GCN Circular 41591
Subject
EP250828a: Optical counterpart detection with Kinder observations
Event
Date
2025-08-29T10:12:24Z (a month ago)
From
Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941@gmail.com>
Via
Web form
A. Aryan, C.-S. Lin (both NCU), Y. J. Yang (NYUAD), A. K. H. Kong (NTHU), T.-W. Chen (NCU), J. Gillanders (Oxford), S. J. Smartt (Oxford/QUB), Y.-H. Lee, A. Sankar. K, Y.-C. Pan, C.-C. Ngeow, M.-H. Lee, C.-H. Lai, W.-J. Hou, H.-C. Lin, H.-Y. Hsiao, 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:
Followed by our r-band observations using the 1m LOT (Aryan et al., GCN 41579), we continued to observe the field of the fast X-ray transient EP250828a (Liu et al., GCN 41574) in i-band using the 40cm SLT at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2025, ApJ, 983, 86).
The first SLT epoch of observations in i-band started at 14:17 UTC on the 28th of August 2025 (MJD 60915.595), 6.74 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. We utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform template subtraction with the Pan-STARRS1 (Chambers et al., 2016 arXiv:1612.05560) image using the 'sfft' (Hu et al., 2022, ApJ, 936,157) algorithm. In the stacked as well as in the difference image, we weakly detected the optical counterpart candidate proposed by Taguchi et al., (GCN 41582). The candidate counterpart was also confirmed by Yao et al. (GCN 41583) and Fu et al. (GCN 41584).
In our i-band difference image, the optical counterpart was detected with an SNR of 2.5. The details of the observations and the measured photometry (in the AB system) are as follows:
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass
SLT | i | 60915.595 | 6.74 | 300 * 24 | 22.46 +/- 0.27 | 1".48 | 1.01
The presented magnitude is calibrated using the field stars from the Pan-STARRS1 catalog and is not corrected for the expected Galactic foreground extinction of A_i = 0.21 mag in the direction of the transient (Schlafly & Finkbeiner 2011).