EP/FXT follow-up observation confirms the new outburst of GX 339-4

ATel #17330; Qing-Chang Zhao (IHEP), Zihan Yang (IHEP), Liang Zhang (IHEP), Lian Tao (IHEP), Hua Feng (IHEP), Shuang-Nan Zhang (IHEP)
on 13 Aug 2025; 08:41 UT
Credential Certification: Liang Zhang (liang.zhang@mail.bnu.edu.cn)

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Following the XB-NEWS report on the detection of the optical rise of GX 339-4 (ATel #17327), we performed a follow-up observation with the EP/FXT instrument. This observation started at 2025-08-12T08:51:54.9 (UTC) and lasted approximately 5600 seconds.

GX 339-4 was significantly detected during this observation. The X-ray spectrum can be well described by an absorbed power-law model with a photon index of 1.7(1). The absorption-corrected flux in the 0.5-10 keV band is 2.9(1)*10^-11 ergs cm^-2 s^-1. No significant quasi-periodic oscillations (QPOs) were detected in the timing analysis.

We encourage continued multi-wavelength follow-up observations.

We thank the Einstein Probe Science Operations team for promptly scheduling this observation. Launched on January 9, 2024, the Einstein Probe (EP) is a space-based X-ray observatory designed to monitor the soft X-ray sky and perform rapid X-ray follow-up observations (Yuan et al. 2025, Science objectives of the Einstein Probe mission).

References: Yuan, W., Dai, L., Feng, H., et al. 2025, SCPMA, 68, 239501