EP261015b
GCN Circular 45970
Subject
EP261015b: J-band upper limit by the SYSU 80cm infrared telescope
Date
2026-10-17T09:09:41Z (6 minutes ago)
From
Jianhong Cao at Sun Yat-sen University <caojh35@mail2.sysu.edu.cn>
Via
Web form
Jian-Hong Cao, Jia-Qi Lin, Zhong-Nan Dong, Jin-Ji Li, Pu Lin, Yun Shi, Hao-Nan Yang, Rui-Chen Gao, Jing-Kai Sun, Hao-Ran Zhang, P. H. Thomas Tam, Rong-Feng Shen, and Bin Ma (Sun Yat-sen University) report on behalf of the SYSU 80cm infrared telescope team:
We observed the field of EP261015b, detected by the Wide-field X-ray Telescope (WXT) on board Einstein Probe (Li et al., GCN 45958), using the Sun Yat-sen University (SYSU) 80cm infrared telescope. Our J-band observations were carried out on 2026 Oct 16 from 16:31 to 17:32 UTC, approximately 26.57–27.59 hours after the start of the WXT observation. We combined 150 exposures of 20 s each, yielding a total exposure time of 3000 s.
We do not detect any new source in the WXT localization in our stacked image, and obtain the following 5-sigma upper limit: J = 19.5 mag (Vega).
The SYSU 80cm infrared telescope is operated and managed by the Department of Astronomy, Sun Yat-sen University.
GCN Circular 45958
Subject
EP261015b: Einstein Probe detection of an X-ray transient
Date
2026-10-16T06:44:01Z (a day ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Z. -X. Li, G. -L. Huang (IHEP), R. -Z. Li (YNAO), M. Nie (YNAO) and H. Sun (NAOC) on behalf of the Einstein Probe (EP) team:
We report on the detection of an X-ray transient, designated EP261015b, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient was detected by EP-WXT during an observation starting from 2026-10-15T13:56:38 (UTC), and identified by ground-based telemetry analysis. The WXT position of the source is R.A. = 0.581 deg, DEC = 30.973 deg (J2000) with an uncertainty of 2.83 arcmin in radius (90% C.L. statistical and systematic). 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 Galactic hydrogen column density of 4.83e20 cm^-2 and a photon index of 2.11 (-0.81/+0.96). The derived average unabsorbed 0.5-4 keV flux is 3.50 (-1.27/+2.03) e-11 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).