EP260628c/EP J223759.5+531421: Optical Upper limits with Kinder observations
ATel #17875; C.-H. Lai, A. Aryan (both NCU), A. K. H. Kong (NTHU), T.-W. Chen, W.-J. Hou (both NCU), S. J. Smartt, J. Gillanders (both Oxford), S. Yang (HNAS), A. Sankar. K, K. N.-T. Ho, A. Dutta, Y.-H. Lee, Y.-C. Pan, C.-C. Ngeow, M.-H. Lee, 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), K. W. Smith, H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, C. Angus, A. Aamer (all QUB), T. Moore (STScI), A. Schultz and M. Huber (both IfA, Hawaii)
on 6 Jul 2026; 08:13 UT
Credential Certification: Albert Kong (akong@phys.nthu.edu.tw)
Subjects: Optical, Neutron Star, Transient, Magnetar
Referred to by ATel #: 17876
We observed the magnetar candidate EP260628c/EP J223759.5+531421 discovered by the Einstein Probe mission (Yang et al. ATel #17859; Rea et al., ATel #17870) using the 1m LOT at the Lulin observatory, as part of the Kinder collaboration (Chen et al. 2025, ApJ, 983, 86). Observations were made with the g, r, i, and z filters each with 120s exposure. The first LOT epoch of observations in g-band started at 15:24 UTC on the 3rd of July 2026 (MJD 61224.641). We did not detect any signature of any new or uncataloged source within the Einstein Probe FXT region of uncertainty (Yang et al., ATel #17859).
We employed AutoPhOT to perform PSF photometry. The details of the observations and the 3-sigma upper limit (in the AB system) are as follows:
Filter | MJD (start) | Magnitude
g | 61224.641 | >20.1
r | 61224.644 | >20.0
i | 61224.645 | >19.9
z | 61224.647 | >19.2
The presented upper limit 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 Galactic extinction. The methodology, details on the Lulin observatory telescopes, and a compilation of our optical follow-up campaign for fast X-ray transients 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.
