GCN Circular 45902
Subject
EP261007b: Second EP-FXT follow-up observation and a possible X-ray counterpart to the optical transient
Event
Date
2026-10-11T15:56:45Z (19 hours 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 was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission on 2026-10-07 (Wen et al., GCN 45847). A refined analysis of the EP-WXT and autonomous EP-FXT observations was subsequently reported by Wen et al. (GCN 45872). Optical and near-infrared 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).
The EP-FXT performed a second follow-up observation of EP261007b beginning at 2026-10-10T04:43:36 (UTC), with an exposure of about 3566 s. No significant X-ray source was detected at the position of EP261007b. We derived a 3σ upper limit on its 0.5–10 keV flux of 2.3 × 10^-13 erg cm^-2 s^-1. This result indicates that the X-ray emission had faded since the initial EP observation.
Quirola-Vasquez et al. (GCN 45864) reported an optical transient outside the original FXT error circle. The transient was subsequently confirmed in archival SVOM/VT observations by Li et al. (GCN 45870). In the second FXT observation, an X-ray source was detected at R.A. = 356.3435 deg, Decl. = 11.3440 deg (J2000), with an offset of 12.6 arcsec from the optical position. The 0.5–10 keV spectrum from FXT-B is reasonably well fitted by an absorbed power-law model with the hydrogen column density fixed at the Galactic value. The best-fit photon index is 3.1 (+2.5/-2.2), corresponding to an unabsorbed 0.5–10 keV flux of 6.27 × 10^-14 erg cm^-2 s^-1.
Given its close proximity, this X-ray source might be associated with the optical source reported in GCN 45864 and GCN 45870, although additional observations are needed to firmly establish the connection.
We encourage further multiwavelength observations of EP261007b and the possible X-ray counterpart to the optical transient.
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).