EP261111a
GCN Circular 46227
Junjie-Jin (NAOC), Qi-Zhao (NAOC), Dong-Xu (NAOC), Yue-Sun (NAOC), Haiyang-Mu (NAOC), Yu-Zhang (NAOC), Pengliang-Du (NAOC), Zhou-Fan (NAOC), Hong-Wu (NAOC) report on behalf of a large collaboration:
The Wide-field X-ray Telescope on Einstein Probe detected the X-ray transient EP261111a (Huang et al., GCN 46226).
We performed optical observations of the field of EP261111a at Xinglong Observatory. Our measurements are summarized below.
Inspection of the stacked science images and the archival-image comparison revealed no candidate optical counterpart within the EP/WXT localization region (3 arcmin radius). The representative 5-sigma point-source limiting magnitudes are listed below.
Time offsets are measured from the EP/WXT trigger at 2026-11-11T13:25:10Z (GCN 46226).
Mid-time (UTC) | Hours after trigger | Exposure (s) | Filter | 5-sigma limiting magnitude | System | Telescope 2026-11-11T13:48:38.264 | 0.391 | 6 x 300 | g | > 20.955 | PS1-AB-calibrated | Tsinghua-NAOC 0.8-m telescope (TNT)
The tabulated times are exposure-weighted mid-times in UTC. The representative depths were estimated from the background noise in the observed stacked images.
Photometry was calibrated using field stars from the BEST catalog (Xiao et al. 2026) on the PS1 photometric system.
References: GCN 46226 · EP261111a: Einstein Probe detection of an X-ray transient — https://gcn.nasa.gov/circulars/46226
GCN Circular 46226
G. L. Huang, Z. X. Li (IHEP, CAS), Y. J. Zhang (THU), W. D. Zhang (NAO, CAS) on behalf of the Einstein Probe (EP) team:
We report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP261111a. The transient triggered EP-WXT (ID: 01709322401) at 2026-11-11T13:25:10 (UTC). The WXT position of the source is R.A. = 43.118 deg, DEC = 44.444 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic).
A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 43.1202 deg, DEC = 44.4365 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic).
Further information will be updated when the telemetry data is received.
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).