TITLE: GCN CIRCULAR NUMBER: 38889 SUBJECT: EP250109a / GRB 250109A: refined EP-WXT analysis and EP-FXT follow-up observation DATE: 25/01/10 09:43:01 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn

R.-Z. Li (YNAO, CAS), X.-L. Chen (YNU), K. Chatterjee (YNU), Y.-L. Hua (PMO, CAS), H. Sun and Y. Liu (NAO, CAS) report on behalf of the Einstein Probe team:

Since 2025-01-09 06:17:58 (UTC, T0), EP250109a (Li et al., GCN 38864) has been detectable with high significance using the WXT onboard the EP, peaking about 60 seconds post-trigger. The observed peak flux in the 0.5–4 keV range is about 2.5 x 10^-9 erg/s/cm^2. The flux then rapidly decayed to background levels within 200 seconds post-trigger. The average 0.5-4 keV WXT spectrum can be fitted with an absorbed power law with a photon index of 2.9 +/- 1.0 (with a fixed Galactic equivalent hydrogen column density of 3.04 x 10^21 cm^-2). The derived lower limit of the intrinsic column density is 5.0 (-0.3, +0.4) x 10^21 cm^-2 when the redshift is fixed at 0. The derived average unabsorbed 0.5-4 keV flux is 2.5 (-1.2, +4.6) x 10^-9 erg/s/cm^2.

In addition to the follow-up observation, we conducted a follow-up observation of EP241217a using the FXT onboard the EP. The observation began at 2025-01-09T08:06:40 (UTC, 1.8 hours post-trigger), with an exposure time of approximately 4.2 ks. The average 0.5–4 keV FXT spectrum can be modeled with an absorbed power law, featuring a photon index of 1.9 (-0.7, +1.9) and a fixed Galactic equivalent hydrogen column density of 3.04 x 10^21 cm^-2). The derived lower limit of the intrinsic column density is about 3 x 10^20 cm^-2 when the redshift is fixed at 0. The estimated average unabsorbed 0.5–4 keV flux is about 4.9 (-1.2, +2.0) x 10^13 erg/s/cm^2.

The detection of EP-WXT/FXT (GCN 38864) and Fermi-GBM (GCN 38873 and GCN 38887), combined with the non-detection by Swift-XRT (GCN 38867), indicates that the flux of this transient has decreased very rapidly.

Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with onboard X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).

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