Kavli Affiliate: Erin Kara
| First 5 Authors: Jingyi Wang, Erin Kara, Jeroen Homan, James F. Steiner, Diego Altamirano
| Summary:
IGR J17091-3624 is a black hole X-ray binary (BHXB), often referred to as the
‘twin’ of GRS 1915+105 because it is the only other known BHXB that can show
exotic ‘heartbeat’-like variability that is highly structured and repeated.
Here we report on observations of IGR J17091-3624 from its 2022 outburst, where
we detect an unusually coherent quasi-periodic oscillation (QPO) when the
broadband variability is low (total fractional rms $lesssim$ 6%) and the
spectrum is dominated by the accretion disk. Such spectral and variability
behavior is characteristic of the soft state of typical BHXBs (i.e., those that
do not show heartbeats), but we also find that this QPO is strongest when there
is some exotic heartbeat-like variability (so-called Class V variability). This
QPO is detected at frequencies between 5 and 8 Hz and has Q-factors (defined as
the QPO frequency divided by the width) $gtrsim$ 50, making it one of the most
highly coherent low-frequency QPO ever seen in a BHXB. The extremely high Q
factor makes this QPO distinct from typical low-frequency QPOs that are
conventionally classified into Type-A/B/C QPOs. Instead, we find evidence that
archival observations of GRS 1915+105 also showed a similarly high-coherence
QPO in the same frequency range, suggesting that this unusually coherent and
strong QPO may be unique to BHXBs that can exhibit ‘heartbeat’-like
variability.
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