TITLE: GCN CIRCULAR NUMBER: 44388 SUBJECT: GRB260421B: J band upper limit by SYSU 80cm infrared telescope DATE: 26/04/22 07:24:57 GMT FROM: shiy228@mail2.sysu.edu.cn Yun Shi, Chun Chen, Duo-Le Cao, Zhong-Nan Dong, Rui-Chen Gao, Wei-Sen Huang, Jia-Qi Lin, Pu Lin, Jin-Ji Li, Hao-Nan Yang, Yan Yu, 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 observed the field of the long GRB 260421B, detected by Fermi-GBM (Fermi GBM team, GCN 44360) and SVOM/ECLAIRs (Zhao et al., GCN 44362), and followed up by Einstein Probe/FXT (Wang et al., GCN 44387), using the Sun Yat-sen University (SYSU) 80 cm infrared telescope. Early near-infrared follow-up observations have also been reported (Mo et al., GCN 44384). Our observations were carried out from 2026 Apr 21 15:25 UT to Apr 21 17:06 UT, corresponding to 11.2–12.9 hours after the GBM trigger (2026-04-21 04:13:21 UT), with a total exposure time of 5200 s in the J band, obtained under poor conditions. We do not detect any near-infrared counterpart at the position of the reported afterglow (He et al., GCN 44363; Sanchez-Alvarez, GCN 44364; Turpin et al., GCN 44365; Wu et al., GCN 44366; Li et al., GCN 44377; Pérez-Fournon et al., GCN 44383; Wang et al., GCN 44387). The 5-sigma limiting magnitude is approximately J ~ 16.9 Vega magnitudes. The SYSU 80 cm infrared telescope is operated and managed by the Department of Astronomy, Sun Yat-sen University. View this GCN Circular online at https://gcn.nasa.gov/circulars/44388. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...