Abstract
Abstract 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.
Highlights
Cygnus X-1 is a bright persistent X-ray binary consisting of a black hole accreting matter from an OB star (Bowyer et al 1965; Bolton 1972; Webster & Murdin 1972)
We report the detection of a short-lived narrow quasi-periodic oscillation (QPO) at ∼88 mHz in an Insight-Hard X-ray Modulation Telescope (HXMT) observation during the soft state of the persistent black hole high-mass X-ray binary Cygnus X-1
We report the detection of a millihertz QPO in one of the Insight-HXMT observations of Cygnus X-1 during the soft state
Summary
Cygnus X-1 is a bright persistent X-ray binary consisting of a black hole accreting matter from an OB star (Bowyer et al 1965; Bolton 1972; Webster & Murdin 1972). Analogous changes have been observed in most later discovered black hole X-ray binaries (BH XRBs) and have been classified into different accretion states Some persistent BH XRBs can exhibit accretion states different from the transient BH XRBs. Cygnus X-1 is one of these sources, as its HSS is characterized by both multitemperature blackbody and Compton power law emission The X-ray timing properties of accretion states in Cygnus X1 differ from the general description mentioned above. Type-C QPOs have never been detected during the LHS of Cygnus X-1 (Grinberg et al 2014; Ingram & Motta 2019) and the HSS shows high variability (fractional rms amplitude ∼25%; Axelsson et al 2005; Grinberg et al 2014).
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