TITLE: GCN CIRCULAR NUMBER: 43279 SUBJECT: GRB 251230A/EP251230a: Optical upper limit with Kinder observations DATE: 25/12/31 10:29:09 GMT FROM: Janet Chen at National Central University <janetstars@gmail.com> A. Sankar.K, A. Aryan, T.-W. Chen, C.-S. Lin (all NCU), A. K. H. Kong (NTHU), J. H. Gillanders, S. J. Smartt (both Oxford), Y. J. Yang (NYUAD), Y.-H. Lee, M.-H. Lee, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, W.-J. Hou, H.-Y. Hsiao, 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 fast X-ray transient GRB 251230A/EP251230a (Lanava et al., GCN#43252; Wang et al., GCN#43259; Osborne et al., GCN#43263; Wang et al., GCN#43272; Sbarrato et al., GCN#43275) 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 18:17 UTC on the 30th of December 2025 (MJD 61039.762), 16.91 hr after the Swift/BAT trigger. 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 frame, we did not detect the proposed optical counterpart candidate (Li et al., GCN#43255; Busmann et al., GCN#43258; Masi et al., GCN#43260; Ghosh et al., GCN#43262; Ma et al., GCN#43265; Breeveld et al., GCN#43271), which has a tentative redshift upper limit of z < 2.2 (He et al., GCN#43269). Moreover, we utilized AutoPhOT to perform the PSF photometry. The details of the observations and the measured 3-sigma upper limit (in the AB system) are as follows: Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass SLT | r | 61039.762 | 16.91 | 300 * 24 | >22.5 | 2".10 | 1.17 Utilizing the rigorous Lulin Observatory optical follow-up of the Einstein-Probe-discovered FXTs within the first year of the mission's operation, we investigated the possible connection between FXTs and GRBs, as reported in Aryan et al. (2025), ApJS, 281, 20. Our upper limits are the deepest and latest among those reported (Starling et al., GCN#43267; Tak et al., GCN#43270). The presented upper limit is calibrated using the field stars from the PanSTARRs DR2 and is not corrected for the expected Galactic foreground extinction of A_r = 0.05 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/43279. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...