TITLE: GCN CIRCULAR NUMBER: 43525 SUBJECT: EP260126a: Optical upper limit with Kinder observations DATE: 26/01/27 12:48:50 GMT FROM: Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941@gmail.com> M.-H. Lee, 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, A. Sankar.K, 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 EP260126a (Fu et al., GCN 43520) 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 in r-band started at 13:53 UTC on the 26 January 2026 (MJD 61066.578), 9.46 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 used AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform PSF photometry. We do not detect any new or uncataloged sources within the 20" EP-FXT localization circle. The details of the observations and the measured 3-sigma upper limits (in the AB system) are as follows: Telescope | Filter | MJD (start)| t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass SLT | r | 61066.578 | 9.46 | 300 * 6 | >21.0 | 2".8 | 1.04 SLT | i | 61066.606 | 10.14 | 300 * 3 | >20.3 | 2".5 | 1.03 The non-detections in our observations are consistent with those reported by Zhu et al. (GCN 43521), Globus et al. (GCN 43522), Eyles-Ferris et al. (GCNs 43523 and 43524). The presented upper limits are calibrated using the field stars from the Pan-STARRS catalog and are not corrected for the expected Galactic foreground extinction of A_r = 0.24 and A_i = 0.18 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/43525. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...