TITLE: GCN CIRCULAR NUMBER: 44070 SUBJECT: EP260321a: Kinder observations detect a blue variable star and set limits on a source from the z =0.034 galaxy within the error circle DATE: 26/03/21 17:18:54 GMT FROM: Janet Chen at National Central University <janetstars@gmail.com> M.-H. Lee, A. Aryan, T.-W. Chen, W.-J. Hou (all NCU), S. Yang (HNAS), S. J. Smartt, J. Gillanders (both Oxford), A. K. H. Kong (NTHU), Y. J. Yang (NYUAD), Y.-H. Lee, A. Sankar.K, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, C.-S. Lin H.-C. Lin, H.-Y. Hsiao, J.-K. Guo (all NCU), Z. N. Wang, D. C. Qiang, L. L. Fan (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 EP260321a (Huang et al., GCN 44068 using the 1m LOT at 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 LOT epoch of observations started at 13:38 UTC on the 21st of March 2026 (MJD 61120.568), 1.13 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 difference image, we detected an obvious residual at RA, DEC = 09:59:42.82, 00:25:05.84. We noticed that this position coincides with a stellar source that sits close on top of a resolved galaxy detected in SDSS, Pan-STARRS1 and Legacy Surveys (SDSS J095942.88+002506.2). The stellar source has varying magnitudes in different catalogs (PS1 r = 20.42 +/- 0.25, SDSS r = 21.07 +/- 0.05, DESI r = 19.07). This is likely an unrelated variable star. Moreover, we used AutoPhOT to perform PSF photometry at the residual's position. 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 | 61120.568 | 1.13 | 300 * 6 | 20.97 +/- 0.04 | 1".02 | 1.09 Our 3-sigma upper limit for the observed field is r = 23.1 mag using the SDSS catalogue as the reference. The galaxy SDSS J095942.88+002506.2 is at z = 0.0343, as measured in 2dF and in the DESI Legacy Survey DR1 spectroscopic catalogue (TARGETID: 39627799320858502) lies within the EP FXT localisation region. If EP260321 is associated with this galaxy, then our detection limits imply there is no new source brighter than an absolute magnitude of M ~ -12.6 mag at t-t0 = 1.13 hours after the FXT trigger. A supernova or kilonova would still be rapidly rising in this phase; further observations are encouraged. 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 = 0.06 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 can be found 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/44070. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...