GCN Circular 45748
GRB 260924A: Optical follow-up observations with Kinder
From: Amar Aryan at National Central University, Institute of Astronomy (NCUIA)
Issued: 2026-09-25 17:19 UTC
Type: GRB, Optical Transient

A. Aryan (NCU), R. Gupta (NASA GSFC), K. N.-T. Ho, T.-W. Chen, C.-S. Lin (all NCU), A. K. H. Kong (NTHU), S. J. Smartt, J. Gillanders (both Oxford), S. Yang(HNAS), Y.-H. Lee, A. Sankar.K, A. Dutta, M.-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 GRB 260924A (The Fermi GBM team, GCN 45712; D'Elia et al., GCN 45713; Evans et al., GCN 45715; Airasca et al., GCN 45721; McDermott et al., GCN 45723; Dichiara et al., GCN 45730; Gupta et al., GCN 45733) using the 40cm SLT and 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 SLT epoch of observations in r-band started at 19:42 UTC on the 24th of September 2026 (MJD 61307.8207), 18.47 hr after the Fermi GBM detection.

For the SLT g-band observations, 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. In the stacked image, we detect the optical counterpart reported by Hu et al., (GCN 45717) and confirmed by Izza et al., (GCN 45718); Vijaykumar et al., (GCN 45734); Klingler et al., (GCN 45735) and Novotny et al. GCN (45743). The NIR detection is reported by Ma et al., GCN (45736). The GRB is also detected by NuSTAR (Waratkar et al., GCN 45732) and SVOM/GRM (SVOM/GRM team, GCN 45741). For the LOT r-band observations, the GRB is detected in all six individual frames, each of 300 seconds.

Moreover, we further used AutoPhOT to perform PSF photometry. 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*
SLT | g | 61307.8207 | 18.47 | 300 * 12 | 19.2 +/- 0.1 | 2".2 | 2.30
LOT | r | 61307.8328 | 18.76 | 300 * 1 | 18.38 +/- 0.04| 1".8 | 2.54

Our detections are in agreement with the reported optical counterpart magnitudes at various epochs. We aim to obtain further observations in the upcoming nights.

*for the single r-band frame, it's simply the airmass at which the frame was observed.

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). The reported magnitudes are not corrected for expected galactic extinction of A_g = 0.140 mag and A_r = 0.097 mag, in the direction of the transient (Schlafly & Finkbeiner 2011). The optical follow-up methodology of transients and details on the Lulin Observatory telescopes are presented in Aryan et al. 2025, ApJS, 281, 20, doi:10.3847/1538-4365/adfc69.

Filter: gamma-ray bursts are posted only if in the top 10% of the past year's — by fluence (total energy received per cm², from Fermi GBM, Konus-Wind, GECAM and similar circulars; now ≥ 3.9×10^-5 erg/cm²) or by the number of GCN Circulars about the burst (now ≥ 14). Thresholds are recalculated monthly.
Relayed automatically from NASA's General Coordinates Network.