Probing the black hole merger history in clusters using stellar tidal disruptions

Abstract

The dynamical assembly of binary black holes (BBHs) in dense star clusters (SCs) is one of the most promising pathways for producing observable gravitational wave (GW) sources, however several other formation scenarios likely operate as well. One of the current outstanding questions is how these different pathways may be distinguished apart. In this paper we suggest a new multimessenger observable that can be used to constrain the formation of BBH mergers originating from SCs: the electromagnetic signal from tidal disruptions (TDs) of stars by BBHs. Such TDs will show variability in their light curve from the orbital motion of the disruptive BBHs, and can therefore be used to map the BBH orbital period distribution, and thereby also the dynamical mechanisms that eventually drive the BBHs to merger. Using an analytical approach including General Relativistic effects, we find that the orbital period distribution of BBHs within globular clusters peaks on timescales of days, which we argue is unique to this assembly pathway. We propose that the search for variable TDs in current and future EM transient surveys might be used to constrain the merger history of BBHs in SCs.

Dissecting the properties of neutron star–black hole mergers originating in dense star clusters

“…2) x 10 -9 yr -1 , compatible with the value expected for TDEs triggered by BH binaries 41,42 . The 90% of disrupted stars have a mass < 0.5 M☉, thus possibly representing white dwarfs or low-mass main sequence stars.…”

Fast Optical Transients from Stellar-mass Black Hole Tidal Disruption Events in Young Star 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). These stellar-mass BH tidal disruption events (TDEs) may occur during close encounters of pairs of single stars (i.e., single-single interactions) and also during small-N (typically three-or four-body) resonant encounters that occur through binary-mediated dynamical interactions (e.g., Fregeau & Rasio 2007).…”

“…Note that this same scaling is found in the equal mass case (e.g., Samsing et al 2017;Samsing 2018). Here, we take P TD = 2R TD /a BBH as in Samsing et al (2019). We can then rewrite Equation (9) as…”

Merger rate of black hole binaries from globular clusters: Theoretical error bars and comparison to gravitational wave data from GWTC-2

“…We obtain the distribution of cluster formation times from the semi-analytical galaxy formation model of El-Badry et al [80]. The same model has also been used in recent work [e.g., 31,81], allowing a direct comparison of our results to literature. El-Badry et al [80] describe the process of GC formation as resulting from star formation activity in the high-density disks of gas-rich galaxies.…”