TITLE: GCN CIRCULAR NUMBER: 40088 SUBJECT: EP250404a / GRB 250404A: J band observations by SYSU 80cm telescope DATE: 25/04/06 12:26:24 GMT FROM: Chun Chen at Sun Yat-sen University, SYSU 80cm telescope team chench386@mail2.sysu.edu.cn

Chun Chen, Xia Li, Zhong-Nan Dong, Jia-Qi Lin, Wei-Sen Huang, Jin-Ji Li, Pu Lin, Hao-Nan Yang, Yan Yu, Hao-Ran Zhang, Hao-Yuan Zheng, P H Thomas Tam, Rong-Feng Shen, Bin Ma (Sun Yat-sen University) report on behalf of the SYSU 80cm telescope team:

We observed the field of EP250404a detected by Einstein Probe (Hu et al., GCN 40051), which is temporally and spatially consistent with GRB250404A (Fermi GBM team, GCN40050; Fermi GBM team, GCN40067), using the Sun Yat-sen University 80cm infrared telescope with 183 x 20 s exposures in J band. The calculated position is R.A. = 125.0575 deg, DEC = 35.5305 deg J2000, from EP/FXT observation. Our observations began at 2025-4-4 15:25:20 UTC, 1.09 hours after the EP trigger.

We detected a counterpart at the position of the optical afterglow (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; Lipunov et al., GCN 40066; He et al., GCN 40069; Perez-Fournon et al., GCN 40071; Jelinek et al., GCN 40072; Rakotondrainibe et al., GCN 40073; Taguchi et al., GCN 40074; Ruocco, GCN 40076; Martin-Carrillo et al., GCN 40080; Pavoni et al., GCN 40083; Li et al., GCN 40084) in the stacked images. The preliminary photometric magnitudes in J band are below:

Start Time (UTC) |Band | Exp | Mag (Vega) --------------------|-----|-------|------------------ 2025/4/4 15:25:20 | J | 200s | 15.30+/-0.08 2025/4/4 15:28:40 | J | 200s | 15.28+/-0.09 2025/4/4 15:32:20 | J | 200s | 15.47+/-0.10 2025/4/4 15:35:40 | J | 200s | 15.76+/-0.11 2025/4/4 15:39:00 | J | 360s | 15.83+/-0.10 2025/4/4 15:50:20 | J | 400s | 16.05+/-0.12 2025/4/4 15:57:00 | J | 400s | 15.99+/-0.12 2025/4/4 16:03:40 | J | 400s | 15.99+/-0.12 2025/4/4 16:10:40 | J | 400s | 16.29+/-0.15 2025/4/4 16:17:40 | J | 1100s | 16.21+/-0.11

The photometric calibration was performed using nearby stars from the 2MASS catalog and the magnitudes are not corrected for Galactic extinction.

View this GCN Circular online at https://gcn.nasa.gov/circulars/40088. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...