GCN Circular 46076
Subject
EP261025a: EP-FXT follow-up observations
Event
Date
2026-10-29T03:14:14Z (4 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
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).