Tidal capture formation of Low Mass X-Ray Binaries from wide binaries in the field

TL;DR

This paper proposes a new formation scenario for field Low Mass X-Ray Binaries involving wide binaries perturbed by stellar fly-bys, leading to tidal interactions and eventual evolution into LMXBs, especially for black holes formed with minimal natal kicks.

Contribution

The study introduces a dynamical formation model for LMXBs from wide binaries perturbed by stellar fly-bys, highlighting the role of direct-collapse black holes with low natal kicks.

Findings

High formation rates for BH LMXBs with direct-collapse models.

Number density of LMXBs correlates with stellar density.

Mass function of BH companions peaks at 0.4-0.6 M_sun.

Abstract

We present a potentially efficient dynamical formation scenario for Low Mass X-ray Binaries (LMXBs) in the field, focusing on black-hole (BH) LMXBs. In this formation channel LMXBs are formed from wide binaries $(>1000$ AU) with a BH component and a stellar companion. The wide binary is perturbed by fly-by's of field stars and its orbit random-walks and changes over time. This diffusion process can drive the binary into a sufficiently eccentric orbit such that the binary components tidally interact at peri-center and the binary evolves to become a short period binary, which eventually evolves into an LMXB. The formation rate of LMXBs through this channel mostly depends on the number of such BH wide binaries progenitors, which in turn depends on the velocity kicks imparted to BHs (or NSs) at birth. We consider several models for the formation and survival of such wide binaries, and calculate the LMXB formation rates for each model. We find that models where BHs form through direct collapse with no/little natal kicks can give rise to high formation rates comparable with those inferred from observations. This formation scenario had several observational signatures: (1) the number density of LMXBs generally follows the background stellar density and (2) the mass function of the BH stellar companion should be comparable to the mass function of the background stellar population, likely peaking at $0.4-0.6$ M$_{\odot}$. The latter aspect, in particular, is unique to this model compared with previously suggested LMXB formation models following common envelope binary stellar evolution. We note that NS LMXBs can similarly form from wide binaries, but their formation rate through this channel is likely significantly smaller due to their much higher natal kicks.