TITLE: GCN CIRCULAR NUMBER: 41070 SUBJECT: EP250711a: EP-WXT detection and FXT follow-up observation of a fast X-ray transient DATE: 25/07/12 07:53:13 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn
Y. Wang (PMO, CAS), D.Y. Li, W.X. Wang, W. Yuan (NAO, CAS) report on behalf of the Einstein Probe team: We 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.
We 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).
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).
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