GCN Circular 42921
Subject
EP251130a: Optical detections with Kinder observations
Event
Date
2025-12-01T14:51:58Z (16 days ago)
From
Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941@gmail.com>
Via
Web form
A. Aryan, T.-W. Chen, H.-Y. Hsiao, C.-S. Lin (all NCU), A. K. H. Kong (NTHU), J. Gillanders, S. J. Smartt (both Oxford), Y. J. Yang (NYUAD), Y.-H. Lee, A. Sankar.K, Y.-C. Pan, C.-C. Ngeow, M.-H. Lee, C.-H. Lai, W.-J. Hou, H.-C. Lin, 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:
We observed the field of the fast X-ray transient EP251130a (Wu et al., GCN 42903; Zhang et al., GCN 42915) using the 1m LOT and 40cm SLT at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2025, ApJ, 983, 86, doi:10.3847/1538-4357/adb428). The first epoch of r-band LOT observations began at 16:49 UTC on November 30, 2025 (MJD 61009.701), 5.70 hours 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 clearly detected the optical counterpart (Dornic et al., GCN 42904; He et al., GCN 42914; Gupta et al., GCN 42916; Zheng et al., GCN 42917; van Dalen et al., GCN 42918; Moskvitin et al., GCN 42919). The optical counterpart lies within the Swift-XRT (Evans et al., GCN 42908) and the refined EP-FXT (Zhang et al., GCN 42915) localizations. Moreover, we utilized AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform PSF photometry on stacked images. The details of the observations and the measured magnitudes (in the AB system) are as follows:
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass
LOT | r | 61009.701 | 5.70 | 300 * 6 | 21.97 +/- 0.08 | 1".16 | 1.03
SLT | i | 61009.703 | 5.74 | 300 * 7 | 20.87 +/- 0.09 | 1".78 | 1.03
We plan to continue following the FXT on a few subsequent days. The presented magnitudes are calibrated using the field stars from the ATLAS-RefCat2 catalog from MAST (Tonry J. L. et al., 2018, ApJ, 867, 105) and are not corrected for the expected Galactic foreground extinction of A_r = 0.26 mag and A_i = 0.19 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 can be found in Aryan et al., 2025, ApJS, 281, 20, doi:10.3847/1538-4365/adfc69.