Suzaku Results on Cygnus X-1 in the Low/Hard State

TL;DR

This study presents Suzaku X-ray observations of Cygnus X-1 in a low/hard state, modeling its spectrum with a cool disk and hot corona, revealing details about the accretion structure and spectral variability.

Contribution

First detailed Suzaku spectral analysis of Cygnus X-1 in low/hard state incorporating a cool disk and a hot corona, with insights into geometry and spectral changes during flares.

Findings

Coronal electron temperature ~100 keV.

Disk temperature ~0.2 keV and protrusion into corona.

Spectral hardening correlates with decreased Comptonization during flares.

Abstract

The black-hole binary Cygnus X-1 was observed for 17 ks with the Suzaku X-ray observatory in 2005 October, while it was in a low/hard state with a 0.7-300 keV luminosity of 4.6 x 10^37 erg/s. The XIS and HXD spectra, spanning 0.7-400 keV, were reproduced successfully incorporating a cool accretion disk and a hot Comptonizing corona. The corona is characterized by an electron temperature of ~100 keV, and two optical depths of ~0.4 and ~1.5 which account for the harder and softer continua, respectively. The disk has the innermost temperature of ~0.2 keV, and is thought to protrude half way into the corona. The disk not only provides seed photons to the Compton cloud, but also produces a soft spectral excess, a mild reflection hump, and a weakly broadened iron line. A comparison with the Suzaku data on GRO J1655-40 reveals several interesting spectral differences, which can mostly be attributed to inclination effects assuming that the disk has a flat geometry while the corona is grossly spherical. An intensity-sorted spectroscopy indicates that the continuum becomes less Comptonized when the source flares up on times scales of 1-200 s, while the underlying disk remains unchanged.