Detection of a low frequency quasi-periodic oscillation in the soft state of Cygnus X-1 with Insight-HXMT

TL;DR

This paper reports the detection of a rare, short-lived 88 mHz QPO in Cygnus X-1 during its soft state, observed across a broad energy range with Insight-HXMT, suggesting a possible link to accretion flow inhomogeneities.

Contribution

First detection of a narrow 88 mHz QPO in Cygnus X-1's soft state using Insight-HXMT, expanding understanding of QPO phenomena in high-mass X-ray binaries.

Findings

QPO detected in 1-250 keV range across all instruments

Fractional rms remains ~5% above 3 keV and decreases at lower energies

QPO differs from typical type-A, -B, -C QPOs in black hole binaries

Abstract

We report the detection of a short-lived narrow quasi-periodic oscillation (QPO) at ~88 mHz in an Insight-HXMT observation during the soft state of the persistent black hole high mass X-ray binary Cygnus X-1. This QPO is significantly detected in all the 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 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.