TITLE: GCN CIRCULAR NUMBER: 43278 SUBJECT: EP251229a: Optical upper limit with Kinder observations DATE: 25/12/31 10:27:16 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. 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 EP251229a (Wang et al., GCN#43253) 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 20:18 UTC on the 29th of December 2025 (MJD 61038.846), 0.32 hr after the EP-WXT 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. Moreover, we utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform template subtraction with the DESI Legacy Survey (Dey et al., 2019, AJ 157, 168) DR10 image using the 'sfft' (Hu et al., 2022, ApJ, 936, 157) algorithm. We did not detect any new and uncataloged source within the EP-FXT localization circle with a radius of 20", either in the stacked or difference image. Moreover, we further 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 | 61038.846 | 0.32 | 300 * 12 | >22.4 | 2".01 | 1.35 Our non-detection is deeper than Lipunov et al. (GCN#43254), consistent with Martin-Carrillo et al. (GCN#43266) and Perez-Garcia et al. (GCN#43268). The absence of any associated optical counterpart down to 22.4 mag with the optical observations starting at 0.32 hours of its X-ray discovery aligns well with the 'Dark' nature of FXTs (Aryan et al., 2025, ApJS, 281, 20). 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 = 0.14 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/43278. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...