Hard and soft spectral states of ULXs

Abstract

AbstractI 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 ≈1–3LEdd. 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 (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)