TITLE:   GCN CIRCULAR
NUMBER:  43813
SUBJECT: EP260221a: Optical upper limit  with Kinder observations
DATE:    26/02/23 09:42:06 GMT
FROM:    Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941@gmail.com>

Y.-H. Lee, A. Aryan, H.-Y. Hsiao, T.-W. Chen (all NCU), A. K. H. Kong (NTHU), J. H. Gillanders, S. J. Smartt (both Oxford), Y. J. Yang (NYUAD), A. Sankar.K, M.-H. Lee, C.-H. Lai, Y.-C. Pan, C.-C. Ngeow, W.-J. Hou, C.-S. Lin, H.-C. Lin, 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 EP260221a (Zhang et al., GCN 43791 and Liu et al., GCN 43802) using the 40cm SLT at the Lulin observatory, as part of the Kinder collaboration (Chen & Yang et al. 2025, ApJ, 983, 86, doi:10.3847/1538-4357/adb428). The first SLT epoch of observations in r-band started at 13:51 UTC on 21st of February 2026 (MJD 61092.577), 2.13 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. Neither in the stacked frame nor in the difference image do we detect the optical counterpart candidate proposed by Watson et al. (GCN 43806) and confirmed by Eyles-Ferris et al. (GCN 43807). Clearly, our non-detection is attributable to the shallower detection limits in our observations. 

Moreover, we used AutoPhOT to perform PSF photometry. The details of the observations and the measured 3-sigma upper limits (in the AB system) are as follows:
 
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass
SLT       | r      | 61092.577   | 2.13      | 300 * 24     | >20.7     | 3".42       | 2.11                    

The non-detection in our observations is consistent with the reports by Mandarakas et al. (GCN 43793), Kumar et al. (GCN 43794), and Min-He et al. (GCN 43798).

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 = 0.075 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 are presented in Aryan et al. 2025, ApJS, 281, 20, doi:10.3847/1538-4365/adfc69.