TITLE:   GCN CIRCULAR
NUMBER:  46227
SUBJECT: EP261111a: Xinglong optical upper limit
DATE:    26/11/11 15:54:58 GMT
FROM:    Xinglong Observatory at National Astronomical Observatories (NAOC), Chinese Academy of Sciences (CAS) <xinglong@nao.cas.cn>

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