TITLE: GCN CIRCULAR NUMBER: 43692 SUBJECT: EP260210a: Optical upper limits with Kinder observations DATE: 26/02/11 08:35:06 GMT FROM: Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941@gmail.com> A. Aryan, C.-S. Lin, T.-W. Chen (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, M.-H. Lee, C.-H. Lai, Y.-C. Pan, C.-C. Ngeow, 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 EP260210a (Shi et al. GCN 43681) using the 40cm SLT and 1m LOT at the 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 13:57 UTC on 10th of February 2026 (MJD 61081.582), 3.57 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 DESI Legacy Survey (Dey et al. 2019, AJ 157, 168) DR10 image (for r-band) and Pan-STARRS1 3Pi (Chambers et al. 2016, arXiv:1612.05560) archive image (for i-band) using the 'SFFT' (Hu et al. 2022, ApJ, 936, 157) algorithm. Moreover, we used AutoPhOT 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 | 61081.582 | 3.57 | 300 * 12 | >22.0 | 1".5 | 1.47 LOT | r | 61081.584 | 3.62 | 300 * 6 | >22.9 | 1".3 | 1.54 SLT | i | 61081.628 | 4.69 | 300 * 20 | >21.6 | 1".4 | 1.18 The non-detections in our observations are consistent with those reported by Kang et al. (GCN 43683), Fortin et al. (GCN 43684), and Li et al. (GCN 43687). The presented magnitude is calibrated using the field stars from the ATLAS-RefCat2 catalog from MAST (Tonry J. L. et al. 2018, ApJ, 867, 105) and is not corrected for the expected Galactic foreground extinction of A_r = 0.034 mag and A_i = 0.026 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/43692. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...