Black Hole Ultracompact X-Ray Binaries as Galactic Low-frequency Gravitational Wave Sources: The He Star Channel

Abstract

Black hole (BH) ultracompact X-ray binaries (UCXBs) are potential Galactic low-frequency gravitational wave (GW) sources. As an alternative channel, BH UCXBs can evolve from BH+He star binaries. In this work, we perform a detailed stellar evolution model for the formation and evolution of BH UCXBs evolving from the He star channel to diagnose their detectability as low-frequency GW sources. Our calculations found that some nascent BH+He star binaries after the common-envelope (CE) phase could evolve into UCXB-LISA sources with a maximum GW frequency of ∼5 mHz, which can be detected in a distance of 10 kpc (or 100 kpc). Once BH+He star systems become UCXBs through mass transfer, they would emit X-ray luminosities of ∼1038 erg s−1, making them ideal multimessenger objects. If the initial He-star masses are ≥0.7 M ⊙, those systems are likely to experience two Roche lobe overflows, and the X-ray luminosity can reach a maximum of 3.5 × 1039 erg s−1 in the second mass-transfer stage. The initial He-star masses and initial orbital periods of progenitors of Galactic BH UCXB-LISA sources are in the range of 0.32–2.9 M ⊙ and 0.02–0.19 days, respectively. Nearly all BH+He star binaries in the above parameter space can evolve into GW sources whose chirp masses can be accurately measured. Employing a population synthesis simulation, we predict the birthrate and detection number of Galactic BH UCXB-LISA sources evolving from the He star channel are R = 2.2 × 10−6 yr−1 and 33 for an optimistic CE parameter, respectively.

Some Old Globular Clusters (and Stars) Inferring That the Universe Is Older Than Commonly Accepted

“…Either the existence of an intermediate black hole in the center of the cluster as it is suggested by Kizilton et al (2017) [30]. Or more recently Ke Qin et al (2023, 2024) [31,32] simulations showed that black hole main-sequence star (BH-MS) binaries with an initial orbital period less than the bifurcation period can evolve into ultra-compact X-ray binaries (UCXBs) that can be detected by LISA challenging the current assumption that its age suggests the Universe is older. Or the age of NGC 104 suggest that the Universe is older than 13.8Gyr.…”

An Alternative Channel to Black Hole Low-mass X-Ray Binaries: Dynamical Friction of Dark Matter?

“…BH ultracompact X-ray binaries evolving from the mainsequence star and the He star channels are potential Galactic low-frequency GW sources (Qin et al 2023(Qin et al , 2024, which could be detected by LISA (Amaro-Seoane et al 2023), TianQin (Luo et al 2016), and Taiji (Ruan et al 2020). Meanwhile, the accurate observations of optical and X-ray observations in BH LMXBs can measure the most important physical parameters, thus, BH LMXBs are the candidate probes for unveiling the dark matter density spike.…”