GCN Circular 42597
Subject
EP251102a: Xinglong 2.16-m clear-band observations of the optical counterpart
Event
Date
2025-11-06T06:42:49Z (4 days ago)
From
Xinglong Observatory at National Astronomical Observatories (NAOC), Chinese Academy of Sciences (CAS) <xinglong@nao.cas.cn>
Via
Web form
Yu-Zhang(NAOC), Junjie-Jin(NAOC), Haiyang-Mu(NAOC), Junjun-Jia(NAOC), Jie-Zheng(NAOC), Yinan-Zhu(NAOC), Zhou-Fan(NAOC), Hong-Wu(NAOC) report on behalf of a large collaboration:
We observed the field of the X-ray transient EP251102a, detected by EP/WXT, using the Xinglong 2.16-m telescope at Xinglong, Hebei, China. We obtained three 300 s clear-band frames with a median time of 2025-11-03T20:56:36, i.e. 9.2 hr after the EP trigger. The stacked clear-band image has a 5σ limiting magnitude of ≈23.0 mag, calibrated against Pan-STARRS g-band field stars. We detect the optical counterpart in the stacked image at
RA (J2000) = 07:32:39.024 = 113.1626 deg
Dec (J2000) = +07:19:18.88 = +7.32191 deg.
This position is consistent with the candidate counterpart identified by Schneider et al. (GCN 42544). The preliminary magnitude of this source is 18.738 ± 0.025 (AB), which is brighter than reported in other follow-up observations (GCN 42544, 42546).
We note the presence of a bright source ≈3 arcsec from the candidate counterpart, whose measured magnitude is consistent with its Pan-STARRS g-band magnitude, indicating that our flux calibration and PSF point-source decomposition are reliable. Therefore, the brightness of EP251102a in the clear band is likely intrinsically higher, or may be attributed to an unusual spectral energy distribution (SED) of the source. Further observations are encouraged.