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

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.


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