CaltechAUTHORS
Published December 20, 2025 | Version Published
Journal Article Open

The 2025 Failed Outburst of IGR J17091−3624: Spectral Evolution and the Role of Ionized Absorbers

Creators

  • 1. ROR icon California Institute of Technology
  • 2. ROR icon Goddard Space Flight Center
  • 3. ROR icon University of Milan
  • 4. ROR icon Istituto Universitario di Studi Superiori di Pavia
  • 5. ROR icon Villanova University
  • 6. ROR icon Harvard-Smithsonian Center for Astrophysics
  • 7. Independent Researcher
  • 8. ROR icon Howard University
  • 9. ROR icon Utah Valley University
  • 10. ROR icon University of Erlangen-Nuremberg
  • 11. ROR icon Newcastle University
  • 12. ROR icon Massachusetts Institute of Technology
  • 13. ROR icon Ehime University
  • 14. ROR icon Jet Propulsion Lab
  • 15. ROR icon University of California, Berkeley

Abstract

IGR J17091−3624 is the only black hole X-ray binary candidate—aside from the well-studied black hole system GRS 1915+105—observed to exhibit a wide range of structured variability patterns in its light curves. In 2025, the source underwent a “failed” outburst: it brightened in the hard state but did not transition to the soft state before returning to quiescence within a few weeks. During this period, IGR J17091−3624 was observed by multiple ground- and space-based facilities. Here, we present results from six pointed NuSTAR observations obtained during the outburst. None of the NuSTAR light curves showed the exotic variability classes typical of the soft state in this source; however, we detected, for the first time, strong dips in the count rate during one epoch, with a total duration of ∼4 ks as seen by NuSTAR. Through spectral and timing analysis of all six epochs, we investigate the hard-state spectral evolution and the nature of the dips. A clear evolution of the coronal properties with luminosity is observed over all six epochs, with clear signatures of relativistic disk reflection that remain largely unchanged across the first five epochs. The first five epochs also show a strong and stable quasiperiodic oscillation feature in the power spectra. The dips observed in Epoch 5 are consistent with partial obscuration by ionized material with a column density NH ≈ 2.0 × 1023 cm−2. We discuss possible origins for this material and place constraints on the orbital parameters and distance of the system.

Copyright and License

© 2025. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Acknowledgement

The authors thank the anonymous referee for comments that improved the clarity of the manuscript. This work was supported under NASA Contract No. NNG08FD60C and made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. G.M. acknowledges financial support from the European Union’s Horizon Europe research and innovation program under the Marie Skłodowska-Curie grant agreement No. 101107057. A.I. acknowledges support from the Royal Society. J.B.C. is supported under 80GSFC21M0006. T.D. acknowledges support from the DFG research unit FOR 5195 (grant No. WI 1860/20-1). The research of E.N. is supported by an appointment to the NASA Postdoctoral Program at the NASA Goddard Space Flight Center, administered by Oak Ridge Associated Universities under contract with NASA. M.P. acknowledges support from the JSPS Postdoctoral Fellowship for Research in Japan, grant No. P24712, as well as the JSPS Grants-in-Aid for Scientific Research—KAKENHI, grant No. J24KF0244.

Facilities

NuSTAR - The NuSTAR (Nuclear Spectroscopic Telescope Array) mission.

Software References

XSPEC (K. A. Arnaud 1996), XSTAR (T. Kallman & M. Bautista 2001), relxill (T. Dauser et al. 2014; J. García et al. 2014).

Files

Adegoke_2025_ApJ_995_143.pdf

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Additional details

Related works

Is new version of
Discussion Paper: arXiv:2510.14134 (arXiv)

Funding

National Aeronautics and Space Administration
NNG08FD60C
European Union
101107057
Royal Society
National Aeronautics and Space Administration
80GSFC21M0006
Deutsche Forschungsgemeinschaft
WI 1860/20-1
National Aeronautics and Space Administration
NASA Postdoctoral Program -
Japan Society for the Promotion of Science
P24712
Japan Society for the Promotion of Science
J24KF0244

Dates

Submitted
2025-08-13
Accepted
2025-10-14
Available
2025-12-11
Published

Caltech Custom Metadata

Caltech groups
Astronomy Department, NuSTAR, Space Radiation Laboratory, Division of Physics, Mathematics and Astronomy (PMA)
Publication Status
Published