2021ApJ...919...46Y

We report the detection of a short-lived narrow quasi-periodic oscillation (QPO) at ∼88 mHz in an Insight-HXMT exposure during the soft state of the persistent black hole high-mass X-ray binary Cygnus X-1. This QPO is significantly detected in all three instruments of Insight-HXMT, so in the broad energy range 1-250 keV. The fractional rms of the QPO does not show significant variations above 3 keV (∼5%) while it decreases at lower energy (∼2%). We show that this QPO is different from the type-A, -B, and -C QPOs usually observed in black hole X-ray binaries. We compare QPOs at similar frequencies that have been previously detected in other persistent high-mass X-ray binaries in the soft state; we speculate that such QPOs might relate to some local inhomogeneity rarely formed in the accretion flow of wind-fed accretion systems.