Reflection and noise in Cygnus X-1

TL;DR

This study analyzes RXTE data of Cygnus X-1, revealing a correlation between noise frequencies, spectral parameters, and reflection, suggesting the inner disk boundary shifts closer to the compact object during state changes.

Contribution

It demonstrates a clear correlation between noise frequencies, spectral steepness, and reflection in Cygnus X-1, linking timing and spectral properties to accretion disk geometry.

Findings

Reflection amplitude increases with noise frequency.

Spectral slope steepens as the inner disk boundary moves inward.

Correlation extends to soft spectral states and frequency-resolved spectra.

Abstract

We analyzed RXTE/PCA observations of Cyg X-1 from 1996-1998. We found a tight correlation between the characteristic noise frequencies (e.g. the break frequency nu_br) and the spectral parameters in the low spectral state. The amplitude of reflection increases and the spectrum of primary radiation steepens as the noise frequency increases (nu_br changes by a factor of ~15 in our sample). This can be understood assuming that increase of the noise frequency is associated with the shift of the inner boundary of the optically thick accretion disk towards the compact object. The related increase of the solid angle, subtended by the disk, and of the influx of the soft photons to the Comptonization region lead to an increase of the amount of reflection and steepening of the Comptonized spectrum. The correlation between the slope of primary radiation and the amplitude of reflection extends to the soft spectral state. A similar correlation between reflection and slope was found for the frequency resolved spectra in the 0.01-15 Hz frequency range. Such a correlation could appear if the longer time scale variations are associated with emission originating closer to the optically thick disk and, therefore, having steeper Comptonized spectra with larger reflection.