TITLE: GCN CIRCULAR NUMBER: 44490 SUBJECT: EP260507a: Kinder optical counterpart candidate DATE: 26/05/07 15:19:55 GMT FROM: Janet Chen at National Central University <janetstars@gmail.com> A. Sankar.K, A. Aryan, Y.-H. Lee, T.-W. Chen, W.-J. Hou (all NCU), A. K. H. Kong (NTHU), S. J. Smartt, J. Gillanders (both Oxford), S. Yang(HNAS), M.-H. Lee, A. Dutta, K. N.-T. Ho, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, H.-C. Lin, H.-Y. Hsiao, C.-S. Lin, J.-K. Guo (all NCU), Z. N. Wang, D. C. Qiang, L. L. Fan (all HNAS), Y. J. Yang (NYUAD), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, K. W. Smith, C. Angus, A. Aamer (all QUB), T. Moore (STScI), A. Schultz and M. Huber (both IfA, Hawaii) report: We observed the field of the fast X-ray transient EP260507a (Fu et al., GCN 44488) using the 40cm SLT at the Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2025, ApJ, 983, 86). The first SLT epoch of observations started at 14:06 UTC on 7th May 2026 (MJD 61167.5837), 0.30 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 using the 'SFFT' (Hu et al. 2022, ApJ, 936, 157) algorithm. In the stacked frame as well as in the difference image, we detect an uncatalogued optical counterpart candidate at RA, DEC = 13:47:35.567, -22:14:08.38 with an uncertainty of 1". The optical counterpart candidate is well inside the EP-FXT error circle of 20". The details of the observations and measured PSF magnitude with template subtraction (in the AB system) of the possible counterpart of EP260507a are as follows: -------------------------------------------------------------------------------------------------- Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass SLT | r | 61167.5837 | 0.30 | 300 * 6 | 20.64 +/- 0.07 | 2".07 | 1.44 The presented magnitudes are calibrated using the field stars from ATLAS-RefCat2 catalog from MAST (Tonry J. L. et al. 2018, ApJ, 867, 105). The reported magnitudes/upper-limits are not corrected for an expected galactic extinction of A_r = 0.16 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/44490. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...