GCN Circular 45855
Subject
GRB 261007A/EP261007a: Optical follow-up observations with Kinder
Event
Date
2026-10-08T09:52:27Z (9 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:
As part of the Kinder collaboration (Chen et al. 2025, ApJ, 983, 86, doi:10.3847/1538-4357/adb428), we conducted follow-up observations of the Gamma-ray Burst, GRB 261007A / fast X-ray transient EP261007a (Goto et al., GCN 45824; Malacaria et al., GCN 45830; Wen et al., GCN 45828). Imaging was acquired in the r-band using the 1m LOT and in the i-band using the 40cm SLT. The first LOT epoch of observations in r-band started at 14:01 UTC on the 07th of October 2026 (MJD 61320.5813), 21.35 hr after the EP-WXT detection.
We utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform template subtraction with the DESI Legacy Survey (Dey et al. 2019, AJ 157, 168) DR10 images using the 'SFFT' (Hu et al. 2022, ApJ, 936, 157) algorithm on individual frames. The optical counterpart (Zheng et al., GCN 45825; Mohan et al., GCN 45832; Saccardi et al., GCN 45837; Shi et al., GCN 45838; An et al., GCN 45840; Arya et al., GCN 45841; Rehemtulla et al., GCN 45843; Ruiz-Laguna et al., GCN 45844; Angulo et al., GCN 45848; Li et al., GCN 45851) with the redshift z=2.101 (Saccardi et al., GCN 45829) was clearly detected in both bands.
Moreover, we further used AutoPhOT to perform PSF photometry. The details of the observations and the measured photometry from the first frame of each band are as follows:
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | Seeing | Airmass
LOT | r | 61320.5813 | 8.25 | 300 * 1 | 20.01 +/- 0.04 | 1".9 | 1.49
SLT | i | 61320.5841 | 8.32 | 300 * 1 | 19.9 +/- 0.1 | 2".1 | 1.46
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 = 0.08 mag and A_i = 0.06 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.