Formation of Low-mass X-ray Binaries. Iii. a New Formation Mechanism: Direct Supernova

We propose a new formation mechanism (direct-supernova) for low-mass X-ray binaries (LMXBs) that does not involve any prior phase of mass transfer. Survival through the supernova (SN) explosion and shrinkage of the orbit is achieved by a kick velocity of appropriate magnitude and direction imparted to the neutron star at its birth. We present analytical population synthesis calculations of LMXBs forming via both the direct-SN and the helium-star SN mechanisms, and compare the results. We find that the direct-SN channel contributes a non-negligible fraction of the total LMXB population, depending strongly on the r.m.s. magnitude of the kick velocity. More importantly, the direct-SN mechanism provides a natural way for the formation of low-mass binary pulsars in nearly circular orbits with orbital periods in excess of ∼ 100d, which cannot have been formed via the helium-star SN mechanism. Subject headings: binaries: close – stars: evolution – stars: neutron – pulsars: general – supernovae: general – X-rays: stars