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 nonnegligible fraction of the total LMXB population, depending strong-ly on the rms 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.