Prof. Phil Charles

High-mass X-ray binaries (HMXBs) were discovered in our Galaxy 50 years ago, and over a hundred such objects are known. The great majority have neutron stars (indicated via X-ray pulsations) accreting from the strong stellar winds of their early-type companions, but some involve Roche-lobe overflow as well. With orbital periods from several days to many weeks, HMXBs have been valuable in providing accurate neutron star mass determinations, but many also display much longer "super-orbital" modulations, the nature of which is still controversial. Similar HMXBs have been found in nearby galaxies via X-ray imaging surveys over the last 30 years, and a sub-set of these off-nuclear sources are described as ultra-luminous X-ray sources (ULXs) because their luminosities can exceed a hundred times the Eddington Limit for a neutron star. Accordingly they were thought to harbour the long-sought class of IMBH - Intermediate Mass Black Hole. But in 2014, one of these ULXs was found to be an X-ray pulsar, which immediately revealed its orbital period, and now more than a half-dozen are known, meaning that, in the right circumstances, neutron stars can become extreme super-Eddington emitters. They also display longer-term variations, and we have now found evidence for a simple linear relationship between the orbital period and super-orbital period in ULX pulsars, akin to that found in HMXBs a decade ago. I will introduce and describe the physical properties of these classes of objects and then describe the various models put forward for such extraordinary behaviour.