Low-mass X-ray binaries from black hole retaining globular clusters

Abstract

Recent studies suggest that globular clusters (GCs) may retain a substantial population of stellar-mass black holes (BHs), in contrast to the long-held belief of a few to zero BHs. We model the population of BH low-mass X-ray binaries (BH-LMXBs), an ideal observable proxy for elusive single BHs, produced from a representative group of Milky Way GCs with variable BH populations. We simulate the formation of BH-binaries in GCs through exchange interactions between binary and single stars in the company of tens to hundreds of BHs. Additionally, we consider the impact of the BH population on the rate of compact binaries undergoing gravitational wave driven mergers. The characteristics of the BH-LMXB population and binary properties are sensitive to the GCs structural parameters as well as its unobservable BH population. We find that GCs retaining ∼ 1000 BHs produce a galactic population of ∼ 150 ejected BH-LMXBs whereas GCs retaining only ∼ 20 BHs produce zero ejected BH-LMXBs. Moreover, we explore the possibility that some of the presently known BH-LMXBs might have originated in GCs and identify five candidate systems.

Fast Optical Transients from Stellar-mass Black Hole Tidal Disruption Events in Young Star Clusters

“…Additionally, stellar-mass BHs are expected to dynamically interact with luminous stars in stellar clusters. BH-star encounters are expected to play a crucial role in the formation of both accreting and detached BH binaries (e.g., Ivanova et al 2010Ivanova et al , 2017Giesler et al 2018;Kremer et al 2018a) with properties similar to the BH candidates detected to date in Milky Way GCs (e.g., Kremer et al 2019a). Additionally, such dynamical encounters may occasionally cause a star to cross a BH within its tidal disruption radius, leading to a tidal disruption of the star (Perets et al 2016;Lopez et al 2020;Kremer et al 2019bKremer et al , 2019cSamsing et al 2019;Fragione et al 2020).…”

Black Hole Mergers from Star Clusters with Top-heavy Initial Mass Functions

“…Among the densest environments in the universe, globular clusters (GCs) are ideal laboratories to investigate the importance of stellar dynamics in forming and evolving compact objects and compact binaries (see, e.g., Heggie & Hut 2003). Frequent dynamical encounters between cluster members are fundamental in creating and explaining the existence of a number of exotic populations, such as X-ray binaries (e.g., Clark 1975;Verbunt et al 1984;Giesler et al 2018;Kremer et al 2018), radio pulsars (e.g., Lyne et al 1987;Sigurdsson & Phinney 1995;Ivanova et al 2008;Ye et al 2019), and gravitational wave (GW) sources (e.g., Rodriguez et al 2015;Askar et al 2017;Banerjee 2017;Fragione & Kocsis 2018;Kremer et al 2019).…”

Spin and eccentricity evolution in triple systems: From the Lidov-Kozai interaction to the final merger of the inner binary

“…The eccentricity is yet another interesting aspect that we explore in this study, as it usually bears unique signatures of a binary's formation channel [33,[54][55][56][57][58][59][60][61][62][63], and it is anticipated to be detectable by future space-based GW observatories in the millihertz and decihertz bands such as LISA [64], TianQin [65], and TianGO [66]. This motivates investigating the limiting eccentricity that can be excited by the LK mechanism and the observational consequences for future space-based and ground GW detectors.…”

“…Given a set of initial conditions for ðJ; L; eÞ, together with a set of constant parameters ðM t ; q; S 1 ; S 2 ; χ eff Þ, one can obtain the averaged orbital evolution in terms of L by solving fhdJ=dLi; hde=dLig using Eqs. (59) and (60). While in this process we lose track of the exact value of S, we nevertheless know its probability density function for each system with ðJ; L; eÞ given by pðSjJ; L; eÞ ¼ 2 τ pre…”

“…This idea has been studied extensively in the context of LISA (see, e.g., Ref. [58,60]). However, for systems reaching the limiting value [Eq.…”