Cygnus X-1: shedding light on the spectral variability of a black hole

TL;DR

This paper develops a classification scheme for the spectral states of the black hole Cygnus X-1 using RXTE and ASM data, improving accuracy over previous methods and enabling better spectral analysis across different states.

Contribution

It introduces a new classification scheme based on RXTE-ASM colors and count rates that accurately classifies Cygnus X-1's spectral states from 1996 to 2011.

Findings

A simple count rate criterion results in higher misclassification.

The new scheme accurately classifies states using RXTE-ASM data.

It allows for spectral analysis with low contamination across states.

Abstract

The knowledge of the spectral state of a black hole is essential for the interpretation of data from black holes in terms of their emission models. Based on pointed observations of Cyg X-1 with the Rossi X-ray timing Explorer (RXTE) that are used to classify simultaneous RXTE-ASM observations, we develop a scheme based on RXTE -ASM colors and count rates that can be used to classify all observations of this canonical black hole that were performed between 1996 and 2011. We show that a simple count rate criterion, as used previously, leads to a significantly higher fraction of misclassified observations. This scheme enables us to classify single INTEGRAL-IBIS science windows and to obtain summed spectra for the soft, intermediate and hard state with low contamination by other states.