EP-WXT detection of on-going X-ray brightening likely from 4U 1850-086

ATel #18006; M. J. Liu (NAO, CAS), B. T. Wang (YNAO, CAS), Y. L. Hua (PMO, CAS), and W. D. Zhang (NAO, CAS) on behalf of the Einstein Probe (EP) team
on 25 Aug 2026; 09:51 UT
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)

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We report on the detection of X-ray brighening, likely from 4U 1850-086, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The onset of the brightening was detected by EP-WXT since 2026-08-14. Before that, the 0.5-4 keV unabsorbed flux of the source as measured by WXT had been more or less stable, as a level of ~2x10^-10 erg/s/cm2. After the onset, the flux has been increasing, to 7.4(+/-0.4)x10^-10 erg/s/cm2 erg/s/cm2 on 2026-08-24 when the most recent WXT observation of the source was taken. The fluxes are derived from preliminary spectral analysis of the WXT data using an absorbed blackbody model. In both the stable and brightening phases, we obtain consistent values of the blackbody temperature (0.5+/-0.1 keV on Aug. 13th vs. 0.50+0.03/-0.02 keV on Aug. 24th) and the neutral hydrogen column density (2.0+2.8/-2.0x10^21 cm^-2 on Aug. 13th vs. 0.7+0.4/-0.3x10^21 cm^-2 on Aug. 24th). The errors are quoted at a 90% confidence level.

Since 4U 1850-086 is located in a globular cluster NGC 6712, and limited by the spatial resolution of WXT (~5 arcmin), we cannot exclude that the brightening is due to another X-ray source. We plan to schedue an EP-FXT follow up observation. Multi-wavelength follow-up observations are encouraged.

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.