Evolution of low-mass X-ray binaries:dependence on the massof the compact object

We perform numerical calculations to simulate the evolution of low-mass X-ray binary systems. For the accreting compact object we consider the initial mass of 1.4, 10, 20, 100, 200, 500 and 1000 M⊙, corresponding to neutron stars (NSs), stellarmass black holes (BHs) and intermediate-mass BHs. Mass transfer in these binaries is driven by nuclear evolution of the donors and/or orbital angular momentum loss due to magnetic braking and gravitational wave radiation. For the different systems, we determine their bifurcation periods Pbif that separate the formation of converging systems from the diverging ones, and show that Pbif changes from ～ 1 d to >～ 3 d for a 1 M⊙ donor star, with increasing initial accretor mass from 1.4 to 1000 M⊙. This means that the dominant mechanism of orbital angular momentum loss changes from magnetic braking to gravitational radiation. As an illustration we compare the evolution of binaries consisting of a secondary star of 1 M⊙ at a fixed initial period of 2 d. In the case of the NS or stellar-mass BH accretor, the system evolves to a well-detached He white dwarf-neutron star/black hole pair, but it evolves to an ultra-compact binary if the compact object is an intermediate-mass BH. Thus the binary evolution heavily depends upon the mass of the compact object. However, we show that the final orbital period-white dwarf mass relation found for NS low-mass X-ray binaries is fairly insensitive to the initial mass of the accreting star, even if it is an intermediate-mass BH.