GCN Circular 45655
Subject
EP260920a: refined analysis of the EP-WXT and autonomous EP-FXT observations
Event
Date
2026-09-20T08:34:10Z (2 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
K. R. Ni (CCNU), S. Q. Zheng, Jin-Rui Liu (BNU), Tian-Yu Liu, G. J. Yang, B. B. Zhang and Z.-X. Ling (NAO, CAS), on behalf of the Einstein Probe (EP) team:
The X-ray transient EP260920a triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission at 2026-09-20T00:06:04 (UTC) (Ni et al., GCN 45649). Subsequent follow-up observations were reported by Liu et al. (GCN 45650, GCN 45653) and Dornic et al. (GCN 45654). The refined analysis of the WXT data shows that the event started at T0 = 2026-09-20T00:00:54 (UTC), approximately 300 s before the WXT trigger. The WXT light curve shows no significant temporal variability during the observation.
The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power-law model, with the hydrogen column density fixed at NH = 1.0 x 10^21 cm^-2, as inferred from the FXT spectral analysis. The best-fit photon index is 2.61 (-0.64/+0.71), and the derived average unabsorbed 0.5-4 keV flux is 1.94 (-0.50/+0.68) x 10^-10 erg s^-1 cm^-2.
The Follow-up X-ray Telescope (FXT) on board EP observed the source autonomously following the WXT trigger. With the telemetry data received, the refined on-ground analysis gives an X-ray source position of R.A. = 26.5281 deg, Dec. = -13.9806 deg (J2000), with an uncertainty of about 10 arcsec in radius (90% C.L. statistical and systematic). The FXT light curve shows an overall declining trend during the observation.
We jointly fitted the FXT-A and FXT-B spectra in the 0.5-10 keV band with an absorbed power-law model. The best-fit hydrogen column density is NH = 1.00 (-0.18/+0.19) x 10^21 cm^-2, and the photon index is 2.00 (-0.06/+0.06). The derived average unabsorbed 0.5-10 keV flux is 5.40 (-0.14/+0.15) x 10^-11 erg s^-1 cm^-2.
An optical transient candidate was reported by Liu et al. (GCN 45650) at R.A. = 01:46:06.71, Dec. = -13:59:06.1 (J2000). This position is about 15 arcsec from the refined FXT position, outside the refined 10-arcsec 90% confidence error circle.
The uncertainties are at the 90% confidence level for the above spectral parameters.
Further multi-wavelength follow-up observations are encouraged to explore the nature of EP260920a.
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).