Formation of ultra-compact X-ray binaries through circum-binary disk-driven mass transfer

TL;DR

This paper proposes a new formation channel for ultra-compact X-ray binaries involving circum-binary disks, which allows for a wider range of initial conditions to produce ultra-short orbital periods.

Contribution

It introduces the role of circum-binary disks in mass transfer, expanding the parameter space for UCXB formation compared to previous models.

Findings

Circum-binary disks significantly enhance angular momentum loss.

Ultra-short orbital periods can be achieved over a broader initial parameter range.

Provides an alternative formation pathway for UCXBs.

Abstract

The formation of ultra-compact X-ray binaries (UCXBs) has not been well understood. Previous works show that ultra-short orbital periods ($<1$ hr) may be reached through mass transfer driven by magnetic braking in normal low/intermediate-mass X-ray binaries (L/IMXBs) only for an extremely small range of initial binary parameters, which makes it difficult to account for the rather large population of UCXBs. In this paper we report the calculated results on mass transfer processes in L/IMXBs with a circum-binary disk. We show that when the orbital angular momentum loss due to a circum-binary disk is included, ultra-short orbital periods could be reached for a relatively wide range of initial binary parameters. The results of our binary models suggest an alternative formation channel for UCXBs.