A study of spectral and timing properties of Cyg X-1 based on a large sample of pointed RXTE observations

TL;DR

This study analyzes RXTE observations of Cyg X-1, exploring spectral states, physical models of Comptonization, and variability patterns to understand accretion processes and spectral state transitions.

Contribution

It provides a comprehensive correlation analysis between spectral properties, Comptonization parameters, and variability, using physical models to interpret accretion rate changes.

Findings

Correlations between spectral color and Comptonization parameters.

Variability dependencies explained by accretion rate and dissipation models.

Spectral state transitions linked to changes in physical parameters.

Abstract

We study a large sample of RXTE PCA/HEXTE observations of Cyg X-1. We characterize the spectra by soft and hard X-ray colours (which define the spectral states), and fit them with a physical model of hybrid, thermal/non-thermal Comptonization. We then fit the power spectra by a sum of Lorentzians. We show the resulting correlations between the spectral colour (defining the hard, intermediate and soft spectral states), the Comptonization amplification factor, the fractional Compton reflection strength, and the peak frequency of the Lorentzians. We also calculate the fractional variability (rms) as a function of photon energy. We fit the obtained rms dependencies by physical models of varying accretion rate, a varying soft-photon input from an outer disc to a hot inner plasma, and a varying dissipation rate in a hot corona above a disc.