TITLE:   GCN CIRCULAR
NUMBER:  40046
SUBJECT: EP250402a: Optical upper limit with Kinder observations
DATE:    25/04/04 12:01:23 GMT
FROM:    Janet Chen at National Central University <janetstars@gmail.com>

A. Aryan, Y.-H. Lee, T.-W. Chen, W.-J. Hou (all NCU), S. Brennan (OKC), A. K. H. Kong (NTHU), J. Gillanders (Oxford), S. J. Smartt (Oxford/QUB), Y. J. Yang, A. Sankar. K, Y.-C. Pan, C.-C. Ngeow, M.-H. Lee, H.-Y. Hsiao, C.-S. Lin, 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 EP250402a (Li et al., GCN 40017, GCN 40038) 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 r-band started at 14:18 UT on the 3rd of April 2025 (MJD = 60768.538), ~24.43 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/EP-FXT localization error circle of radius 2.5 arcminutes/10 arcseconds. The field of FXT is only covered by SkyMapper. 

We further employed the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, 62) 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 | r | 60768.538 | 24.43 | 300 * 24 | >20.5 | 1".83 | 3.11   

The presented upper limit was calibrated using the field stars from the SkyMapper catalog and was not corrected for the expected Galactic foreground extinction corresponding to a reddening of A_r = 0.38 mag, respectively, in the direction of the transient (Schlafly & Finkbeiner 2011).