GCN Circular 45872
Subject
EP261007b: Refined analysis of the EP-WXT and autonomous EP-FXT observations
Event
Date
2026-10-09T11:18:00Z (a day ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
W.F. Wen (SZTU), Q.H. Lao (NAO, CAS), L.N. Xu (USTC), H.Y. Liu (NAO, CAS), X.H. Tang (THU), and C.C. Jin (NAO, CAS) report on behalf of the Einstein Probe (EP) team:
The X-ray transient EP261007b triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission at 2026-10-07T15:05:16
(Wen et al., GCN 45847). Subsequent follow-up observations were reported
by Ma et al. (GCN 45849), Zhang et al. (GCN 45860), Mohan et al. (GCN 45861), Shi et al. (GCN 45862), Quirola-Vasquez et al. (GCN 45864), and Li et al. (GCN 45870). Here we present the analysis of the EP-WXT and autonomous EP-FXT observations of EP261007b.
The time-averaged WXT spectrum in the 0.5–4 keV band was fitted with an absorbed power-law model. The hydrogen column density was fixed at the Galactic value of NH = 3.59 × 10^20 cm^-2 at the source position. The best-fit photon index is 1.9 (+0.6/-0.4). The average unabsorbed 0.5–4 keV flux is 7.9 (+2.8/-2.3) × 10^-11 erg s^-1 cm^-2.
The Follow-up X-ray Telescope (FXT) on board EP began its autonomous observation at 2026-10-07T15:10:24 UTC, 308 s after the WXT trigger. FXT-A detected two X-ray sources within the WXT error region reported in GCN 45847:
EPF_J234524.0+112110: R.A. = 356.3500 deg, Dec. = 11.3526 deg (J2000).
EPF_J234521.3+112424: R.A. = 356.3388 deg, Dec. = 11.4067 deg (J2000).
Each position has a 90% confidence error radius of approximately 10 arcsec, including statistical and systematic uncertainties.
For EPF_J234524.0+112110, we jointly fitted the FXT-A and FXT-B spectra in the 0.5–10 keV band with an absorbed power-law model. The hydrogen column density was fixed at the Galactic value. The best-fit photon index is 2.0 ± 0.4. The average unabsorbed 0.5–10 keV flux is 3.5 (+1.3/-0.8) × 10^-13 erg s^-1 cm^-2.
For EPF_J234521.3+112424, we fitted the FXT-B spectrum in the 0.5–10 keV band with an absorbed power-law model, fixing NH at the Galactic value. The best-fit photon index is 1.8 (+1.3/-1.0). The average unabsorbed 0.5–10 keV flux is 6.3 (+12.4/-3.8) × 10^-14 erg s^-1 cm^-2.
We note that a variable source was detected outside the FXT error circle by NOT (Quirola Vasquez et al., GCN 45864) and SVOM/VT (Li et al., GCN 45870). We have examined the FXT images and found no X ray source present at this position. More follow-up observations are encouraged.
All quoted uncertainties on the spectral parameters and fluxes are at the 90% confidence level.
Launched on January 9, 2024, EP is a space-based X-ray observatory designed to monitor the soft X-ray sky and perform X-ray follow-up observations (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).