TITLE: GCN CIRCULAR NUMBER: 39534 SUBJECT: EP250227a: Optical upper limits with Kinder observations DATE: 25/02/28 16:56:52 GMT FROM: Amar Aryan at National Central University, Institute of Astronomy (NCUIA) amararyan941@gmail.com

A. Aryan (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), Y. J. Yang, 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 EP250227a (Wen et al., GCN 39532) 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 15:53 UT on the 27th of February 2025 (MJD = 60733.662), ~11.22 hrs after the EP trigger, while the first SLT epoch of observations in the r band started at 16:28 UT on the 27th of February 2025 (MJD = 60733.686), ~11.79 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. We did not detect any uncataloged optical counterpart candidate within the EP-WXT localization error circle of radius 2.7 arcminutes.

We further employed the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform PSF photometry on our stacked frames. The details of the observations and measured three 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 | 60733.662 | 11.22 | 300 * 6 | >21.1 | 1".47 | 1.80 SLT | r | 60733.686 | 11.79 | 300 * 12 | >21.5 | 1".46 | 1.49

The non-detection of any prominent optical counterpart is consistent with Malesani et al., (GCN 39533).

The presented upper limits were calibrated using the field stars from the Pan-STARRS1 catalog and were not corrected for the expected Galactic foreground extinction corresponding to a reddening of A_i = 0.12 mag and A_r = 0.16 mag, respectively, in the direction of the transient (Schlafly & Finkbeiner 2011).

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