Einstein Probe detection of an X-ray Transient EP250630b

ATel #17306; R.-D. Liang, H.-W. Pan (NAOC), Y.-L. Hua (PMO), T. Xiao (GXU), X. Fan (WHU), S.-E. Xu (WHU), Y. Liu, W. Yuan (NAOC) report on behalf of the Einstein Probe team:
on 25 Jul 2025; 08:54 UT
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)

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We report on the detection of a long-term X-ray transient by EP-WXT through ground data analysis, designated EP250630b. The transient was firstly detected at 2025-06-30T09:23:01 (UTC) and reached a peak flux of 4.12 x 10^-11 erg/cm^2/s at 2025-07-14, then decreased to 2.64 x 10^-11 erg/cm^/s at 2025-07-23. The WXT position of the source is R.A. = 75.863 deg, DEC = -69.382 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic).

A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed at 2025-07-23T11:57:12. Within the WXT error circle, a bright, uncatalogued X-ray source was detected within the WXT uncertainty region. The FXT position is RA, Dec = 76.9061 deg, -69.4031 deg with an uncertainty of 10 arcsec (90% C.L. statistical and systematic). The averaged 0.5-10 keV FXT spectrum is well described by an absorbed blackbody model, with a best-fit temperature of approximately 4.4(+/-0.3) x 10^-2 keV, and the unabsorbed flux is 8.3(+/-1.7) x 10^-12 erg/cm^2/s.Its position is spatially coincident with the optical transient candidate AT 2025ggn, reported by ASAS-SN as a possible nova outburst at 2025-03-30 in the LMC. The corresponding X-ray luminosity is estimated to be 2.5 x 10^36 erg/s, assuming the distance to the LMC. Another possible counterpart, the Be star candidate 2MASS J05033739-6924111, located approximately 2 arcsec from AT 2025ggn, cannot be ruled out at this stage.

We plan to continue monitoring the source with FXT, and will provide further updates as the analysis progresses.

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.