Black-hole states in external galaxies

TL;DR

This paper reviews the current understanding of black-hole states in external galaxies, comparing ultra-luminous X-ray sources with stellar-mass black holes to shed light on their nature.

Contribution

It provides a summary of phenomenological knowledge and approaches for comparing black-hole states in external galaxies with those of stellar-mass black holes.

Findings

Different states identified from spectra and variability

Comparison methods discussed

Insights into black-hole nature suggested

Abstract

A large number of Ultra-Luminous X-ray Sources in external galaxies is now known, but the discussion about their nature is still unsettled: intermediate-mass black holes or stellar-mass black holes accreting above the Eddington limit? A promising path that can provide an answer is through comparison with "normal" stellar-mass black holes. These display separate states, identified from both their energy spectra and their fast variability. Although the nature of these states and their transitions is not clear, such a comparison can be extremely useful. In this paper, I briefly outline our current phenomenological knowledge of these states and present a (partial) overview of the different approaches followed to make these comparisons.