TITLE: GCN CIRCULAR NUMBER: 39815 SUBJECT: EP250321a: Kinder optical follow-up observations DATE: 25/03/21 14:45:24 GMT FROM: Janet Chen at National Central University janetstars@gmail.com
Y.-H. Lee, A. Aryan, T.-W. Chen, Y. J. Yang, W.-J. Hou, H.-Y. Hsiao (all NCU), A. K. H. Kong (NTHU), J. Gillanders (Oxford), S. J. Smartt (Oxford/QUB), M.-H. Lee, Y.-C. Pan, C.-C. Ngeow, A. Sankar. K, C.-H. Lai, C.-S. Lin, H.-C. Lin, 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 EP250321a (Hu et al., GCN 39800) using the 1m LOT (in g and r band) and 40cm SLT (in i and z band) at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al. 2024, arXiv:2406.09270). The first LOT epoch of observations in the r band started at 11:57 UT on the 21st of March 2025 (MJD 60755.498), ~5.78 hrs after the EP trigger, while the first SLT epoch of observations in the i band started at 12:23 UT on the 21st of March 2025 (MJD 60755.516), ~6.21 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 images, we clearly detected the optical counterpart candidate proposed by Fu et al. (GCN 39804) and confirmed by several other observations (e.g., Perez-Garcia et al al., GCN 39805; Brivio et al., GCN 39807; Zhu et al., GCN 39809; Becerra et al., GCN 39810; Pérez-Fournon et al., GCN 39812).
We employed the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform PSF photometry on individual frames. The details of the observation and measured photometry from the first frame (in the AB system) were as follows:
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | Seeing | Airmass LOT | r | 60755.498 | 5.78 | 300 * 6 | 21.86 +/- 0.07 | 1".1 | 1.63 SLT | i | 60755.516 | 6.21 | 300 * 12 | 20.75 +/- 0.10 | 1".4 | 1.36 LOT | g | 60755.520 | 6.31 | 300 * 6 | >22.04 | 1".5 | 1.41
The presented magnitudes 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_r = 0.12 mag, A_i = 0.09 mag, and A_g = 0.17 mag, respectively, in the direction of the transient (Schlafly & Finkbeiner 2011).
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