TITLE: GCN CIRCULAR NUMBER: 44002 SUBJECT: GRB 260310A / AT2026fgk: Multiband optical follow-up with Kinder observations DATE: 26/03/14 08:19:29 GMT FROM: Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941@gmail.com> C.-H. Lai, A. Aryan, Y.-H. Lee, C.-S. Lin, T.-W. Chen (all NCU), A. K. H. Kong (NTHU), J. H. Gillanders, S. J. Smartt (both Oxford), Y. J. Yang (NYUAD), A. Sankar.K, M.-H. Lee, Y.-C. Pan, C.-C. Ngeow, W.-J. Hou, H.-Y. Hsiao, 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 performed multiband optical observations of the field of the GRB 260310A (Fermi GBM team, GCN 43951, Hamburg et al., GCN 43975) 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 g-band started at 13:45 UTC on 13th of March 2026 (MJD 61092.577), 3.37 days after the Fermi GBM detection. The reported optical counterpart, AT 2026fgk (Konno et al., GCN 43974; Hinds et al., GCN 43977; Lipunov et al., GCN 43978; Martin-Carrillo et al., GCN 43979; Moreno Méndez et al., GCN 43980; Ugarte Postigo et al., GCN 43984; Hsu et al., GCN 43986; Pursiainen et al., GCN 43990; Becerra et al., GCN 43991; Martin-Carrillo et al., GCN 44000; Watson et al., GCN 44001), is clearly visible in each individual frame. The GRB also had X-ray (Salunke et al., GCN 43958; Jayaraman et al., GCN 43994) and NIR (Li et al., GCN 43993; Stein et al., GCN 43996) detections. We utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform template subtractions with the Pan-STARRS1 3Pi archive images (Chambers et al. 2016 arXiv:1612.05560) using the 'SFFT' (Hu et al. 2022, ApJ, 936, 157) algorithm. Moreover, we further used AutoPhOT to perform PSF photometry. The details of the observations and the measured photometry (in the AB system) are as follows: Telescope | Filter | MJD (start) | t-t0 (d) | Exposure (s) | Magnitude | avg.Seeing | Airmass LOT | g | 61112.573 | 3.367 | 300 * 1 | 18.72 +/- 0.02 | 1".94 | 2.35 LOT | r | 61112.576 | 3.371 | 300 * 1 | 18.42 +/- 0.02 | 1".70 | 2.32 LOT | i | 61112.581 | 3.374 | 300 * 1 | 18.26 +/- 0.02 | 1".77 | 2.28 LOT | z | 61112.584 | 3.378 | 300 * 1 | 17.96 +/- 0.05 | 1".90 | 2.25 In multiband photometry, the optical counterpart appears red. 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). In the direction of the transient, these magnitudes are not corrected for the expected Galactic foreground extinctions of 0.09, 0.06, 0.04, and 0.03 mags (Schlafly & Finkbeiner 2011) for the g, r, i, and z bands, respectively. The details on the Lulin observatory telescopes and the follow-up methodology for such transients are presented in Aryan et al. (2025, ApJS, 281, 20, doi:10.3847/1538-4365/adfc69). View this GCN Circular online at https://gcn.nasa.gov/circulars/44002. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...