Small field with a large impact

Abstract

The nature of ultraluminous X-ray astronomical sources has long been unclear. The latest observations of these rare systems provide some crucial clues, but still leave theorists scratching their heads. See Letters p.198 & p.202 Ultraluminous X-ray sources (ULXs) are non-nuclear point sources that are widely believed to contain either intermediate mass black holes or smaller, stellar mass black holes accreting from a binary companion. The study of ULXs provides information about black hole formation and/or modes of high Eddington rate accretion. Two papers in this issue of Nature describe pulsating ULXs with unusual properties. Christian Motch et al. find that source P13 in the galaxy NGC 7793 is in a ∼64 day period binary system. By modelling the strong optical and UV modulations arising from X-ray heating of the B9Ia donor star, they constrain the black hole mass to be less than 15 solar masses. Matteo Bachetti et al. observe a source in the galaxy M82 that, the pulsation data imply, harbours a neutron star rather than a black hole, raising doubts over the assumption that black holes power the most luminous X-ray binaries.