The spectral-timing analysis of Cygnus X-1 with Insight-HXMT

TL;DR

This study analyzes Cygnus X-1 using Insight-HXMT data, revealing how spectral states influence timing properties across a broad energy range, and extending previous findings with new energy-dependent insights.

Contribution

It provides a comprehensive spectral-timing analysis of Cygnus X-1 across 1-150 keV, highlighting state-dependent variability and spectral features with data extending beyond previous RXTE studies.

Findings

Reflection component is stronger in the soft state.

Fractional rms evolution depends on energy band and spectral shape.

Short-term variability features correlate with spectral parameters.

Abstract

Cygnus X-1, as the first discovered black hole binary, is a key source for understanding the mechanisms of state transitions, and the scenarios of accretion in extreme gravity fields. We present a spectral-timing analysis of observations taken with the Insight-HXMT mission, focusing on the spectral-state dependent timing properties in the broad energy range of 1--150 keV, thus extending previous RXTE-based studies to both lower and higher energies. Our main results are the following: a) We successfully use a simple empirical model to fit all spectra, confirming that the reflection component is stronger in the soft state than in the hard state; b) The evolution of the total fractional root mean square (rms) depends on the selected energy band and the spectral shape, which is a direct result of the evolution of the power spectral densities (PSDs); c) In the hard/intermediate state, we see clear short-term variability features and a positive correlation between central frequencies of the variability components and the soft photon index $\Gamma_1$, also at energies above 15 keV. The power spectrum is dominated by red noise in the soft state instead. These behaviors can be traced to at least 90 keV; d) The coherence and the phase-lag spectra show different behaviors dependent on different spectral shapes.