Evolution of Accretion Modes between Spectral States Inferred from Spectral and Timing Analysis of Cygnus X-1 with Insight-HXMT observations

TL;DR

This study analyzes Cygnus X-1's spectral and timing properties across states, revealing that the hot corona dominates X-ray variations and proposing a state-dependent corona geometry evolution.

Contribution

It provides a detailed spectral-timing analysis with a state-dependent corona geometry model based on observations from Insight-HXMT.

Findings

The accretion disk's inner boundary moves inward as the source softens.

X-ray variations are primarily generated in the hot corona, not the disk.

Different corona geometries are inferred for each spectral state.

Abstract

We execute a detailed spectral-timing study of Cygnus X-1 in the low/hard, intermediate and high/soft states with the Hard X-ray Modulation Telescope observations. The broad band energy spectra fit well with the "truncated disk model" with the inner boundary of the accretion disk stays within $\sim$10 \textit R$_{\rm g}$ and moves inward as the source softens. Through studying of PDS, rms and Fourier-frequency component resolved spectroscopy, we find that the X-ray variations are generated in two different regions for each state. We discover that the major role that contributes to the X-ray variation is the hot corona rather than the accretion disk. We suggest a scenario with different corona geometry for each state based on the truncated disk geometry in which the corona wraps up the disk to form a sandwich geometry in the low/hard state, and then gradually moves away from the disk in direction that is perpendicular to the disk until forming a jet-like geometry in the high/soft state.