EP260917b
GCN Circular 45652
Subject
EP260917b: EP-FXT non-detection and upper limit
Date
2026-09-20T04:06:01Z (2 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
B. Zhang, J. P. Feng (USTC), G. Y. Zhao (QTNU), Z.-C. Zou (NJU), W. D. Zhang (NAO,CAS) on behalf of the Einstein Probe (EP) team:
EP-FXT performed a follow-up observation of EP260917b (Zhang et al., GCN 45629) starting at 2026-09-19T07:09:10 (UTC), approximately 40 hours after the WXT detection. With an exposure time of 2961s, no significant X-ray source was detected within the 2.6 arcmin radius of the EP-WXT position. The derived 0.5-10 keV upper limit is about 3.32e-14 erg cm^-2 s^-1.
Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).
GCN Circular 45629
Subject
EP260917b: EP-WXT detection of an X-ray transient
Date
2026-09-18T12:45:28Z (4 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
B.Zhang, J. P. Feng (USTC), G. Y. Zhao (QTNU), Z.-C. Zou (NJU), W. D. Zhang (NAO,CAS) on behalf of the Einstein Probe (EP) team:
We report on the detection of a fast X-ray transient, designated EP260917b, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient was independently detected by two CMOS detectors during an observation starting from 2026-09-17T15:28:53 (UTC), and identified by ground-based telemetry analysis. The WXT position of the source is R.A. = 281.628 deg, DEC = -17.562 deg (J2000) with an uncertainty of 2.636 arcmin in radius (90% C.L. statistical and systematic). No flaring activity or strong variability is seen in its lightcurve. No cataloged X-ray sources have been found within the WXT error circle.
The average WXT 0.5-4 keV spectrum can be fitted by an absorbed power-law model with a fixed Galactic hydrogen column density of 1.61×10^21 cm^-2 and a photon index of -0.27 (-0.63/+0.57). The derived average unabsorbed 0.5-4 keV flux is 1.31 (-0.37/+0.44) × 10^-10 erg cm^-2 s^-1. All uncertainties are reported in 90% C.L.
We have arranged a follow-up observation with the Follow-up X-ray Telescope onboard EP and will update the information upon receiving the data. Multi-wavelength observations are strongly encouraged.
Launched on 2024 January 9, Einstein Probe is a space-based X-ray observatory designed to monitor the dynamic X-ray sky and perform rapid follow-up observations of newly discovered transients (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).