TITLE: GCN CIRCULAR NUMBER: 44171 SUBJECT: EP260321a :Xinglong optical observations DATE: 26/04/01 05:05:57 GMT FROM: Xinglong Observatory at National Astronomical Observatories (NAOC), Chinese Academy of Sciences (CAS) <xinglong@nao.cas.cn> Yu-Zhang(NAOC), Junjie-Jin(NAOC), Haiyang-Mu(NAOC), Yuguang-Sun(NAOC), Pengliang-Du (NAOC), Jie-Zheng(NAOC), Zhou-Fan(NAOC), Hong-Wu(NAOC) report on behalf of a large collaboration: We performed optical observations of the field of a fast X-Ray transient EP260321a(Q.J. Huang et al., GCN 44068) in the g, r and i filter using Tsinghua-NAOC 0.8-m telescope (TNT) located at Xinglong, Hebei, China. A series of 300-s exposures were obtained on three epochs (March 25, 26, and 27, 2026), with the first observation at 2026-03-25T14:37:57, approximately 4 days after the EP FXT trigger (2026-03-21T12:30:18). We measure a preliminary magnitude calibrated against nearby stars from the Pan-STARRS catalog. We summarize our observation results as follows: Obs. No. | Time (UTC) | Exposure Time (s) | Filter | Apparent mag (AB) | Telescope Name 1 | 2026-03-25T14:37:57 | 300 s | g |17.70 +/- 0.24| Tsinghua-NAOC 0.8-m telescope (TNT) 2 | 2026-03-25T14:53:18 | 300 s | r |17.69 +/- 0.20| Tsinghua-NAOC 0.8-m telescope (TNT) 3 | 2026-03-25T15:08:39 | 300 s | i |17.42 +/- 0.25| Tsinghua-NAOC 0.8-m telescope (TNT) 4 | 2026-03-26T16:00:16 | 300 s | g |17.17 +/- 0.24| Tsinghua-NAOC 0.8-m telescope (TNT) 5 | 2026-03-26T16:15:37 | 300 s | r |17.30 +/- 0.23| Tsinghua-NAOC 0.8-m telescope (TNT) 6 | 2026-03-26T16:40:01 | 300 s | i |17.38 +/- 0.30| Tsinghua-NAOC 0.8-m telescope (TNT) 7 | 2026-03-27T13:25:21 | 300 s | g |16.61 +/- 0.01| Tsinghua-NAOC 0.8-m telescope (TNT) 8 | 2026-03-27T13:40:42 | 300 s | r |16.53 +/- 0.01| Tsinghua-NAOC 0.8-m telescope (TNT) 9 | 2026-03-27T13:56:03 | 300 s | i |16.43 +/- 0.02| Tsinghua-NAOC 0.8-m telescope (TNT) View this GCN Circular online at https://gcn.nasa.gov/circulars/44171. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...