TITLE: GCN CIRCULAR NUMBER: 41801 SUBJECT: EP250911a: Optical detection with Kinder observations DATE: 25/09/11 17:32:52 GMT FROM: Janet Chen at National Central University <janetstars@gmail.com> C.-H. Lai, A. Aryan, T.-W. Chen, C.-S. Lin (all NCU), Y.-C. Shiau (FEMH), J. Gillanders, S. J. Smartt (both Oxford), Y. J. Yang (NYUAD), A. K. H. Kong (NTHU), Y.-H. Lee, A. Sankar. K, Y.-C. Pan, C.-C. Ngeow, M.-H. Lee, W.-J. Hou, H.-C. Lin, H.-Y. Hsiao, 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 EP250911a (Liang et al., GCN 41788) using the 1m LOT and 40cm SLT at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2025, ApJ, 983, 86). The first LOT epoch of observations in r-band started at 13:17 UTC on the 11th of September 2025 (MJD 60929.554), 4.31 hr after the EP WXT trigger. Moreover, the first SLT epoch of observations in i-band started at 13:18 UTC on the 11th of September 2025 (MJD 60929.554), 4.33 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. In the stacked frames of r, i and z bands from LOT and i-band from SLT, we clearly detected the optical counterpart candidate proposed by Quirola-Vasquez et al. (GCN 41790) and confirmed by Angulo et al. (GCN 41793). Moreover, we further utilized AutoPhOT to perform the PSF photometry. The details of the observations and the measured magnitude (in the AB system) are as follows: Telescope | Filter | MJD (start) | t-t0 (hr | Exposure (s) | Magnitude | avg. Seeing | med. Airmass LOT | r | 60929.554 | 4.31 | 300 * 6 | 21.30 +/- 0.05 | 0".74 | 1.29 SLT | i | 60929.554 | 4.33 | 300 * 24 | 20.44 +/- 0.11| 1".3 | 1.14 LOT | i | 60929.601 | 5.46 | 300 * 6 | 20.71 +/- 0.07 | 0".73 | 1.10 LOT | z | 60929.642 | 6.43 | 300 * 6 | 20.50 +/- 0.09 | 0".97| 1.02 The presented magnitudes are calibrated using the field stars from the SDSS catalog and are not corrected for the expected Galactic foreground reddening of E(B-V) = 0.05 mag in the direction of the transient (Schlafly & Finkbeiner 2011). View this GCN Circular online at https://gcn.nasa.gov/circulars/41801. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...