Einstein Probe detection of an X-ray Transient EP250623a (EP J171203.4-333221.5)

ATel #17247; K.-R. Ni (CCNU), J.-H. Wu (GZHU), W.-F. Wen (SZTU), W. Yuan (NAO, CAS)
on 26 Jun 2025; 16:18 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)

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We report on the detection of an X-ray transient by the Einstein Probe (EP), designated as EP250623a (EP J171203.4-333221.5). The source was first detected by the Wide-field X-ray Telescope (WXT) on board EP at 2025-06-23T18:47:06 (UTC), during an observation with an exposure time of 5.4 ks. Stacking of the WXT data shows that the source was already detected about 2 days before. Following the WXT detection, we performed a target-of-opportunity observation with the Follow-up X-ray Telescope (FXT) on board EP, starting at 2025-06-25T13:58:42 (UTC), with an exposure time of 3060 s. An uncatalogued X-ray source was detected at R.A. = 257.9974 deg, Dec. = -33.5482 deg (Galactic coordinates l = 352.1797 deg, b= 3.4316 deg), with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). The WXT spectrum, when fitted with an absorbed power-law model, has a photon index of 1.23 (-0.86, +1.11) and a NH of 4.2 (-3.3, +5.2) e21 cm^-2, and an unabsorbed X-ray flux of 0.7 (-0.2, +0.6) e-10 ergs/s/cm^2 in 0.5-4 keV.

During the FXT observation, a flare was observed in the light curve, lasting for about 200 seconds. A preliminary analysis shows that the FXT spectrum can be better fitted with an absorbed powerlaw plus a blackbody model (chi2/dof=281.47/244) than an absorbed powerlaw, with a photon index of 1.02+/-0.02, and a blackbody temperature of 0.16 (-0.15, +0.02) keV, and a NH of 2.57(+/-0.11) e21 cm^-2. The derived unabsorbed flux in 0.5-10 keV is 1.59(+/-0.03) e-10 ergs/s/cm^2.

Further monitoring observations on this source will be performed with EP-FXT.

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). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE and CNES.