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

TL;DR

This study analyzes the 2025 failed outburst of the black hole X-ray binary IGR J17091-3624 using six NuSTAR epochs to track hard-state spectral evolution and to investigate newly detected dips. Through broadband reflection modeling with a lamp-post corona and ionized-absorber analysis via XSTAR, the authors show a disk likely near the ISCO and a corona that evolves as luminosity decreases, while one epoch exhibits dips consistent with an ionized obscurer of $N_{ m H}\approx2\times10^{23}\ \mathrm{cm^{-2}}$ and $\,\log\xi\sim2.2$. Timing analysis reveals a stable QPO at $f_{ m QPO}\sim0.21\,\mathrm{Hz}$ across the first five epochs, with dips increasing low-frequency variability but not altering the QPO. The results place constraints on the system’s inclination ($i\sim40^\\circ$) and suggest possible misalignment between inner and outer disks, offering insights into the geometry of accretion and disk winds in BHXBs. Overall, the work links the peculiar variability of IGR J17091-3624 in the hard state to standard accretion physics and geometric obscuration, expanding our understanding of outburst behavior in black hole binaries.

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 $\sim4\,\mathrm{ks}$ 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 which remain largely unchanged across the first five epochs. The first five epochs also show a strong and stable quasi-periodic oscillation (QPO) feature in the power spectra. The dips observed in Epoch 5 are consistent with partial obscuration by ionized material with a column density $N_{\mathrm{H}} \approx 2.0 \times 10^{23}\,\mathrm{cm^{-2}}$. We discuss possible origins for this material and place constraints on the orbital parameters and distance of the system.

Figures (10)

• Figure 1: Exposure-corrected NuSTAR image of IGR J17091-3624 from Epoch 4 (The color map is logarithmically scaled). The source and background regions, each extracted from a 100" radius, are captured by the white and red circles, respectively. The color scale of the image is in counts per pixel. The horizontal and vertical axes are the J2000 coordinates of the system.
• Figure 2: NuSTAR FPMB light curves for all six epochs combined (topmost panel) and plotted separately (lower panels). All the light curves have been binned to $100\,\mathrm{s}$. For Epoch 5, the dashed horizontal line separates the persistent interval from the dip interval.
• Figure 3: Energy-resolved light curve for Epoch 5.
• Figure 4: Comptonized disk blackbody model fit to data from all six epochs using the model cons*TBabs*simplcut*diskbb. Relativistic reflection features are evident in all six epochs. For Epoch 5, only the persistent spectra are plotted.
• Figure 5: Residuals from joint fit to the data from Epochs 1-6 using both phenomenological and physical models. The data points shown are from FPMB. In all cases, only data from the persistent spectra are considered for Epoch 5.