TITLE: GCN CIRCULAR NUMBER: 40089 SUBJECT: GRB 250404A / EP250404a: SVOM/GWAC-F60 and F50 observations DATE: 25/04/06 12:27:20 GMT FROM: Chen Liang-Jun at Guangxi University chenlj@st.gxu.edu.cn
L. J. Chen(GXU), L. P. Xin(NAOC), Y. G. Yang(HNU), X. H. Han, C. WU, Y. L. Qiu, J. Wang, H. L. Li, X .M. Lu, R. S. Zhang, Y. Xu, L. Huang, H. B. Cai, Y. J. Xiao, P. P. Zhang, L. Lan, W. J. Xie, Z. H. Yao, J. Y. Wei(NAOC), X. G. Wang, E. W. Liang(GXU) report on behalf of the SVOM follow-up team:
We observed the field of EP250404a detected by Einstein Probe (Hu et al., GCN 40051), which is temporally and spatially consistent with GRB 250404A (Fermi team, GCN 40050), with the GWAC-F50A and GWAC-F60 at Xinglong Observatory, China. F50 started observing at 2025-04-04T14:25:34 UTC and lasted until 2025-04-04T15:36:36 with 9 frames in R-band and 15 frames in I-band. F60A started observing at 2025-04-04T14:24:56 UTC and lasted until 2025-04-04T15:24:21 with 25 frames in R-band. F60B started observing at 2025-04-04T14:25:05 UTC and lasted until 2025-04-04T15:24:28 with 25 frames in R-band.
The optical counterpart of GRB 250404A / EP250404a (Jiang et al., GCN 40052; Perez-Garcia et al., GCN 40053; Du et al., GCN 40058; Zhu et al., GCN 40061; Konno et al., GCN 40063; Odeh et al., GCN 40064; He et al., GCN 40069; Pérez-Fournon et al., GCN 40071; Jelinek et al., GCN 40072; Rakotondrainibe et al., GCN 40073; Taguchi et al., GCN 40074; Brivio et al., GCN 40075; Ruocco et al., GCN 40076; Martin-Carrillo et al., GCN 40080; Pavoni et al., GCN 40083; Li et al., GCN 40084; Pankov et al., GCN 40086), was clearly detected in our images.
The afterglow shows a first brightening and then a single decay with a peak magnitude of R~14.3 mag, calibrated to the USNO B1.0 catalog, at about 1000 sec after the burst.
Two 60cm GWAC-F60(A/B) are operated by Guangxi University and NAOC, CAS, at Xinglong Observatory, China. The field of view is 19*19 arcmin. The 50cm telescope (F50A) is operated by Huaibei Normal University and NAOC, CAS, at Xinglong Observatory, China. The field of view is 27*27 arcmin.
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