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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 showexotic 'heartbeat'-like variability that is highly structured and repeated.Here we report on observations of IGR J17091-3624 from its 2022 outburst, wherewe detect an unusually coherent quasi-periodic oscillation (QPO) when thebroadband variability is low (total fractional rms $\lesssim$ 6%) and thespectrum is dominated by the accretion disk. Such spectral and variabilitybehavior is characteristic of the soft state of typical BHXBs (i.e., those thatdo not show heartbeats), but we also find that this QPO is strongest when thereis some exotic heartbeat-like variability (so-called Class V variability). ThisQPO is detected at frequencies between 5 and 8 Hz and has Q-factors (defined asthe QPO frequency divided by the width) $\gtrsim$ 50, making it one of the mosthighly coherent low-frequency QPO ever seen in a BHXB. The extremely high Qfactor makes this QPO distinct from typical low-frequency QPOs that areconventionally classified into Type-A/B/C QPOs. Instead, we find evidence thatarchival observations of GRS 1915+105 also showed a similarly high-coherenceQPO in the same frequency range, suggesting that this unusually coherent andstrong QPO may be unique to BHXBs that can exhibit 'heartbeat'-likevariability.

Highly-coherent quasi-periodic oscillations in the 'heartbeat' black hole X-ray binary IGR J17091-3624