TITLE:   GCN CIRCULAR
NUMBER:  45987
SUBJECT: GRB 261017A: Luminance-band upper limit from a 28cm telescope in Chengde
DATE:    26/10/19 11:45:33 GMT
FROM:    Chenxi Bao at Peking University <2300011608@stu.pku.edu.cn>

Chenxi Bao (PKU), Yacheng Kang (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 261017A, detected by Fermi/GBM (GCN 45973; T0 = 2026-10-17T16:05:05) and Swift (Siegel et al., GCN 45974), using a 28cm optical telescope located in Chengde, Hebei, China. Observations began on 2026-10-18 at 19:18:43 UT, approximately 1.1 days after the trigger.

We obtained the Luminance-band image with a total exposure time of 5400 s. No significant source was detected at the position of the reported X-ray counterpart (Siegel et al., GCN 45974; Beardmore et al., GCN 45977; Sbarrato et al., GCN 45979; Barthelmy et al., GCN 45985). We derive a 5-sigma upper limit of L > 21.0 mag at a median time of approximately 1.17 days after the trigger.

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 existing optical/infrared photometric constraints (Fu et al., GCN 45978; Hagio et al., GCN 45980; Tan et al., GCN 45982; Pawar et al., GCN 45986). Given the proposed possibility that GRB 261017A may be a merger-driven long-duration GRB (Maccary et al., GCN 45984), 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.