TITLE: GCN CIRCULAR NUMBER: 39032 SUBJECT: EP250125a: Optical upper limit with Kinder observations DATE: 25/01/25 18:40:30 GMT FROM: Janet Chen at National Central University janetstars@gmail.com

A. Aryan, T.-W. Chen, Y.-H. Lee, W.-J. Hou, C.-S. Lin (all NCU), A. K. H. Kong (NTHU), S. J. Smartt (Oxford/QUB), J. Gillanders (Oxford), A. Sankar.K, Y. J. Yang, M.-H. Lee, H.-Y. Miao, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, H.-Y. Hsiao, 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 EP250125a (Wu et al., GCN 39028) using the 1m LOT at the Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2024 arXiv:2406.09270). The first LOT epoch of observations started at 16:39 UTC on 25th Januaryr 2025 (MJD 60700.694), 14.04 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. In the stacked frame, we did not detect the optical counterpart candidate suggested by Levan et al., (GCN 39027) and Ferro et al., (GCN39031). Considering the decay rate by two above-mentioned optical detections, probably, the optical counterpart candidate faded beyond the detection limit in our observations.

We utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform PSF photometry on our stacked frame. The details of the observations and 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 LOT | r | 60700.694 | 14.04 | 300 * 6 | >22.5 | 1".47 | 1.83

The presented magnitudes were calibrated using the field stars from the Pan-STARRS1 catalog (Chambers et al., 2016 arXiv:1612.05560) and were not corrected for the expected Galactic foreground extinction corresponding to a reddening of A_r = 0.112 mag in the direction of the transient (Schlafly & Finkbeiner 2011).

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