The Effects of High-Velocity Supernova Kicks on the Orbital Properties and Sky Distributions of Neutron Star Binaries

Abstract

We systematically investigate the effects of high supernova kick velocities on the orbital parameters of post-supernova neutron-star binaries. Using Monte- Carlo simulations, we determine the post-supernova distributions of orbital parameters for progeneitors of HMXBs and LMXBs. With the recent distribution of pulsar birth velocities by Lyne & Lorimer (1994), only about 27% of massive systems remain bound after the supernova, of which about 26% immediately experience dynamical mass transfer and possibly merge to become Thorne-Zytkow objects. The correlations between various orbital parameters can be compared with observational samples to yield information about supernova kick velocities and pre-supernova orbital-period distributions. After the supernova, the spins of most stars in massive systems have large inclinations with respect to their orbital axes, and a significant fraction of systems (about 20%) contain stars with retrograde spins. In the case of low-mass companions, we find that about 19% of systems remain bound after the supernova, of which about 57% experience immediate dynamical mass transfer. There is a tight correlation between the eccentricity and the post-supernova orbital period in LMXB progeneitors. All LMXBs with post-supernova periods longer than a few days initially have large eccentricities (e.g. Cir X-1 is discussed in this context). We use the results of these calculations to simulate the sky distributions of HMXBs and LMXBs. The simulated distributions agree with observed samples. Most importantly, the distribution of Galactic LMXBs is consistent with an ordinary Galactic disk population which has been widened because of large supernova kicks and does not require a special population of progenitors.