TITLE:   GCN CIRCULAR
NUMBER:  44605
SUBJECT: EP260507a: Xinglong Schmidt optical counterpart detection
DATE:    26/05/14 05:07:46 GMT
FROM:    Wenxiong Li at NAOC <liwenxiong1992@gmail.com>

Zhen Wang (SDU), Wenxiong Li, Niu Li, Hu Zou, and Yunao Xiao (NAOC) report:
We observed the field of the fast X-ray transient EP260507a (Fu et al., GCN 44488) with the Schmidt telescope at Xinglong Observatory, China. Observations started at May 7 14:09:41 UTC, 0.36 hr after the EP trigger. We obtained a series of 120 s clear-band exposures. The images were processed and stacked with the ASTERIS framework (Guo et al. 2026). PSF photometry was performed using AutoPhOT (Brennan & Fraser 2022). The photometry was calibrated against stars from the DESI Legacy Imaging Surveys catalog in the r band.
We detected the optical counterpart reported by Corcoran et al. (GCN 44489), Sankar et al. (GCN 44490), Kumar et al. (GCN 44492), Bochenek et al. (GCN 44494), Eyles-Ferris et al. (GCN 44495), Jiang et al. (GCN 44496), Li et al. (GCN 44497), and Globus et al. (GCN 44498).
The measured magnitudes are:
T_median (MJD)    T-T0 (hr)    Exposure (s)     Mag (r)
61167.596         0.50         8 x 120      20.37 +/- 0.06
61167.608         0.81         8 x 120      20.55 +/- 0.06
61167.621         1.11         8 x 120      20.94 +/- 0.08
61167.634         1.42         8 x 120      21.08 +/- 0.09
61167.652         1.85         8 x 120      >22.3 (3σ)
61167.685         2.64         12 x 120     21.75 +/- 0.10

The 60/90-cm Schmidt telescope situated at the Xinglong station is supported by the CAS Special Funds for Observatory and Facility Development, alongside a generous contribution from the Zhengjia Enterprise Group.