GCN Circular 45996
EP261007b: Further EP-FXT follow-up observations
From: EP Team at NAOC/CAS
Issued: 2026-10-20 14:17 UTC
Type: X-ray Transient

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:
We report on a further observation of the EP261007b field with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) mission. The observation began at 2026-10-18T18:39:14 UTC, with an exposure of approximately 2978s.
EP261007b was initially detected by EP-WXT (Wen et al., GCN 45847), with subsequent X-ray analysis reported by Wen et al. (GCN 45872). An optical transient within the WXT localization, but outside the original FXT error circle, was reported by Quirola-Vasquez et al. (GCN 45864) and Li et al. (GCN 45870). Our FXT observation on 2026-10-10 revealed a possible X-ray counterpart located 12.6 arcsec from this optical transient (Wen et al., GCN 45902). However, this X-ray counterpart was not detected in the latest FXT observation. We derived a 3 sigma upper limit on the 0.5–10 keV flux of 9.64E-14 erg cm^-2 s^-1.
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).

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