TITLE: GCN CIRCULAR NUMBER: 39308 SUBJECT: EP250212a: EP-WXT detection and FXT follow-up observation of a fast X-ray transient DATE: 25/02/13 03:17:51 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn

D. Y. Li (NAOC, CAS), Y. J. Zhang(THU), Y. C. Fu (BNU), W. Yuan (NAOC, CAS) report on behalf of the Einstein Probe team:

We report on the detection of a fast X-ray transient by EP-WXT, designated EP250212a (trigger ID: 10202399251). The WXT position of the source is R.A. = 114.949 deg, DEC = 60.493 deg (J2000) with an uncertainty of 2.9 arcmin in radius (90% C.L. statistical and systematic). The transient started at 2025-02-12T07:44:07 (UTC) and lasted for about 360 seconds seen from WXT light curve. The average 0.5-4 keV spectrum can be fitted with an absorbed powerlaw model, with a NH of 0.5 (-0.3, +0.3) x 10^22 cm^-2 and a photon index of 2.2 (-0.9, +1.0). The derived average unabsorbed 0.5-4 keV flux is 0.9 (-0.3, +1.0) x10^-9 erg/s/cm2. The peak flux is about 6 x 10^-9 erg/s/cm2. The average spectrum also can be fitted with an absorbed blackbody model, with a NH upper limit of 0.45 x 10^22 cm^-2 and a temperature of 0.6 (-0.1, +0.2) keV.

We performed a follow-up target of opportunity observation with EP-FXT. The observation began at 2025-02-12T13:20:16 (UTC) with an exposure time of 5970 seconds, about 5.6 hours after the burst detected by EP-WXT. Within the WXT error cirlce, an uncatalogued X-ray source was detected at R.A. = 114.9813 deg, DEC = 60.4848 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). The X-ray spectrum can be fitted with an absorbed powerlaw with the absorption fixed at the Galactic value of 6.3 x 10^20 cm^-2, and a photon index of 2.1 (-0.3, +0.3). The derived unabsorbed 0.5-10 keV flux is 3.2 (-0.7, +0.8) x 10^-13 erg/s/cm2. The uncertainties are at the 90% confidence level for the above parameters. The FXT spectrum can not be well fitted with a black body model. A J-band upper limit of 20.9 mag was given by a NOT observation performed about 12.3 hour after the EP trigger (GCN 39306).

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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