GCN Circular 43235
Subject
EP251222a: Optical upper limit with Kinder observations
Event
Date
2025-12-25T09:50:48Z (7 hours ago)
From
Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941@gmail.com>
Via
Web form
M.-H. Lee, A. Aryan, T.-W. Chen, W.-J. Hou, H.-Y. Hsiao (all NCU), A. K. H. Kong (NTHU), J. Gillanders, S. J. Smartt (both Oxford), 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, 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 EP251222a (Huang et al., GCN#43200; Guo et al., GCN#43216) using the 40cm SLT 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 SLT epoch of observations started at 11:44 UTC on the 23rd of December 2025 (MJD 61032.489), 20.43 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. Moreover, 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 did not detect the optical counterpart candidate (Li et al., GCN#43205; Malesani et al., GCN#43211; Busmann et al., GCN#43232) proposed at a redshift of z = 0.505 (An et al., GCN#43227).
Moreover, we further utilized AutoPhOT to perform the PSF photometry. The details of the observations and the measured 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
SLT | i | 61032.489 | 20.43 | 300 * 24 | >21.4 | 2".8 | 1.16
The non-detection in our observations can be attributed to the relatively shallow limit and late follow-up. Our non-detection is consistent with Kumar et al. (GCN#43210) and Lipunov et al. (GCN#43219).
The presented upper limit is calibrated using the field stars from the ATLAS-RefCat2 catalog from MAST (Tonry et al., 2018, ApJ, 867, 105) and is not corrected for the expected Galactic foreground extinction of A_i = 0.23 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.