TITLE: GCN CIRCULAR NUMBER: 44147 SUBJECT: EP260329b: Optical follow-up with Kinder observations DATE: 26/03/29 14:47:29 GMT FROM: Janet Chen at National Central University <janetstars@gmail.com> A. Aryan, A. Sankar.K, T.-W. Chen, C.-S. Lin (all NCU), A. K. H. Kong (NTHU), S. J. Smartt, J. Gillanders (both Oxford), M. Nicholl (QUB), M.-H. Lee, A. Dutta, Y.-H. Lee, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, H.-C. Lin, W.-J. Hou, H.-Y. Hsiao, J.-K. Guo (all NCU), S. Yang, 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. 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 EP260329b (Song et al., GCN 44142) using the 1m LOT at the Lulin observatory, as part of the Kinder collaboration (Chen et al. 2025, ApJ, 983, 86, doi:10.3847/1538-4357/adb428). The first LOT epoch of observations in r-band started at 12:01 UTC on 29th of March 2026 (MJD 61128.501), 1.42 hr after the EP-WXT detection. 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. Our images were strongly affected by moonlight, and are therefore relatively shallow. In the stacked r-band image at t_mid - t_0 =1.80 hr, we did detect a very faint source (SNR 3.4) at the reported position of the optical counterpart candidate (Wenxiong et al., GCN 44139; Zhu et al., GCN 44140; and Eyles-Ferris et al., GCN 44144). Moreover, we further used AutoPhOT to perform PSF photometry. The details of the observations and measured photometry/the 3-sigma upper limit (in the AB system) are as follows: Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | Med. Airmass LOT | r | 61128.501 | 1.42 | 300 * 8 | 20.19+/-0.14 |1".71 | 1.54 LOT | g | 61128.536 | 2.26 | 300 * 7 | >19.9 |1".50 | 1.34 Our observations confirm the rapidly fading nature of the optical counterpart candidate. 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 are not corrected for an expected galactic extinction of A_g = 0.06 mag and A_r = 0.04, 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/44147. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...