TITLE: GCN CIRCULAR NUMBER: 42824 SUBJECT: EP251124a: Optical upper limit with Kinder observations DATE: 25/11/24 17:15:34 GMT FROM: Janet Chen at National Central University <janetstars@gmail.com> Z. N. Wang (HNAS), A. Aryan, T.-W. Chen, W.-J. Hou, Y.-H. Lee (all NCU), A. K. H. Kong (NTHU), J. H. Gillanders, S. J. Smartt (both Oxford), Y. J. Yang (NYUAD), A. Sankar.K, Y.-C. Pan, C.-C. Ngeow, M.-H. Lee, C.-H. Lai, H.-C. Lin, C.-S. Lin, H.-Y. Hsiao, J.-K. Guo (all NCU), S. Yang, 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 EP251124a (Zhou et al., GCN 42816) 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). Our SLT observation started at 16:21 UTC on the 24th of November 2025 (MJD 61003.477), 0.27 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 utilised the Python-based package AutoPhOT (Brennan & Fraser 2022, A&A, 667, A62) to perform template subtraction with the Pan-STARRS1 3Pi archive image (Chambers et al. 2016, arXiv:1612.05560) using the 'hotpants' (Becker 2015, ascl.soft. ascl:1504.004) algorithm. In the difference image, we do not detect any uncataloged sources within the 20" error circle, for each of the three EP-FXT positions reported. We note that the position of FXTB-2 is not covered by Pan-STARRS1. Although this field is included in SDSS DR14 (Abolfathi et al. 2018, arXiv:1707.09322), we do not find any new or uncataloged sources by eye within the 20″ error circle of FXTB-2. Moreover, we further utilized AutoPhOT to perform the PSF photometry. The details of the observations and the 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 | 61003.477 | 0.27 | 300 * 24 | >21.7 | 1".40 | 1.19 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_r = 0.27 mag in the direction of the transient (Schlafly & Finkbeiner 2011). Our observation agrees with Mohan et al. (GCN 42817) and provides a deeper constraint. 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, ApJS, 281, 20. doi:10.3847/1538-4365/adfc69. View this GCN Circular online at https://gcn.nasa.gov/circulars/42824. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...