Extreme mass-ratio gravitational-wave sources: mass segregation and post binary tidal-disruption captures

Abstract

The gravitational-wave (GW) inspirals of stellar-mass compact objects on to a supermassive black hole (SMBH), are some of the most promising GW sources detectable by next-generation space-born GW-detectors. The rates and characteristics of such extreme mass ratio inspirals (EMRIs) sources are highly uncertain. They are determined by the dynamics of stars near MBHs and the rate at which compacts objects are driven to the close proximity of the MBH. Here, we consider weakly and strongly mass-segregated nuclear clusters and the evolution of stars captured into highly eccentric orbits following binary disruptions by the MBH. We make use of a Monte Carlo approach to model the diffusion of both captured objects and compact-objects brought through two-body relaxation processes. We calculate the rates of GW-inspirals resulting from relaxation-driven objects and characterize EMRIs properties. We correct previous studies and show that relaxation-driven sources produce GW-sources with lower-eccentricity than previously found and provide the detailed EMRI eccentricity distribution in the weak and strong mass-segregation regimes. We also show that binary-disruption captured-stars could introduce low-eccentricity GW-sources of stellar BH EMRIs in mass-segregated clusters. The eccentricities of the GW-sources from the capture channel, however, are strongly affected by relaxation processes and are significantly higher than previously suggested. We find that both the rate and eccentricity distribution of EMRIs could probe the dynamics near MBHs, and the contribution of captured stars, characterize the mass-function of stellar compact objects, and verify whether weak or strong mass-segregation processes take place near MBHs.

The Combined Effects of Two-Body Relaxation Processes and the Eccentric Kozai-Lidov Mechanism on the EMRI Rate

“…Additionally, weak two-body interactions can also lead to mass segregation if the BH is more massive than the surrounding population of stars (e.g., Hopman & Alexander 2006;Alexander & Hopman 2009;Preto & Amaro-Seoane 2010;Amaro-Seoane & Preto 2011;Chen & Han 2018). Other physical processes have also been suggested to contribute to the formation of EMRIs, for example, the tidal separation of BH binaries by SMBHs was suggested to form a low eccentricity LISA event (e.g., Miller et al 2005;Raveh & Perets 2021, the latter also include the effects of mass segregation). Furthermore, accretion disks around SMBHs in active galactic nuclei (AGN) have been suggested to further increase the EMRI rate (e.g., Pan & Yang 2021).…”

Mass-gap extreme mass ratio inspirals

“…We propose that future detections of EMRIs and especially massgap EMRIs by spaceborne GW detectors, may be a useful probe for the origins of MGOs, thanks to the limited number of EMRI formation channels and distinct signatures of EMRI sources from different channels. There are two main channels of EMRI formation: the dry loss-cone channel [30][31][32][33][34][35][36][37][38] and the wet active galactic nucleus (AGN) disk channel [39][40][41][42][43][44] (other processes involving tidal disruption or tidal capture of binary sBHs, or tidal stripping of giant stars [45][46][47][48] may also contribute to EMRI formation). In the dry channel, a stellarmass black hole (sBH) is scattered by stars in the nuclear stellar cluster and gravitationally captued by the MBH.…”