TITLE: GCN CIRCULAR NUMBER: 44739 SUBJECT: EP260527a/GRB 260527A: optical upper limit with Kinder observations DATE: 26/05/29 15:40:44 GMT FROM: Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941@gmail.com> K. N.-T. Ho, A. Aryan, T.-W. Chen, W.-J. Hou, H.-Y. Hsiao (all NCU), A. K. H. Kong (NTHU), S. J. Smartt, J. Gillanders (both Oxford), S. Yang(HNAS), Y.-H. Lee, A. Sankar.K, M.-H. Lee, A. Dutta, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, H.-C. Lin, 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 EP260527a (Yang et al., GCN 44718, GCN 44725; Svinkin, GCN 44722; Yu et al., GCN 44724; Kawakubo et al., GCN 44726; Wang et al., GCN 44735) using the 1m LOT 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 16:02 UTC on 28th May 2026 (MJD 61188.6683), 33.71 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. Neither in the individual frames nor in the stacked frame did we detect the reported optical/NIR counterpart candidate (Aguilar-Ruiz et al., GCN 44719; Li et al., GCN 44720; Mo et al., GCN 44727; Busmann et al., GCN 44728; Corcoran et al., GCN 44729; Kabir et al., GCN 44730; O'Connor et al., GCN 44731; Yang et al., GCN 44736). 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 LOT | r | 61188.6683 | 33.71 | 120 * 15 | >21.1 | 1".24 | 2.02 The presented magnitudes are calibrated using the field stars from the Pan-STARRS catalogue. The reported upper limit is not corrected for an expected galactic extinction of A_r = 0.04 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/44739. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...