TITLE:   GCN CIRCULAR
NUMBER:  45906
SUBJECT: GRB 261011A: Luminance-band upper limit from a 28cm telescope in Chengde
DATE:    26/10/11 18:53:04 GMT
FROM:    Y.C. Kang at Peking University <yckang@stu.pku.edu.cn>

Chenxi Bao (PKU), Yacheng Kang (PKU), Xinmiao Zhao (PKU), and Zhuokai Liu (PKU) report on behalf of the Interestar Collaboration and the PKU HiTF (High-energy Transients Follow-up) group:

We observed the field of GRB 261011A, detected by Swift/BAT (Cenko et al., GCN 45896; T0 = 2026-10-11T13:46:41) and SVOM/ECLAIRs (Huang et al, GCN 45895), using a 28cm optical telescope located in Chengde, Hebei, China. The observation began on 2026-10-11 at 14:33:32 UT, approximately 0.78 hr after the Swift/BAT trigger. 

We obtained the Luminance-band image with a total exposure time of 1800 s. No significant source is detected at the position of the reported optical counterpart (Cenko et al., GCN 45896; O'Neill et al., GCN 45897; Fu et al., GCN 45898; Izzo et al., GCN 45899; Wu et al., GCN 45900; Yin et al., GCN 45901; Shi et al., GCN 45903; Aryan et al., GCN 45904; Buckley et al., GCN 45905). We derive a 5-sigma upper limit of L > 19.65 mag. 

Preliminary photometric calibration was performed using nearby catalogued field stars. The reported limit is an approximate AB-equivalent magnitude and has not been corrected for Galactic extinction. Our non-detection is consistent with the reported fading behavior of the optical counterpart. Further optical follow-up observations are encouraged.

The 28cm optical telescope is operated by the Interestar Collaboration, which contributes to the rapid follow-up of high-energy transients as part of the PKU HiTF group.