TITLE: GCN CIRCULAR NUMBER: 40038 SUBJECT: EP250402a: EP-FXT follow-up observations DATE: 25/04/04 08:39:01 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn

R.-Z. Li (YNAO, CAS), H. Q. Cheng (NAO, CAS), Q. C. Zhao, Z. H. Yang (IHEP, CAS), Y. Liu (NAO, CAS) report on behalf of the Einstein Probe team:

The X-ray transient EP250402a was detected by EP-WXT (Li et al., GCN 40017). Two follow-up observations were conducted by EP-FXT.

The first follow-up observation was performed at 2025-04-03T07:25:06 (UTC), about 18.93 hours after the trigger, with an exposure time of 1 ks. On-ground analysis of the EP-FXT data identified an uncatalogued X-ray source at the coordinates (J2000): R.A., Dec. = 172.1798, -47.2954 deg, with an uncertainty of about 10 arcsec (radius, 90% confidence level, including both statistical and systematic errors). This source lies within the EP-WXT error circle (Li et al., GCN 40017). The 0.5-10 keV spectrum can be fitted with an absorbed power-law model, with the column density nH fixed at the Galactic value of 1.43e21 cm^-2, and a photon index of 2.9 (+1.2, -1.0). The unabsorbed 0.5-10 keV flux is 1.9 (+1.2, -0.7) e-13 erg/s/cm^2.

The second follow-up observation was performed at 2025-04-03T17:11:45 (UTC), about 28.71 hours after the trigger, with an exposure time of 4.4 ks. The 0.5-10 keV spectrum can also be fitted with an absorbed power-law model, with nH fixed at the Galactic value of 1.43e21 cm^-2, and a photon index fixed at 2.9. The unabsorbed 0.5-10 keV flux is 9.1 (+5.0, -3.5) e-14 erg/s/cm^2.

All uncertainties quoted above are at the 90% confidence level.

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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