Black Hole Ultra-compact X-ray Binaries: Galactic Low-Frequency Gravitational Wave Sources
Abstract
In the Galaxy, close binaries with compact objects are important low-frequency gravitational wave (GW) sources. As potential low-frequency GW sources, neutron star/white dwarf (WD) ultra-compact X-ray binaries (UCXBs) have been investigated extensively. Using the MESA code, we systematically explored the evolution of black hole (BH)-main sequence star (MS) binaries to diagnose whether their descendants can be detected by space-borne GW detectors. Our simulations show that BH-MS binaries with an initial orbital period less than the bifurcation period can evolve into BH UCXBs that can be detected by LISA. Such an evolutionary channel would form compact mass-transferring BH-WD systems rather than detached BH-WD systems. The calculated X-ray luminosities of BH UCXBs detected by LISA at a distance d=1 kpc are 1033-1035~ erg\,s-1 (1034-1035~ erg\,s-1 for d=10 kpc), hence it is possible to detect their electromagnetic counterparts. It is worth emphasizing only some BH-MS systems with an initial orbital period very close to the bifurcation period can evolve toward low-frequency GW sources whose chirp masses can be measured. The maximum GW frequency of BH UCXBs forming by BH-MS pathway is about 3 mHz, which is smaller than the minimum GW frequency (6.4 mHz) of mass-transferring BH-WD originating from a dynamic process. Furthermore, we obtain an initial parameter space (donor-star masses and orbital periods) of progenitors of BH UCXB-GW sources, which can be applied to future population synthesis simulations. By a rough estimation, we predict that LISA could detect only a few BH UCXB-GW sources forming by the BH-MS channel.