Hard and soft spectral states of ULXs

TL;DR

This paper examines the spectral states of ultraluminous X-ray sources (ULXs), comparing them to Galactic black hole states, and proposes new interpretations for their spectral behaviors and states at high luminosities.

Contribution

It introduces alternative models for ULX spectral states, including bulk motion Comptonization, and suggests the existence of a high/hard state at high accretion rates not seen in Galactic black holes.

Findings

ULXs with curved spectra are likely extensions of the very high state.

Some ULXs remain in a hard power-law state over a wide luminosity range.

A proposed high/hard state may exist at high accretion rates, overlapping with low/hard states.

Abstract

I discuss some differences between the observed spectral states of ultraluminous X-ray sources (ULXs) and the canonical scheme of spectral states defined in Galactic black holes. The standard interpretation of ULXs with a curved spectrum, or a moderately steep power-law with soft excess and high-energy downturn, is that they are an extension of the very high state, up to luminosities ~ 1 to 3 L_{Edd}. Two competing models are Comptonization in a warm corona, and slim disk; I suggest bulk motion Comptonization in the radiatively-driven outflow as another possibility. The interpretation of ULXs with a hard power-law spectrum is more problematic. Some of them remain in that state over a large range of luminosities; others switch directly to a curved state without going through a canonical high/soft state. I suggest that those ULXs are in a high/hard state not seen in Galactic black holes; that state may overlap with the low/hard state at lower accretion rates, and extend all the way to Eddington accretion rates. If some black holes can reach Eddington accretion rates without switching to a standard-disk-dominated state, it is also possible that they never quench their steady jets.