The Microquasar Cyg X-1: A Short Review

TL;DR

This review summarizes spectral observations of Cygnus X-1 across different states, highlighting the accretion disk properties, ionized absorption effects, and potential black hole spin evidence.

Contribution

It provides a comprehensive analysis of multi-satellite spectral data, emphasizing the complex astrophysics and accretion disk characteristics of Cyg X-1.

Findings

Hard state disk inner edge ~40 GM/c^2

Soft state shows prominent disk component

Evidence suggests a spinning black hole

Abstract

We review the spectral properties of the black hole candidate Cygnus X-1. Specifically, we discuss two recent sets of multi-satellite observations. One comprises a 0.5-500 keV spectrum, obtained with every flying X-ray satellite at that time, that is among the hardest Cyg X-1 spectra observed to date. The second set is comprised of 0.5-40 keV Chandra-HETG plus RXTE-PCA spectra from a radio-quiet, spectrally soft state. We first discuss the "messy astrophysics" often neglected in the study of Cyg X-1, i.e., ionized absorption from the wind of the secondary and the foreground dust scattering halo. We then discuss components common to both state extremes: a low temperature accretion disk, and a relativistically broadened Fe line and reflection. Hard state spectral models indicate that the disk inner edge does not extend beyond ~40 GM/c^2, and may even approach as close as ~6 GM/c^2. The soft state exhibits a much more prominent disk component; however, its very low normalization plausibly indicates a spinning black hole in the Cyg X-1 system.