Einstein Probe detection of the brightening of the black hole X-ray binary GS 1354-64/BW Cir
ATel #17650; J. W. Hu, X. Mao (NAO, CAS), Y. L. Hua (PMO, CAS), H. Sun (NAO, CAS) on behalf of the Einstein Probe team
on 7 Feb 2026; 07:10 UT
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)
Subjects: X-ray, Binary, Black Hole
We report on the detection of the brightening of black hole X-ray binary GS 1354-64/BW Cir by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. Multiwavelength detections of GS 1354-64/BW Cir have been reported since December 25, 2025 (ATel #17563, #17582, #17583, #17586, #17611, #17612, #17618, #17625, #17638), indicating the renewed activity of the source and a transition from the hard to the soft state. The last WXT coverage of this source in August 2025 resulted in a non-detection. Due to the Sun-angle constraint, it has only become visible to WXT since February 4, 2026. The source was detected by WXT in a 360 s exposure observation at 2026-02-04T09:29:24 (UTC). The WXT spectrum can be fitted by an absorbed power law with a photon index of 2.2(+/-0.3) and an absorption column density of NH=9.6(+1.6, -1.5) e+21 cm^-2. The derived unabsorbed flux in 0.5-4 keV is 1.1(+0.3, -0.2) x 10^-8 ergs/s/cm^2, consistent with the Swift observation on 28 January 2026 (ATel #17638).
The source has now re-entered the EP-WXT visible window and will be continuously monitored in the survey mode in the coming period.
Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE and CNES.
