TITLE: GCN CIRCULAR NUMBER: 39543 SUBJECT: EP250228a: Optical observations with Kinder and Swift/UVOT DATE: 25/03/01 13:45:29 GMT FROM: Amar Aryan at National Central University, Institute of Astronomy (NCUIA) amararyan941@gmail.com

A. Aryan, Y. J. Yang (both NCU), A. K. H. Kong (NTHU), T.-W. Chen, H.-Y. Hsiao, C.-S. Lin (all NCU), J. Gillanders (Oxford), S. J. Smartt (Oxford/QUB), A. Sankar. K, Y.-C. Pan, C.-C. Ngeow, W.-J. Hou, M.-H. Lee, H.-C. Lin, C.-H. Lai, J.-K. Guo (all NCU), S. Yang, L. L. Fan, Z. N. Wang, 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 EP250228a (Liu et al., GCN 39525; Liu et al., GCN39531) using the 40cm SLT telescope at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al. 2024, arXiv:2406.09270). The first SLT epoch of observations in the i band started at 10:54 UT on the 28th of February 2025 (MJD = 60734.454), ~6.91 hrs after the EP 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 any uncataloged optical counterpart candidate within the EP-FXT localization error circle of radius 20 arcseconds.

Moreover, we employed the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform template subtraction utilizing the "sfft" (Hu, 2022, ApJ, 936, 157) and "hotpants" (Becker A., 2015, ascl.soft. ascl:1504.004) algorithms using templates from SkyMapper DR4 (Onken et., 2024, PASA, 41, e061). We found no evidence of any prominent candidate optical counterpart in the difference images as well.

We further employed AutoPhOT to perform the PSF photometry. The details of the observations and measured 3-sigma upper limits (in the AB system) were as follows:

Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass SLT | i | 60734.454 | 6.91 | 300 * 12 | >20.2 | 1".20 | 2.44 SLT | r | 60734.503 | 8.08 | 300 * 24 | >19.9 | 1".43 | 2.53

The X-ray transient was also detected in Swift-XRT (Page et al., GCN39527). The Swift-XRT position coincided with a star (Gaia DR3 5563280076339409920; as updated by Liu et al., GCN39531), but we probably missed the flaring stage as we did not see any credible excess flux in the difference image. For this star, we measured an r-band magnitude of 15.33 +/- 0.03 mag and an i-band magnitude of 13.65 +/- 0.03 mag in our stacked frames. Our measured magnitudes for this source align perfectly with the archival values in the SkyMapper catalog. In addition, a preliminary U-band magnitude of 17.61 +/- 0.05 (stat) +/- 0.02 (sys) was measured from the quick-look Swift-UVOT observation.

The presented measurements were calibrated using the field stars from the SkyMapper catalog and were not corrected for the expected Galactic foreground extinction corresponding to a reddening of A_i = 0.20 mag, and A_r = 0.27 mag in the direction of the transient (Schlafly & Finkbeiner 2011).

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