TITLE:   GCN CIRCULAR
NUMBER:  46076
SUBJECT: EP261025a: EP-FXT follow-up observations
DATE:    26/10/29 03:14:14 GMT
FROM:    EP Team at NAOC/CAS <ep_ta@bao.ac.cn>

Q. Y. Wu, G. J. Yang (NAO, CAS), S. Q. Zheng, J. R. Liu (BNU), and Y. Liu (NAO, CAS) report on behalf of the Einstein Probe (EP) team:

We report the analysis of two observations of EP261025a with the Follow-up X-ray Telescope (FXT) on board EP. The transient was discovered by EP-WXT, with the flare beginning at 2026-10-25T07:19:57.4 UTC (Yang et al., GCN 46025. Optical and near-infrared upper limits were subsequently reported by Mandarakas et al., GCN 46026, Mohan et al., GCN 46028, and Manyan et al., GCN 46055.

The first FXT observation began at 2026-10-25T13:37:13 UTC, approximately 6.29 hours after the WXT flare onset, with a net exposure of 2.94 ks. The X-ray counterpart was detected, and its refined position is R.A. = 2.5438 deg, Dec. = +60.7671 deg (J2000), with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). We jointly fitted the FXT-A and FXT-B spectra over 0.3-10 keV using an absorbed power-law model. With the hydrogen column density fixed to the Galactic value, N_H = 7.06 x 10^21 cm^-2, we obtained a photon index of 1.68 (+0.62/-0.60) and an unabsorbed 0.3-10 keV flux of 5.93 (+2.91/-1.53) x 10^-13 erg cm^-2 s^-1. All quoted uncertainties on the spectral parameters and fluxes are at the 90% confidence level.

The second FXT observation began at 2026-10-26T20:12:07 UTC, approximately 36.87 hours after the WXT flare onset, with a net exposure of 3.49 ks. No significant X-ray emission from the counterpart was detected. Combining the FXT-A and FXT-B data, we derived a 90% upper limit of 1.24 x 10^-13 erg cm^-2 s^-1 on the unabsorbed 0.3-10 keV flux, assuming the same absorption and photon index as in the first observation. 

We encourage further multiwavelength observations of EP261025a.

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).