Dynamical formation of black hole low-mass X-ray binaries in the field: an alternative to the common envelope

Abstract

Very wide binaries (> 500 AU) are subject to numerous encounters with flying-by stars in the Galactic field and can be perturbated into highly eccentric orbits (e ~ 0.99). For such systems tidal interactions at close pericenter passages can lead to orbit circularization and possibly mass transfer, consequently producing X-Ray binaries without the need for common envelope. We test this scenario for the case of Black Hole Low-Mass X-Ray Binaries (BH LMXBs) by performing a population synthesis from primordial binaries with numerical treatment of random stellar encounters. We test various models for the threshold pericenter distance under which tidal forces cause circularization. We estimate that fly-by interactions can produce a current population of ~ 60$-$220 BH LMXBs in the Galactic field. The results are sensitive to the assumption on tidal circularization efficiency and zero to very small BH natal kicks of a few km/s are required. We show that the most likely donors are low-mass stars (< 1 Msun; at the onset of mass transfer) as observed in the population of known sources (~ 20). However, the low number of systems formed along this route is in tension with most recent observational estimate of the number of dormant BH LMXBs in the Galaxy 10$^4-$10$^8$ (Tetarenko et al. 2016). If indeed the numbers are so high, alternative formation channels of BHs with low-mass donors need to be identified.