TITLE: GCN CIRCULAR NUMBER: 44327 SUBJECT: EP260324a / AT2026hir: Further multiband optical follow-up with the PKU 60cm Telescope DATE: 26/04/17 13:01:53 GMT FROM: Qiang Wang at Peking University <2501110280@stu.pku.edu.cn> Qiang Wang, Yacheng Kang, Ruize Shi, Xinmiao Zhao, and Fangzhou Ren (PKU) report on behalf of the PKU HiTF (High-energy Transients Follow-up) group: We observed the field of EP260324a (Wu et al., ATel 17728; T0 = 2026-03-24T07:04:20) using the PKU 60cm telescope at Xinglong Observatory, NAOC. Observations were conducted over multiple epochs between 2026 Mar 30 and Apr 11, corresponding to ~ 6.3–18.3 days after the EP-WXT detection, in the g, r, and i bands. In the stacked images, we clearly detect the optical counterpart in the difference images at a position consistent with that of ZTF26aapviim / AT2026hir (Anumarlapudi et al., GCN 44120; Mohan et al., GCN 44128; Kang et al., GCN 44156; Stein et al., GCN 44194; Globus et al., GCN 44196; Watson et al., GCN 44216). Preliminary photometry is calibrated against nearby Pan-STARRS field stars and reported in the AB magnitude system, without correction for Galactic extinction. Combining these data with our earlier observations on 2026 March 27 (Kang et al., GCN 44156), we find that the optical emission likely peaked at ~ 1 week after the EP trigger. Following the peak, we estimate the temporal decay using a simple power-law model, yielding indices of alpha_g ~ 1.13 and alpha_r ~ 0.93. The observation log is summarized below following the time sequence: | Date | Start_UT | T_mid − T0 (days) | Filter | Exposure (s) | Magnitude (AB) | | :--------: | :---------: | :---------------: | :----: | :----------: | :------------: | | 2026-03-30 | 14:03:01.11 | 6.30 | g | 2 * 600 | 17.40 +/- 0.03 | | 2026-03-30 | 14:25:47.00 | 6.31 | r | 2 * 600 | 17.62 +/- 0.04 | | 2026-03-31 | 13:25:47.93 | 7.27 | g | 2 * 600 | 17.31 +/- 0.03 | | 2026-03-31 | 13:48:41.86 | 7.29 | r | 2 * 600 | 17.64 +/- 0.04 | | 2026-03-31 | 14:11:31.90 | 7.34 | i | 12 * 600 | 17.87 +/- 0.03 | | 2026-04-06 | 14:02:29.94 | 13.30 | i | 4 * 600 | 18.11 +/- 0.03 | | 2026-04-06 | 14:45:34.47 | 13.34 | g | 4 * 600 | 17.97 +/- 0.03 | | 2026-04-07 | 13:49:09.03 | 14.30 | g | 4 * 600 | 18.06 +/- 0.03 | | 2026-04-07 | 14:34:49.59 | 14.33 | r | 4 * 600 | 18.27 +/- 0.04 | | 2026-04-10 | 14:10:56.15 | 17.31 | g | 5 * 600 | 18.42 +/- 0.03 | | 2026-04-10 | 15:04:02.07 | 17.35 | r | 5 * 600 | 18.54 +/- 0.04 | | 2026-04-11 | 13:05:42.64 | 18.28 | r | 6 * 600 | 18.57 +/- 0.05 | Further analysis and follow-up observations are ongoing. The PKU 60cm Telescope is operated by the Department of Astronomy, Peking University. The PKU HiTF group is dedicated to rapid follow-up observations of high-energy transients. View this GCN Circular online at https://gcn.nasa.gov/circulars/44327. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...