GCN 44081: EP260321a: further Kinder optical observations

A. Aryan, M.-H. Lee, T.-W. Chen, W.-J. Hou (all NCU), S. Yang (HNAS), A. K. H. Kong (NTHU), T. Moriya (NAOJ), S. J. Smartt, J. Gillanders (both Oxford), Y. J. Yang (NYUAD), A. Dutta, 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 et al., 2025, ApJ, 983, 86, doi:10.3847/1538-4357/adb428). The first LOT epoch of observations started at 11:34 UTC on the 22nd of March 2026 (MJD 61121.482), 23.07 hr after the EP-WXT trigger. 

We used Kinder pipeline (Yang et al. A&A 646, A22) to perform template subtraction with the DESI Legacy Survey (Dey et al. 2019, AJ 157, 168) DR10 image. In the difference image, we still detected this stellar source SDSS J095942.88+002506.2 (Lee et al., GCN 44070), but no other new transient has emerged.

Moreover, we used AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform PSF photometry for this variable star (without template subtraction). 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/avg.) Airmass
LOT       | r      | 61121.482   | 23.07     | 300  * 6     | 19.18 +/- 0.04 | 0".96  | 1.27
LOT       | g      | 61121.520   | 23.99     | 300  * 2     | 18.77 +/- 0.04 | 1".11  | 1.17
Our 3-sigma upper limit for the observed field is r = 23.4 mag.

EP260321a was identified as a possible supernova shock-breakout candidate in the EP refined analysis (Huang et al., GCN 44075). However, no optical counterpart has been reported to date (Lee et al., GCN 44070; Ma et al., GCN 44074; Aguilar-Ruiz et al., GCN 44076; Liang et al., GCN 44079). For comparison, in SN 2008D the optical counterpart was detected only ~1.4 hr after the initial X-ray detection (Soderberg et al. 2008). Given the similar X-ray duration to SN 2008D and the continued absence of any optical counterpart, we suspect that the EP260321a X-ray emission more likely originates from the variable star itself. If it is a Galactic X-ray binary, the spectrum suggests a supersoft (WD) accretor, but the X-ray flux is weak; a strong local stellar flare is also plausible.

The presented magnitudes are calibrated using the field stars from the ATLAS-RefCat2 catalog from MAST (Tonry J. L. et al. 2018, ApJ, 867, 105) and are not corrected for the expected Galactic foreground extinctions of A_r = 0.06 mag and A_g = 0.08 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.