GCN Circular 45981
Subject
EP261015c: refined analysis of the EP-FXT observation favors a stellar flare interpretation
Event
Date
2026-10-18T08:34:18Z (21 hours ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
G. -L. Huang, Z.-X. Li (IHEP), R.-Z. Li (YNAO), M. Nie (YNAO) and H. Sun (NAOC) on behalf of the Einstein Probe (EP) team:
The fast X-ray transient EP261015b was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Huang et al., GCN 45959). The source exhibited a flare that began at approximately 2026-10-15T17:37:52 UTC and lasted for approximately 700 s (Huang et al., GCN 45959). A follow-up observation with the Follow-up X-ray Telescope (FXT) on board EP began at 2026-10-17T17:08:30 UTC, approximately 48 hours after the onset of the flare, with an exposure time of approximately 6000 s.
The FXT detected an uncatalogued X-ray source within the WXT error circle at R.A. = 21.9613 deg and Dec. = 66.6144 deg (l = 126.5406 deg, b = 3.9981 deg), with an uncertainty radius of 10 arcsec (90% confidence level, including both statistical and systematic uncertainties). We fitted the FXT spectrum with a simple absorbed power-law model and obtained an equivalent hydrogen column density of 0.4 (-/+0.2), a photon index of 3.9 (-0.7/+0.8), and an unabsorbed 0.5-10 keV flux of 1.1 (-0.4/+1.0) x 10^-12 erg cm^-2 s^-1.
We note that three Gaia sources are located within the FXT error circle: Gaia DR3 525425756171519616, Gaia DR3 525425756171519488, and Gaia DR3 525425756167531904, at distances of approximately 104, 105, and 2962 pc, respectively. We also fitted the FXT spectrum with an apec model and obtained kT = 1.5 (-0.2/+0.3) keV. If the event was due to a stellar flare, the flare luminosities estimated using the unabsorbed WXT flare flux would be approximately 1.9 (-0.8/+1.6) x 10^32, 2.0 (-0.8/+1.6) x 10^32, and 1.6 (-0.6/+1.3) x 10^35 erg s^-1, respectively.
Given the source’s low Galactic latitude, the relatively soft WXT and FXT spectra, and the approximately two-order-of-magnitude decline in flux between the WXT observation and the FXT observation two days later, we tend to interprete the event as a stellar flare.
Launched on January 9, 2024, EP is a space X-ray observatory designed to monitor the soft X-ray sky with rapid X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE, and CNES.