Kavli Affiliate: Erin Kara
| First 5 Authors: Oluwashina K. Adegoke, Oluwashina K. Adegoke, , ,
| Summary:
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 $sim4,mathrmks$ 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 $N_mathrmH approx 2.0 times
10^23,mathrmcm^-2$. We discuss possible origins for this material and
place constraints on the orbital parameters and distance of the system.
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