GCN Circular 46167
Subject
EP261103a: refined WXT analysis and FXT follow-up observations
Event
Date
2026-11-06T06:53:31Z (14 hours ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
H. Y. Ren (NAO, CAS), B. T. Wang, M. Nie (YNAO, CAS),L.-N. Xu (USTC),X. Tian, S. X. Ren(GXU) and H. W. Pan (NAO, CAS) on behalf of the Einstein Probe (EP) team:
EP261103a triggered EP-WXT at 2026-11-03 20:18:26 UTC (Ren et al., GCN 46142). Our temporal analysis yields a T100 interval beginning at 2026-11-03 20:17:40 UTC, 46 s before the trigger, and lasting 89.55 s. The measured T90 is approximately 52 s. An absorbed power-law fit to the time-averaged 0.5–4 keV spectrum, with the Galactic column density fixed at 3.86 × 10^20 cm^-2, gave a photon index of 2.23 (−0.86/+1.00) and an additional column density of 4.47 (−2.79/+3.24) × 10^21 cm^-2. The mean unabsorbed 0.5–4 keV flux was 3.33 (−1.08/+2.66) × 10^-9 erg cm^-2 s^-1.
The autonomous FXT observation began at 2026-11-03 20:20:45 UTC, approximately 140 s after the WXT trigger. A joint absorbed power-law fit to the FXT-A and FXT-B spectra gave a photon index of 1.95 (−0.40/+0.76) and a mean unabsorbed 0.5–10 keV flux of 4.15 (−1.22/+1.87) × 10^-13 erg cm^-2 s^-1.
EP-FXT performed a further follow-up observation of EP261103a. The observation started at 2026-11-04 19:04:44 UTC, about 23.3 h after the WXT detection, with a total exposure time of around 1.7 ks. On-ground analysis of the FXT data found a weak X-ray source only in FXT-B, with an S/N of approximately 2.4, about 9 arcsec from the optical counterpart candidate reported by NOT (Jiang et al., GCN 46143). An absorbed power-law fit to the 0.5–10 keV FXT-B spectrum gave a best-fit photon index of 5.34 and an unabsorbed flux of approximately 8.7 × 10^-14 erg cm^-2 s^-1. With only 9 source-region counts, the photon index and additional absorption are poorly constrained.
The optical counterpart candidate was initially reported at r = 23.86 ± 0.25 mag with NOT/ALFOSC (Jiang et al., GCN 46143). SVOM/VT detected the candidate at VT_R = 23.4 ± 0.3 AB mag (Ma et al., GCN 46145), and NTT/EFOSC2 measured Gunn r = 24.10 ± 0.13 AB mag (Corcoran et al., GCN 46148).
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).