Modelling hard and soft state of Cygnus X-1 with propagating mass accretion rate fluctuations

TL;DR

This study uses the PROPFLUC model to analyze X-ray timing data of Cygnus X-1, revealing different accretion flow geometries in soft and hard states and suggesting black hole spin alignment.

Contribution

It applies the PROPFLUC model to multiple states of Cygnus X-1, providing insights into accretion flow structures and black hole spin orientation.

Findings

Good model-data agreement in soft and hard states

Soft state involves an extended hot flow above the disc

Hard state consistent with a truncated disc geometry

Abstract

We present a timing analysis of threeRossi X-ray Timing Explorer observations of the black hole binary Cygnus X-1 with the propagating mass accretion rate fluctuations model PROPFLUC. The model simultaneously predicts power spectra, time lags, and coherence of the variability as a function of energy. The observations cover the soft and the hard state of the source, and the transition between the two. We find good agreement between model pre-dictions and data in the hard and in the soft state. Our analysis suggests that in the soft state the fluctuations propagate in an optically thin hot flow extending up to large radii above and below a stable optically thick disc. In the hard state, our results are consistent with a truncated disc geometry, where the hot flow extends radially inside the inner radius of the disc. In the transition from soft to hard state, the characteristics of the rapid variability are too complex to be successfully described with PROPFLUC. The surface density profile of the hot flow predicted by our model and the lack of QPOs in the soft and hard state, suggest that the spin of the black hole is aligned with the inner accretion disc and therefore probably with the rotational axis of the binary system.