{
  "circularId": 41070,
  "bibcode": "2025GCN.41070....1W",
  "eventId": "EP250711a",
  "createdOn": 1752306793654,
  "editedOn": 1752341512414,
  "body": "Y. Wang (PMO, CAS), D.Y. Li, W.X. Wang, W. Yuan (NAO, CAS) report on behalf of the Einstein Probe team:\n\nWe report on the detection of a fast X-ray transient by EP-WXT, designated EP250711a (trigger ID: 08500000365). The source was detected on ground from the telemetry data, causing a significant delay from the occuring time of the event. The onboard alert system was not triggered due to the low Galactic lattitude of the source, and no automated follow-up was performed with EP-FXT due to the observation was an non-interruptible one.The WXT position of the source is R.A. = 292.153 deg, DEC = 4.707 deg (J2000) with an uncertainty of 2.2 arcmin in radius (90% C.L. statistical and systematic). The observation begun at 2025-07-11T17:10:56 (UTC), and as suggested from the EP-WXT light curve, the transient was already present at the start time of the observation. The light curve exihbits a double-peak profile, and lasted for about 1300 seconds. The average 0.5-4 keV spectrum can be fitted with an absorbed powerlaw model, with the absorption fixed at the Galactic value of 3.78 x 10^22 cm^-2, and a photon index of 2.5 (+/-0.5). The derived average unabsorbed 0.5-4 keV flux is 2.1 (+/-0.4) x 10^-10 erg/s/cm2. The peak flux is about 8 x 10^(-10) erg/s/cm2.  The uncertainties are at the 90% confidence level for the above parameters.\n\nWe performed a follow-up target of opportunity observation with EP-FXT, starting at 2025-07-12T04:02:18Z, about 11 hours after the WXT detection. Within the WXT error circle, an uncataloggued X-ray source was detected at R.A. = 292.1614 , Dec.= 4.7035 with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic). Further analysis will be updated after receiving the telemetry data. An i band upper limit of 21.3 magnitude was given by a COLIBRÍ observation performed about 10.5 hours after the EP-WXT trigger (GCN 41069).\n\nLaunched 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).\n",
  "editedBy": "Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
  "subject": "EP250711a: EP-WXT detection and FXT follow-up observation of a fast X-ray transient",
  "version": 2,
  "submittedHow": "web",
  "submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
  "format": "text/plain"
}