States and transitions in black-hole binaries

TL;DR

This paper reviews the complex phenomenology of black-hole transients, proposing a new state classification based on spectral hardness and variability, emphasizing the importance of rapid transitions for understanding accretion physics.

Contribution

It introduces a novel state classification for black-hole binaries that incorporates observed transitions and unifies transients, other binaries, and AGN within a common framework.

Findings

Fast variability transitions are key to understanding state evolution.

A new state classification aligns with observed phenomenology.

The framework applies to transients, persistent binaries, and AGN.

Abstract

With the availability of the large database of black-hole transients from the Rossi X-Ray Timing Explorer, the observed phenomenology has become very complex. The original classification of the properties of these systems in a series of static states sorted by mass accretion rate proved not to be able to encompass the new picture. I outline here a summary of the current situation and show that a coherent picture emerges when simple properties such as X-ray spectral hardness and fractional variability are considered. In particular, fast transition in the properties of the fast time variability appear to be crucial to describe the evolution of black-hole transients. Based on this picture, I present a state-classification which takes into account the observed transitions. I show that, in addition to transients systems, other black-hole binaries and Active Galactic Nuclei can be interpreted within this framework. The association between these states and the physics of the accretion flow around black holes will be possible only through modeling of the full time evolution of galactic transient systems.