The formation of low-mass transient X-ray binaries

We consider constraints on the formation of low-mass X-ray binaries containing neutron stars (NLMXBs) arising from the presence of soft X-ray transients among these systems. For a neutron star of mass M-1 similar or equal to 1.4 M-circle dot at formation, we show that in short-period (less than or similar to 1-2 day) systems driven by angular 1momentum loss these constraints require the secondary at the beginning of mass transfer to have a mass of 1.3 M-circle dot less than or similar to M-2 less than or similar to 1.5 M-circle dot and to be significantly nuclear evolved, provided that supernova (SN) kick velocities are generally small compared with the pre-ShT orbital velocity. As a consequence, a comgaratively large fraction of such systems appear as soft X-ray transients even at short periods, as observed. Moreover, the large initial secondary masses account for the rarity of NLMXBs at periods P less than or similar to 3 hr. In contrast, NLMXB populations forming with large kick velocities would not have these properties, suggesting that the kick velocity is generally small compared with the pre-SN orbital velocity in a large fraction of systems, consistent with a recent reevaluation of pulsar proper motions. The results also place tight constraints on the strength of magnetic braking: if magnetic braking is significantly stronger than the standard form, too many unevolved NLMXBs would form; if it is slower by only a factor of similar or equal to 4, no short-period NLMXBs would form at all in the absence of a kick velocity. The narrow range for M-2 found for negligible kick velocity implies restricted ranges near 4 M-circle dot for ihe helium star antecedent of the neutron star and near 4 M-circle dot for the original main-sequence progenitor. The pre-common-envelope period must lie near 4 M-circle dot and we estimate the short-period NLMXB formation rate in the disk of the Galaxy as similar to 2 x 10(-7) yr(-1). Our results show that the neutron star mass at short-period NLMXB formation cannot be significantly larger than 1.4 M-circle dot. Systems with formation masses of M-1 less than or similar to 1.2 M-circle dot would have disrupted, so observations implying M-1 similar to 1.4 M-circle dot in some NLMXBs suggest that much of the transferred mass is lost from these systems.