The Brightest Black Holes

I suggest that there are two classes of ultraluminous X–ray sources (ULXs), corresponding to super–Eddington mass inflow in two situations: (a) thermal–timescale mass transfer in high–mass X–ray binaries, and (b) long–lasting transient outbursts in low–mass X–ray binaries. These two classes are exemplified by SS433 and microquasars like GRS 1915+105 respectively. The observed ULX population is a varying mixture of the two, depending on the star formation history of the host galaxy. ULXs in galaxies with vigorous star formation (such as the Antennae) are generally SS433–like, while ULXs in elliptical galaxies must be of the microquasar type. The latter probably have significantly anisotropic radiation patterns. They should also be variable, but demonstrating this may require observations over decades. The close analogy between models of X–ray binaries and active galactic nuclei (AGN) suggests that there should exist an apparently super–Eddington class of the latter, which may be the ultrasoft AGN, and a set of X–ray binaries with Doppler–boosted X–ray emission. These are presumably a subset of the ULXs, but remain as yet unidentified.