X-ray detection of the black hole transient IGR J17091-3624 during its recent optical brightening with Einstein Probe

ATel #17919; L. Zhang, Z. Li (IHEP, CAS), A. Marino, F. Coti Zelati, G. Illiano, N. Rea, M. Imbrogno, Y. L. Wang (ICE-CSIC), D. M. Russell (NYU Abu Dhabi), L. Tao, H. Feng (IHEP, CAS), W. Yuan, H. Sun, C. Jin (NAO, CAS), J. U. Ness (ESA)
on 22 Jul 2026; 01:24 UT
Credential Certification: Liang Zhang (liang.zhang@mail.bnu.edu.cn)

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Following the XB-NEWS report of optical brightening in the black hole transient IGR J17091-3624 (D. Russell et al., 2026, Atel#17907), we have performed two ~3-ks observations with the Follow-up X-ray Telescope (FXT) on board Einstein Probe (Yuan et al., 2025) on 2026-07-18T10:21:20 UTC and 2026-07-21T00:44:51 UTC. A source consistent with the nominal position of IGR J17091-3624 is clearly detected in the FoV at high (>24 SNR) significance.

In the first observation, the source is characterized by a relatively hard X-ray spectrum, with a photon index of Gamma=1.2+/-0.4 and an absorption column density of NH=(4+/-1)*10^22 cm^-2, obtained from a fit with the XSPEC model tbabs*powerlaw (uncertainties reported at 1-sigma confidence). The estimated (unabsorbed) flux in the 0.3-10 keV range is about 7e-12 erg/cm^2/s. The spectrum from the second observation shows a similar spectral shape, with a slight increase in source flux to 1e-11 erg/cm^2/s. This corresponds to an X-ray luminosity of about 1e35 erg/s at a distance of 10 kpc (e.g., Wang et al., 2022 and ref. therein). These results confirm the ongoing renewed, although faint, activity of the system and might imply that a new full outburst from the source is imminent. Multi-wavelength observations are encouraged.

Launched on January 9, 2024, EP is a space X-ray observatory designed to monitor the soft X-ray sky with rapid 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.