GCN Circular 44354
Subject
EP260416a: Optical follow-up observations with Kinder
Event
Date
2026-04-20T08:42:49Z (6 months ago)
From
Janet Chen at National Central University <janetstars@gmail.com>
Via
Web form
K. N.-T. Ho, A. Aryan, T.-W. Chen, W.-J. Hou (all NCU), A. K. H. Kong (NTHU), S. J. Smartt, J. Gillanders (both Oxford), S. Yang(HNAS), A. Sankar.K, M.-H. Lee, A. Dutta, Y.-H. Lee, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, H.-C. Lin, H.-Y. Hsiao, C.-S. Lin, J.-K. Guo (all NCU), Z. N. Wang, D. C. Qiang, L. L. Fan (all HNAS), Y. J. Yang (NYUAD), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, K. W. Smith, 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 EP260416a (Hu et al., GCN 44307) using the 40cm SLT 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 SLT epoch of observations in r-band started at 11:48 UTC on the 16th of April 2026 (MJD 61146.4918), 6.32 hr after the EP-WXT detection.
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. Owing to the faint optical counterpart candidate (Ducoin et al., GCN 44310), the transient was not detected in our stacked image.
After the reported optical rebrightening (Hua et al., GCN 44318), we performed another SLT epoch of observations starting at UTC 12:33 on April 18th, 2026 (61148.5228), 55.05 hr post the WXT discovery. The optical counterpart (Ducoin et al., GCN 44310; Li et al., GCN 44315; Hua et al., GCN 44318; Lee et al., GCN 44319; Zheng et al., GCN 44321; Martin-Carrillo et al., GCN 44328; Li et al., GCN 44334, He et al. GCN 44336, An et al. GCN 44339, Jelinek et al. GCN 44341; and Guziy et al., GCN 44351) was detected in the stacked images.
We observed the transient for one more epoch, starting at UTC 11:30 on April 19th, 2026 (61149.4790), about 78.0 hr since the WXT discovery. The optical counterpart candidate was not detected in the stacked image.
Moreover, we further used AutoPhOT to perform PSF photometry. The details of the observations and measured photometry/the 3-sigma upper limit (in the AB system) are as follows:
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass
SLT | r | 61146.4918 | 6.31 | 300 * 24 | >21.3 | 2".13 | 1.26
SLT | r | 61148.5228 | 55.05 | 300 * 19 | 21.16 +/- 0.08 | 2".05 | 1.13
SLT | r | 61149.4790 | 78.00 | 300 * 18 | >20.9 | 2".66 | 1.35
The presented magnitudes are calibrated using the field stars from ATLAS-RefCat2 catalog from MAST (Tonry J. L. et al. 2018, ApJ, 867, 105). The reported magnitudes/upper-limits are not corrected for an expected galactic extinction of A_r = 0.04 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.