2 The gamma-ray binary

We start with a close binary where a low-mass black hole is accompanied by the helium star. The configuration results from the spiral-in of a compact object in a giant, the progenitor of the helium star (see Portegies Zwart 1998, hereafter PZ).

The collapse of the helium core results in the formation of a neutron star. The sudden mass loss in the supernova and the velocity kick imparted to the neutron star may dissociate the binary. If the system remains bound a neutron star - black hole binary is formed.

The separation between the two compact stars decreases due to gravitational wave radiation (see Peters & Mathews 1963). At an orbital separation of a few tens of kilometers the neutron star fills its Roche-lobe and starts to transfer mass to the black hole. Mass transfer from the neutron star to the black hole is driven by the emission of gravitational waves but can be stabilized by the redistribution of mass in the binary system. If the mass of the black hole $\ {\raise-.5ex\hbox{$\buildrel<\over\sim$}}\2.2$ ${M}_\odot$ coalescence follows within a few orbital revolutions owing the Darwin-Riemann instability (Clarck et al. 1977). If the mass of the black hole $\ {\raise-.5ex\hbox{$\buildrel\gt\over\sim$}}\5.5$ ${M}_\odot$ the binary is gravitationally unstable (Lattimer & Schram 1976); the event horizon of the hole is then larger than than the orbital separation. Only in the small mass ranges from 2.2 ${M}_\odot$ to 5.5 ${M}_\odot$ stable mass transfer is possible (see PZ).

The entire episode of mass transfer lasts for several seconds. Mass transfer becomes unstable if the neutron star starts to expand rapidly as soon as its mass drops below the stability limit of $\sim 0.1$ ${M}_\odot$. Initially the neutron star's material falls in the black hole almost radially but at a later stage an accretion disc can be formed (see PZ for details).