GCN Circular 45854
Subject
EP261006a: Optical follow-up observations with Kinder
Event
Date
2026-10-08T09:49:40Z (18 hours ago)
From
Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941@gmail.com>
Via
Web form
Z.-Y. Chen, A. Aryan, T.-W. Chen, C.-S. Lin (all NCU), A. K. H. Kong (NTHU), S. J. Smartt, J. H. Gillanders (both Oxford), S. Yang (HNAS), A. Sankar.K, K. N.-T. Ho, A. Dutta, M.-H. Lee, Y.-H. Lee, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, H.-C. Lin, H.-Y. Hsiao, W.-J. Hou, J.-K. Guo (all NCU), Z. N. Wang, D. C. Qiang, L. L. Fan (all HNAS), Y. J. Yang (NYUAD), H.-W. Lin (UMich), K. W. Smith, H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, C. Angus, A. Aamer (all QUB), T. Moore (STScI), A. Schultz and M. Huber (both IfA, Hawaii) report:
We observed the field of the fast X-ray transient EP261006a (Lao et al., GCN 45826) using the 1m LOT at the Lulin observatory, as part of the Kinder collaboration (Chen et al. 2025, ApJ, 983, 86, doi:10.3847/1538-4357/adb428). The first LOT epoch of observations in r-band started at 15:11 UTC on the 07th of October 2026 (MJD 61320.6327), 21.35 hr after the EP-WXT detection.
We used the Python-based AutoPhOT package (Brennan & Fraser 2022, A&A, 667, A62) to perform template subtraction with the Pan-STARRS1 3Pi archive image (Chambers et al. 2016 arXiv:1612.05560) using the 'SFFT' (Hu et al. 2022, ApJ, 936, 157) algorithm on individual frames. The optical counterpart (Eyles-Ferris et al., GCN 45831; Wu et al., GCN 45852; Bochenek et al., GCN 45853) was detected in each individual frame.
Moreover, we further used AutoPhOT to perform PSF photometry. The details of the observations and the measured photometry from the first frame are as follows:
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | Seeing | Airmass
LOT | r | 61320.6327 | 21.35 | 300 * 1 | 19.096 +/- 0.039 | 1".3 | 1.06
Our measurements confirm the reported rise by Eyles-Ferris et al. (GCN 45831); however, the source does not vary rapidly over 25 minutes of observations, remaining at ~19.1 mag in the r-band.
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_r = 0.096 mag in the direction of the transient (Schlafly & Finkbeiner 2011). The methodology, details on the Lulin observatory telescopes, and a compilation of our optical follow-up campaign for FXTs discovered within the first year of operation of the Einstein-Probe mission are presented in Aryan et al. 2025, ApJS, 281, 20, doi:10.3847/1538-4365/adfc69.