Power-Colours: Simple X-ray Binary Variability Comparison

TL;DR

This paper introduces a simple, model-independent method using power colours to classify and analyze the variability states of black hole and neutron star X-ray binaries, demonstrating consistent patterns across multiple sources.

Contribution

The study presents a new power colour-based technique for classifying X-ray binary states, showing its effectiveness across diverse sources and states.

Findings

Power colour diagrams differentiate black hole states during outburst.

Consistent evolution paths observed across multiple black hole binaries.

Similar spectral evolution found in Cyg X-1 and transient black holes.

Abstract

We demonstrate a new method of variability classification using observations of black hole X-ray binaries. Using `power colours' -- ratios of integrated power in different Fourier frequency bands -- we can clearly differentiate different canonical black hole states as the objects evolve during outburst. We analyse (~ 2400) Rossi X-ray Timing Explorer observations of 12 transient low mass black hole X-ray binaries and find that the path taken around the power colour-colour diagram as the sources evolve is highly consistent from object to object. We discuss how the consistency observed in the power colour-colour diagram between different objects allows for easy state classification based on only a few observations, and show how the power-spectral shapes can be simply classified using a single parameter, the power-spectral `hue'. To illustrate the benefits of our simple model-independent approach, we show that the persistent high mass X-ray binary Cyg X-1 shows very similar power-spectral evolution to the transient black hole sources, with the main difference being caused by a combination of a lack of quasi-periodic oscillations and an excess of low-frequency power-law noise in the Cyg X-1 power spectra during the transitional state. We also compare the transient objects to the neutron star atoll source Aquila X-1, demonstrating that it traces a different path in the power colour-colour plot. Thus, power-colours could be an effective method to classify newly discovered X-ray binaries.