GCN Circular 40069
Subject
EP250404a: Xinglong decaying optical counterpart
Date
2025-04-04T22:17:12Z (10 days ago)
From
Xinglong Observatory at National Astronomical Observatories (NAOC), Chinese Academy of Sciences (CAS) <xinglong@nao.cas.cn>
Via
Web form
Min-He (NAOC), Yiming-Mao (NAOC), Junjie-Jin (NAOC), Haiyang-Mu (NAOC), Jinlei-Zhang (NAOC), Junjun-Jia (NAOC), Pengliang-Du (NAOC), Zhou-Fan (NAOC), Hong-Wu (NAOC) report:
We observed the field of the X-ray transient, EP 250404a, detected by EP/WXT using the Xinglong 0.8-m, 2.16-m telescope located at Xinglong, Hebei, China. We obtained 6 x 300 s g'-band frames for 0.8-m telescope, 5 * 300s clear-band frames for 2.16-m telescope during the time between 2025-04-04T14:32:21.91 and 2025-04-04T17:08:42 as soon as the EP trigger. We have found the optical counterpart of EP 250404a in these images, and the light of this object had been decreased rapidly. The magnitude of this object firstly be ~ 15.68+/- 0.014 at 2025-04-04T14:32:21.91 with 0.8-m telescope in g'-band, and then decrease to ~ 19.7+/-0.102 with 2.16-m telescope at a median time of 2025-04-04T17:03:42, which measured by a stacked clear-band image and calibrated with the Pan-STARRS r' band field. The stacked clear-band image using the 2.16-m telescope has a 5-sigma limiting magnitude of ~21.64 mag, calibrated with the Pan-STARRS r'-band field.
Within the 20 arcsec error circle of the FXT, we identified an optical transient (OT) localized at R. A. = 08:20:11.41 , Dec. = 35:31:27.68 (J2000).
We summarize our observation results as follows:
Obs. No. | Start Time (UTC) | End Time (UTC) | Exposure Time (s) | Filter | Apparent mag (AB) |Telescope Name
1 | 2025-04-04 14:32:21.91 | 2025-04-04 14:37:21.91 | 300s | g' | 15.681 +/- 0.014 | Xinglong 0.8-m Telescope.
2 | 2025-04-04 14:37:27.81 | 2025-04-04 14:42:27.81 | 300s | g' | 15.717 +/- 0.013 | Xinglong 0.8-m Telescope.
3 | 2025-04-04 14:42:33.76 | 2025-04-04 14:47:33.76 | 300s | g' | 16.124 +/- 0.019 | Xinglong 0.8-m Telescope.
4 | 2025-04-04 15:11:06.35 | 2025-04-04 15:16:06.35 | 300s | g' | 17.864 +/- 0.070 | Xinglong 0.8-m Telescope.
5 | 2025-04-04 15:16:12.26 | 2025-04-04 15:21:12.26 | 300s | g' | 18.149 +/- 0.088 | Xinglong 0.8-m Telescope.
6 | 2025-04-04 15:21:18.16 | 2025-04-04 15:26:18.16 | 300s | g' | 19.015 +/- 0.112 | Xinglong 0.8-m Telescope.
7 | 2025-04-04 14:37:24 | 2025-04-04 14:42:24 | 300s |clear| 14.882 +/- 0.001 (calibrated to r') | Xinglong 2.16-m Telescope.
8 | 2025-04-04 14:42:32 | 2025-04-04 14:47:32 | 300s |clear| 15.363 +/- 0.002 (calibrated to r')| Xinglong 2.16-m Telescope.
9 | 2025-04-04 16:58:22 | 2025-04-04 17:13:42 | 3x300s |clear| 19.703 +/- 0.102 (calibrated to r')| Xinglong 2.16-m Telescope
Further observations are encouraged.