probe mission are presented in Aryan et al., 2025, ApJS, 281, 20, doi:10.3847/1538-4365/adfc69. View this GCN Circular online at https://gcn.nasa.gov/circulars/43236. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...