Detection of a Low-frequency Quasi-periodic Oscillation in the Soft State of Cygnus X-1 with Insight-HXMT

We report the detection of a short-lived narrow quasi-periodic oscillation (QPO) at similar to 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 (similar to 5%) while it decreases at lower energy (similar to 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.

Automated summary

The study reports the detection of a rare narrow quasi-periodic oscillation in a persistent black hole high-mass X-ray binary, which differs from previously observed types of QPOs and may be related to local inhomogeneities in the accretion flow.