{
  "version": 2,
  "circularId": 36209,
  "editedOn": 1713710817281,
  "eventId": "EP240420a",
  "editedBy": "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>",
  "submitter": "Dr Jie Zheng at Xinglong Observatory, National Astronomical Observatories ( NAOC), CAS <jiezheng@nao.cas.cn>",
  "body": "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:\n\nWe 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.\n\nWithin the 10 arcsec error circle of the EP/FXT, we identified an optical transient (OT) localized at\nR. A. = 15:14:55.072 , Dec. = 14:48:18.096 (J2000).\nThis OT has m = 20.657 +/- 0.4 (AB) in the Xinglong 0.8-m telescope image calibrated with the Pan-STARRS g band field.\n\nWe summarize our observation results as follows:\n\nObs. No. |     Start Time (UTC) | End Time (UTC) | Exposure Time (s) | Apparent mag (AB) | Telescope Name\n1 | 2024-04-20 13:00:00 | 2024-04-20 15:00:00 |   | > 19.5 | Xinglong Schmidt 60/90-cm Telescope\n2 | 2024-04-20 14:38:24 | 2024-04-20 14:58:20 | 3x600 s | 20.657 +/- 0.4 | Xinglong 0.8-m Telescope\n3 | 2024-04-20 20:17:20 | 2024-04-20 20:23:41 | 3x180 s | > 21.7 | Xinglong 2.16-m Telescope\n\nFurther observations are encouraged.",
  "bibcode": "2024GCN.36209....1W",
  "submittedHow": "web",
  "subject": "EP240420a: Xinglong decaying optical counterpart",
  "createdOn": 1713694764408
}