Einstein Probe detection of a new outburst from GRS 1739-278
ATel #17238; 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:
on 19 Jun 2025; 17:03 UT
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)
Subjects: X-ray, Binary, Black Hole
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).
A follow-up observation with the Follow-up X-ray Telescope (FXT) on board EP was performed automatically around 2 minutes after the trigger. 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. We have not received the telemetry data of FXT at the time of writing; only onboard alert data of FXT are available, from which we obtain the source count rate and estimate a source flux in 0.5-10 keV of around 2 x 10^-10 erg/s/cm^2 (by assuming an absorbed powerlaw spectrum with a neutral hydrogen column density of 6 x 10^20 cm^-2 and a photon index of 2).
We fit the spectrum of the latest WXT observation on June 19, and find the spectrum to be best fit by an absorbed power-law with a photon index of 2.0 (+0.9/-0.8) and a neutral hydrogen column density of 1.7 (+0.8/-0.6) x 10^22 cm^-2. The derived unabsorbed flux in 0.5-4 keV is 1.2 (+1.0/-0.6) 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).
