PREPRINT
C70F42FE-1A30-4C73-99DF-206345AED281

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

M. Zhou, V. Grinberg, Q. -C. Bu, A. Santangelo, F. Cangemi, C. M. Diez, O. König, L. Ji, M. A. Nowak, K. Pottschmidt, J. Rodriguez, J. Wilms, S. Zhang, J. -L. Qu, S. -N. Zhang

Submitted on 13 September 2022

## 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 ${\mathrm{\Gamma }}_{1}$, 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.

## Preprint

Comment: 18 pages, 16 figures, 1 table. accepted for publication in Astronomy & Astrophysics

Subject: Astrophysics - High Energy Astrophysical Phenomena