Supernova Kicks, Magnetic Braking, and Neutron Star Binaries

Abstract

We consider the formation of low-mass X-ray binaries (LMXBs) containing accreting neutron stars via the helium star supernova channel. The predicted relative number of short-period transients provides a sensitive test of the input physics in this process. We investigate the e †ect of varying mean kick velocities , orbital angular momentum loss efficiencies, and common-envelope ejection efficiencies on the sub-population of short-period systems, both transient and persistent. Guided by the thermal-viscous disk instability model in irradiation-dominated disks, we posit that short-period transients have donors close to the end of core hydrogen burning. We Ðnd that with increasing mean kick velocity the overall short-period fraction, s, grows, while the fraction r of systems with evolved donors among short-period systems drops. This e †ect, acting in opposite directions on these two fractions, allows us to constrain models of LMXB formation through comparison with observational estimates of s and r. Without Ðne tuning or extreme assumptions about evolutionary parameters, consistency between models and current observations is achieved for a regime of intermediate average kick magnitudes of about 100È200 km s~1, provided that (1) orbital braking for systems with donor masses in the range 1È1.5 is weak, i.e., M _ much less e †ective than a simple extrapolation of standard magnetic braking beyond 1.0 would M _ suggest, and (2) the efficiency of common-envelope ejection is low.