TITLE:   GCN CIRCULAR
NUMBER:  42902
SUBJECT: EP251129a/GRB 251129A: EP-FXT follow-up observations
DATE:    25/11/30 06:51:00 GMT
FROM:    EP Team at NAOC/CAS <ep_ta@bao.ac.cn>

Y. H. Jiang, Y. Wu, C. Y. Dai (NJU), Y. Wang (PMO), H. C. Ding, T. Wu (AHNU), W. J. Zhang, Y. Liu (NAO, CAS), on behalf of the Einstein Probe (EP) team:

A follow-up observation of EP251129a / GRB 251129A (Wu et al., GCN 42884, Tan et al., GCN 42877) with the Follow-up X-ray Telescope (FXT) was performed automatically at 2025-11-29T03:16:08 (UTC),  i.e. T0+2643 s, where T0 is reported by SVOM (Tan et al., GCN 42877) , with an exposure time of 5,003 s. The average FXT 0.5-10 keV spectrum of the observation can be fitted with an absorbed power law, with a fixed Galactic hydrogen column density of 2.94 x 10^20 cm^-2, an intrinsic hydrogen column density of 1.40 (-0.05/+0.05) x 10^22 cm^-2, and a photon index of 1.98 (-0.02/+0.02) . The derived average unabsorbed 0.5-10 keV flux is 1.74 (-0.02/+0.02) x 10^(-9) erg/s/cm^2.

We performed two target-of-opportunity (ToO) follow-up observations with FXT at 2025-11-29T05:26:09 (UTC, T0+10,444 s) and 2025-11-29T12:34:30 (UTC, T0+36,145 s), with an exposure time of 2,786 s and 3,091 s, respectively.

The average FXT 0.5-10 keV spectrum of the first ToO observation can be fitted with an absorbed power law, with a fixed Galactic hydrogen column density of 2.94 x 10^20 cm^-2, an intrinsic hydrogen column density of 1.04 (-0.11/+0.11) x 10^22 cm^-2, and a photon index of 2.45 (-0.05/+0.05) . The derived average unabsorbed 0.5-10 keV flux is 1.56 (-0.04/+0.04) x 10^(-10) erg/s/cm^2.

The average FXT 0.5-10 keV spectrum of the second ToO observation can be fitted with an absorbed power law, with a fixed Galactic hydrogen column density of 2.94 x 10^20 cm^-2, an intrinsic hydrogen column density of 1.49 (-0.46/+0.46) x 10^22 cm^-2, and a photon index of 3.49 (-0.23/+0.23). The derived average unabsorbed 0.5-10 keV flux is 9.20 (-0.75/+0.82) x 10^(-13) erg/s/cm^2.

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).