Einstein Probe detection of an X-ray transient EP250321b

ATel #17103; H. Q. Cheng (NAOC, CAS), D. F. Hu (PMO, CAS), Q. C. Zhao, Z. H. Yang (IHEP, CAS), L. Chen, W. D. Zhang (NAOC, CAS) on behalf of the Einstein Probe team
on 23 Mar 2025; 13:40 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, designated EP250321b, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient was first detected by WXT at 2025-03-21T00:03:32.900 (UTC). The average 0.5-4 keV spectrum can be fitted by an absorbed power law (with the column density fixed at the Galactic value of 1.42 x 10^21 cm^-2) with a photon index of 1.1(-0.9, +1.0). The derived unabsorbed flux in 0.5-4 keV is 7.6(-3.0, +5.3) x 10^(-11) erg/s/cm^2.

We conducted a follow-up observation with the X-Ray Telescope (XRT) onboard the Neils Gehrels Swift Observatory, which started at 2025-03-22T19:49:03 (UTC), about 43.8 hours after EP-WXT detection. The observation, with an exposure time of 2.9 ks, unveiled an uncatalogued X-ray source located at R.A. = 244.1632 deg, DEC = -30.7073 deg (J2000) with an uncertainty of 3.1 arcsec in radius (90% C.L. statistical and systematic). The Galactic coordinates are l = 346.7390 deg, b = 14.2142 deg. The average 0.3-10 keV spectrum can be fitted by an absorbed power law (with the column density fixed at the Galactic value of 1.42 x 10^21 cm^-2) with a photon index of 1.72(-0.34, +0.35), giving an average unabsorbed 0.3-10 keV flux of 1.5(-0.3, +0.4) x 10^(-12) erg/s/cm^2. The source thus exhibits a rapid diminishing in its X-ray brightness within two days. The uncertainties are at the 90% confidence level for all spectral parameters.

No known X-ray sources were found within the error circle of the XRT position. We also requested Swift and NICER monitoring observations to track the optical/UV and X-ray evolution of the source. More multi-wavelength follow-up observations are encouraged to explore this new source.

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.