{
  "submittedHow": "web",
  "eventType": [
    "X-ray Transient"
  ],
  "eventId": "EP261007b",
  "subject": "EP261007b: Refined analysis of the EP-WXT and autonomous EP-FXT observations",
  "submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
  "circularId": 45872,
  "createdOn": 1791544680565,
  "format": "text/plain",
  "body": "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:\n\nThe 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\n (Wen et al., GCN 45847). Subsequent follow-up observations were reported \nby 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.\nThe 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.\nThe 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:\nEPF_J234524.0+112110: R.A. = 356.3500 deg, Dec. = 11.3526 deg (J2000).\nEPF_J234521.3+112424: R.A. = 356.3388 deg, Dec. = 11.4067 deg (J2000).\nEach position has a 90% confidence error radius of approximately 10 arcsec, including statistical and systematic uncertainties.\nFor 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.\nFor 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.\nWe note that a variable source was detected outside the FXT error circle by NOT (Quirola\u001eVasquez et al., GCN 45864) and SVOM/VT (Li et al., GCN 45870). We have examined the FXT images and found no X\u001eray source present at this position. More follow-up observations are encouraged. \nAll quoted uncertainties on the spectral parameters and fluxes are at the 90% confidence level.\nLaunched 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)."
}