TITLE: GCN CIRCULAR NUMBER: 40790 SUBJECT: EP-WXT trigger 01709179036: a new outburst of GRS 1739-278 DATE: 25/06/20 05:30:21 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn

H. Sun (NAO, CAS), J. Yang (NJU), H. Z. Wu (HUST), X. L. Chen (YNU), W. D. Zhang (NAO, CAS) on behalf of the Einstein Probe (EP) team:

We report on the detection of a new X-ray outburst from the black hole low-mass X-ray binary GRS 1739-278, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The source exhibited a rapid rise in its 0.5-4 keV flux since June 15th, 2025 and triggered EP-WXT (ID: 01709179036) at 2025-06-19T14:54:11 (UTC, ATel #17238).

A follow-up observation with the Follow-up X-ray Telescope (FXT) on board EP was performed automatically around 2 minutes after the trigger, with an exposure time of 1.2 ks. A bright X-ray source, whose position is consistent with GRS 1739-278, was detected by FXT, thus confirming the new outburst of GRS 1739-278. The FXT spectrum can be fitted by an absorbed power-law with a photon index of 2.1 (+/-0.1) and a neutral hydrogen column density of 2.3 (+/-0.1) x 10^22 cm^-2. The derived unabsorbed flux in 0.5-10 keV is 1.8 (+/-0.1) x 10^-9 erg/s/cm^2.

Multi-wavelength follow-up observations are strongly 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).

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