Evidence for a Cool Disk and Inhomogeneous Coronae from Wide-band Temporal Spectroscopy of Cygnus X-1 with Suzaku

TL;DR

This study uses broadband Suzaku data to analyze the spectral and timing properties of Cygnus X-1, revealing a complex, multi-component emission structure consistent with a truncated disk and inhomogeneous coronae, varying over different timescales.

Contribution

It provides a model-independent analysis of Cygnus X-1's broadband spectral and timing behavior, identifying distinct emission components and their evolution across states.

Findings

Identification of three separate emission components: constant below 2 keV, broad soft in 2-10 keV, hard in 10-300 keV.

Long-term spectral evolution linked to changes in disk temperature and truncation radius.

Hard intermediate state dominated by soft Compton component; low/hard state dominated by hard Compton component.

Abstract

Unified X-ray spectral and timing studies of Cygnus X-1 in the low/hard and hard intermediate state were conducted in a model-independent manner, using broadband Suzaku data acquired on 25 occasions from 2005 to 2009 with a total exposure of ~ 450 ks. The unabsorbed 0.1--500 keV source luminosity changed over 0.8--2.8% of the Eddington limit for 14.8 solar masses. Variations on short (1--2 seconds) and long (days to months) time scales require at least three separate components: a constant component localized below ~2 keV, a broad soft one dominating in the 2--10 keV range, and a hard one mostly seen in 10--300 keV range. In view of the truncated disk/hot inner flow picture, these are respectively interpreted as emission from the truncated cool disk, a soft Compton component, and a hard Compton component. Long-term spectral evolution can be produced by the constant disk increasing in temperature and luminosity as the truncation radius decreases. The soft Compton component likewise increases, but the hard Compton does not, so that the spectrum in the hard intermediate state is dominated by the soft Compton component; on the other hand, the hard Compton component dominates the spectrum in the dim low/hard state, probably associated with a variable soft emission providing seed photons for the Comptonization.