{
  "format": "text/plain",
  "circularId": 45971,
  "submittedHow": "web",
  "subject": "EP261015b: Refined analysis of EP-FXT observations",
  "submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
  "body": "Z. -X. Li, G. -L. Huang (IHEP), R. -Z. Li (YNAO), M. Nie (YNAO) and H. Sun (NAOC) on behalf of the Einstein Probe (EP) team:\nThe X-ray transient EP261015b triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission at 2026-10-15T13:56:38 UTC (Li et al., GCN 45958). A subsequent observation was reported by Cao et al. (GCN 45970). Follow-up observation with Follow-up X-ray Telescope (FXT) on board EP began at 2026-10-16T20:20:28 UTC, approximately 30.40 hours after the WXT trigger, with an exposure time of 5129 s.\nFXT-B detected two X-ray sources within the WXT error region reported in GCN  45958:\nEPF_J000211.4+305730: R.A. = 0.5465 deg, Dec. = 30.9576 deg (J2000).\nEPF_J000231.0+305716: R.A. = 0.6320 deg, Dec. = 30.9546 deg (J2000).\nEach position has a 90% confidence error radius of approximately 10 arcsec, including statistical and systematic uncertainties.\nFor EPF_J000211.4+305730, 2.04 arcsec away from the position of a high proper motion star: UCAC4 605-000101, we fitted the spectra in the 0.5-10 keV band with an absorbed APEC model. The hydrogen column density was fixed at the Galactic value of 4.83 × 10^20 cm2. The best-fit plasma temperature is 1.44 (+0.77/-0.25) keV. The unabsorbed 0.5–10 keV flux is 1.19 (+0.57/-0.35) × 10^-13 erg s^-1 cm^-2.\nFor EPF_J000231.0+305716, 11.50 arcsec away from the position of a Quasar SDSS J000230.78+305717.6, we fitted the spectra 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 2.84 (+1.96/-1.78). The average unabsorbed 0.5–10 keV flux has an upper limit of 8.43 × 10^-14 erg/s/cm2.\nWe tend to interpret EP261015b as a stellar flare from the high proper motion star UCAC4 605-000101. All 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).",
  "eventId": "EP261015b",
  "createdOn": 1792235429385,
  "eventType": [
    "X-ray Transient"
  ]
}