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GCN Circular 36209

Subject
EP240420a: Xinglong decaying optical counterpart
Date
2024-04-21T10:19:24Z (3 months ago)
Edited On
2024-04-21T14:46:57Z (3 months ago)
From
Dr Jie Zheng at Xinglong Observatory, National Astronomical Observatories ( NAOC), CAS <jiezheng@nao.cas.cn>
Edited By
Leo P. Singer at NASA/GSFC <leo.p.singer@nasa.gov> on behalf of Dr Jie Zheng at Xinglong Observatory, National Astronomical Observatories ( NAOC), CAS <jiezheng@nao.cas.cn>
Via
Web form
B. C. Wang, J. J. Jin, Z. Fan, H. Wu, M. He, H. Y. Mu, L. Ge, J. B. Zhang, J. Zheng, Y. Zhang, X. Li, Y. G. Sun, J. J. Jia, W. X. Li, S. J. Xue, H. Zou, X. N. Guan, D. Xu report on behalf of a large collaboration:

We observed the field of the X-ray transient, EP 240420a, detected by EP/WXT using the Xinglong 0.8-m telescope located at Xinglong, Hebei, China. We obtained 3 x 600 s clear-band frames with a median time of 2024-04-20T14:48:24, i.e., 2.7 hr after the EP trigger, we luckily found the optical counterpart of EP 240420a in the stacked image. The stacked clear-band image using a 0.8-m telescope has a 3-sigma limiting magnitude of ~ 21 mag, calibrated with the Pan-STARRS g band field. Inspection of the whole EP/WXT 3 arcmin error circle produces a few low S/N optical counterpart candidates. We detected no optical transient candidate during April 20T13:00-15:00 with the 60/90 cm Schmidt Telescope in Xinglong, Hebei, China. The upper limit is ~19.5 m in the g-band. Then we performed clear-band photometry on the Xinglong 2.16-m telescope located at Xinglong, Hebei, China, and obtained 3x180s clear-band frames with a median time of 2024-04-20T20:20:30, i.e., 8.2 hr after the EP trigger. The stacked clear-band image using the 2.16-m telescope has a 5-sigma limiting magnitude of ~21.7 mag, calibrated with the Pan-STARRS g-band field. Inspection of the whole EP/WXT 3 arcmin error circle produces no optical counterpart candidates.

Within the 10 arcsec error circle of the EP/FXT, we identified an optical transient (OT) localized at
R. A. = 15:14:55.072 , Dec. = 14:48:18.096 (J2000).
This OT has m = 20.657 +/- 0.4 (AB) in the Xinglong 0.8-m telescope image calibrated with the Pan-STARRS g band field.

We summarize our observation results as follows:

Obs. No. |     Start Time (UTC) | End Time (UTC) | Exposure Time (s) | Apparent mag (AB) | Telescope Name
1 | 2024-04-20 13:00:00 | 2024-04-20 15:00:00 |   | > 19.5 | Xinglong Schmidt 60/90-cm Telescope
2 | 2024-04-20 14:38:24 | 2024-04-20 14:58:20 | 3x600 s | 20.657 +/- 0.4 | Xinglong 0.8-m Telescope
3 | 2024-04-20 20:17:20 | 2024-04-20 20:23:41 | 3x180 s | > 21.7 | Xinglong 2.16-m Telescope

Further observations are encouraged.
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