These outbursts last for months, but may take as long as 50 years or more to recur. The outbursts are powered by the gravitational energy released when huge masses of gas from the companion star fall on to the star astronomers believe is a black hole. The gas is being steadily torn off the companion star by the black hole's gravitational pull. However it does not fall directly on to the black hole, but orbits it in the form of a large flat disc of gas, rather like Saturn's rings. The outbursts occur when this disc becomes unstable, and its gas suddenly falls towards the black hole.
Gas can build up in the disc and cause periodic outbursts only if the disc is normally fairly cool.
Professor King's team has shown that, if there is a hot star at the centre of the disc, it will keep the disc hot. Under these circumstances, the instability never gets a chance to build up because material can flow steadily in towards the central star. The system then appears as a steady source of X- rays instead of having outbursts. Neutron stars are almost as dense as black holes, but, crucially, they are just extended enough to have a real stellar surface. This surface is heated by the steady infall of gas from the disc, keeping the disc hot and stable, which in turn maintains the steady inflow heating the stellar surface in the first place.