EP261015b
GCN Circular 45971
Subject
EP261015b: Refined analysis of EP-FXT observations
Date
2026-10-17T11:10:29Z (10 hours 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:
The X-ray transient EP261015b triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission at 2026-10-15T13:56:38 UTC (Li et al., GCN 45958). A subsequent observation was reported by Cao et al. (GCN 45970). Follow-up observation with Follow-up X-ray Telescope (FXT) on board EP began at 2026-10-16T20:20:28 UTC, approximately 30.40 hours after the WXT trigger, with an exposure time of 5129 s.
FXT-B detected two X-ray sources within the WXT error region reported in GCN 45958:
EPF_J000211.4+305730: R.A. = 0.5465 deg, Dec. = 30.9576 deg (J2000).
EPF_J000231.0+305716: R.A. = 0.6320 deg, Dec. = 30.9546 deg (J2000).
Each position has a 90% confidence error radius of approximately 10 arcsec, including statistical and systematic uncertainties.
For EPF_J000211.4+305730, 2.04 arcsec away from the position of a high proper motion star: UCAC4 605-000101, we fitted the spectra in the 0.5-10 keV band with an absorbed APEC model. The hydrogen column density was fixed at the Galactic value of 4.83 × 10^20 cm2. The best-fit plasma temperature is 1.44 (+0.77/-0.25) keV. The unabsorbed 0.5–10 keV flux is 1.19 (+0.57/-0.35) × 10^-13 erg s^-1 cm^-2.
For EPF_J000231.0+305716, 11.50 arcsec away from the position of a Quasar SDSS J000230.78+305717.6, we fitted the spectra in the 0.5-10 keV band with an absorbed power-law model, fixing NH at the Galactic value. The best-fit photon index is 2.84 (+1.96/-1.78). The average unabsorbed 0.5–10 keV flux has an upper limit of 8.43 × 10^-14 erg/s/cm2.
We tend to interpret EP261015b as a stellar flare from the high proper motion star UCAC4 605-000101. All quoted uncertainties on the spectral parameters and fluxes are at the 90% confidence level.
Launched on January 9, 2024, EP is a space-based X-ray observatory designed to monitor the soft X-ray sky and perform X-ray follow-up observations (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).
GCN Circular 45970
Subject
EP261015b: J-band upper limit by the SYSU 80cm infrared telescope
Date
2026-10-17T09:09:41Z (12 hours 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 (2 days 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).