TITLE: GCN CIRCULAR NUMBER: 41635 SUBJECT: EP250827b: Contemporaneous detection of EP X-ray flare EP250827b and ZTF transient ZTF25abmpngy/AT2025wkm DATE: 25/09/01 19:20:45 GMT FROM: Tomas Ahumada at Caltech <tfahumada94@gmail.com> G. Schroeder (Cornell), T. Ahumada (Caltech), A. Y. Q. Ho (Cornell), M. Kasliwal (Caltech), L.Yan (Caltech), G. Srinivasaragavan (UMD), X. J. Hall (CMU), M. Coughlin (UMN), D. Y. Li (NAO, CAS), J. Yang (ZZU), X. Tian (GXU), K.R., Ni, (CCNU), H. L., Peng (NJNU), H. Y, Liu, Y. J., Song, H. Sun, Y. Liu, C.C.Jin, W. Yuan (NAO, CAS) on behalf of the Zwicky Transient Facility and Einstein Probe (EP) Partnership We report the contemporaneous detection of the X-ray flare EP250827b associated with the optical transient ZTF25abmpngy/AT 2025wkm (RA = 02:26:20.64, Dec = +37:29:59.90). This event was highlighted in the cross-matching of ZTF alerts with EP alerts, an experiment first announced in Ahumada et al (GCN 39791). ZTF25abmpngy/AT 2025wkm was first detected on 2025-08-27T09:38:04 UT, 3.27 hrs after the X-ray flare, and with an r-band apparent magnitude of r = 20.33 +/- 0.11 mag. We note the transient sits on top of a potential host galaxy with no archival spectroscopic redshift. EP250827b was first detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission at RA.= 36.581, DEC.= 37.499, with an uncertainty of 2.4 arcmin in radius (90% C.L. statistical and systematic). The X-ray detection is 14.7 arcseconds away from the optical position. The X-ray transient started at 2025-08-27T06:22:27 (UT), and lasted for more than 1000 seconds as seen from the WXT light curve. The X-ray spectrum can be fitted by an absorbed power law model, with the absorption fixed at the Galactic value of 5.4x10^20 cm^-2 , and a photon index of 2.2 (-0.7, +0.8). The average unabsorbed 0.5-4keV flux is 2.6 (-0.9, +1.3) x 10^-11 erg/s/cm2. In the follow-up WXT coverage approximately 5 hours later at the position, the source was no longer detected, indicating a fast decay in the soft X-ray band. We performed a follow-up target of opportunity observation with EP-FXT. The observation began at 2025-09-01T02:35:49 with an exposure time of 2995 seconds, about 5 days after the burst detected by EP-WXT. Within the WXT error circle, no X-ray source was detected at the optical position, setting a flux upper limit at 90% confidence level around 5 x 10^-14 erg/s/cm2. Based on the current temporal and spatial consistency, EP250827b is likely associated with AT2025wkn. However, due to the significant systematic uncertainty in the WXT position and the non-detection in the follow-up FXT observation, this association cannot be fully confirmed. We are pursuing and encouraging follow-up observations to classify the source and validate the X-ray and optical association. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). Based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Award #2407588 and a partnership including Caltech, USA; Caltech/IPAC, USA; University of Maryland, USA; University of California, Berkeley, USA; University of Wisconsin at Milwaukee, USA; Cornell University, USA; Drexel University, USA; University of North Carolina at Chapel Hill, USA; Institute of Science and Technology, Austria; National Central University, Taiwan; Operations are conducted by Caltech's Optical Observatory (COO) and Caltech/IPAC. View this GCN Circular online at https://gcn.nasa.gov/circulars/41635. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...