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 tr ansfer. 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 analy tical population synthesis calculations of LMXBs forming via both the direct-SN and the helium star SN mechanisms and compare the results. W e find that the direct-SN channel contributes a nonnegligible fraction of the total LMXB population, depending strongly on the rms magnitude of the kick velocity. More importantly, the direct-SN mechanism provi des a natural way for the formation of low-mass binary pulsars in near ly circular orbits with orbital periods in excess of similar to 100(d) , which cannot have been formed via the helium star SN mechanism.