TITLE: GCN CIRCULAR NUMBER: 43135 SUBJECT: EP251214a: Optical upper limit with Kinder observations DATE: 25/12/16 04:56:08 GMT FROM: Janet Chen at National Central University <janetstars@gmail.com> A. Aryan, T.-W. Chen, H.-Y. Hsiao, C.-S. Lin (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, M.-H. Lee, C.-H. Lai, W.-J. Hou, 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 EP251214a (Yin et al., GCN 43087; GCN 43102) using the 40 cm 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 16:25 UTC on the 14th of December 2025 (MJD 61023.684), 13.97 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. We utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform template subtraction with the Pan-STARRS1 3Pi (Chambers et al., 2016 arXiv:1612.05560) image using the 'sfft' (Hu et al., 2022, ApJ, 936, 157) algorithm. We do not find any evidence of a new and uncataloged optical counterpart candidate within the 3.1 arcminute error circle of the EP-WXT localization in either the stacked or the difference image. Moreover, we further utilized AutoPhOT to perform PSF photometry. The details of the observations and the measured upper limit (in the AB system) are as follows: Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass SLT | i | 61023.684 | 13.97 | 300 * 24 | >21.9 | 1".3 | 1.30 Our non-detection is consistent with Basa et al. (GCN 43092); Lipunov et al. (GCN 43099); Zheng et al. (GCN 43110). The absence of any associated optical counterpart down to 21.9 mag with the optical observations starting within 24 hours of its X-ray discovery aligns well with the 'Dark' nature of FXTs (Aryan et al., 2025, ApJS, 281, 20, doi:10.3847/1538-4365/adfc69). The presented magnitude is calibrated using the field stars from the Pan-STARRS DR2 catalog and is not corrected for the expected Galactic foreground extinction of A_i = 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 can be found in Aryan et al. (2025). View this GCN Circular online at https://gcn.nasa.gov/circulars/43135. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...