GCN Circular 45758
Jin-Ji Li, Zhong-Nan Dong, Jia-Qi Lin, Pu Lin, Yun Shi, Hao-Nan Yang, Hao-Ran Zhang, P H Thomas Tam, Rong-Feng Shen, Bin Ma (Sun Yat-sen University) report on behalf of the SYSU 80cm infrared telescope team:
We carried out a second epoch of J-band observations of GRB 260924A using the Sun Yat-sen University (SYSU) 80 cm infrared telescope, following our previous detection reported in Ma et al. (GCN 45736). Our second-epoch observations started on 2026 September 25 at 22:00 UT, approximately 44.8 hours after the Swift/BAT trigger, and continued until 23:03 UT in the J band.
We do not detect the source at the position of the counterpart (Hu et al., GCN 45717; Izzo et al., GCN 45718; Vijaykumar et al., GCN 45734; Klingler et al., GCN 45735; Ma et al., GCN 45736; Castro-Tirado et al., GCN 45740; Yu et al., GCN 45741; Novotny et al., GCN 45743; Aryan et al., GCN 45748) in the stacked image, down to a 5-sigma limiting magnitude of J > 18.5 mag (Vega).
The photometry was calibrated against nearby stars from the 2MASS catalog and is reported in the Vega system, without correction for Galactic extinction. The SYSU 80 cm infrared telescope is operated and managed by the Department of Astronomy, Sun Yat-sen University.
Update (2026 September 28): As the observations were affected by intermittent cloud cover, we reprocessed the data using a refined image-selection and stacking procedure. We retained the 110 highest-quality frames, applied photometric normalization and sub-pixel alignment, and combined them with weights based on transparency, sky noise, and image FWHM, while rejecting anomalous pixels. In the resulting stacked image, we detect a source at the reported counterpart position with a magnitude of J = 18.60 ± 0.13 mag (Vega). The photometry was calibrated against nearby 2MASS stars and has not been corrected for Galactic extinction. We therefore revise our previously reported upper limit to this measurement.