The Close Stellar Companions to Intermediate Mass Black Holes

Abstract

When embedded in dense cluster cores, intermediate mass black holes (IMBHs) acquire close stellar or stellar-remnant companions. These companions are not only gravitationally bound, they tend to hierarchically isolate from other cluster stars through series of multibody encounters. In this paper, we study the demographics of IMBH companions in compact star clusters through direct $N$-body simulation. We study clusters initially composed of $10^5$ or $2\times 10^5$ stars with IMBHs of 75 and 150 solar masses, and follow their evolution for 6-10 Gyr. A tight innermost binary pair of IMBH and stellar object rapidly forms. The IMBH has a companion with orbital semi-major axis at least three times tighter than the second-most bound object over 90% of the time. These companionships have typical periods of order years and are subject to cycles of exchange and destruction. The most frequently observed, long-lived pairings persist for $\sim 10^7$ yr. The demographics of IMBH companions in clusters are diverse; they include both main sequence, giant stars, and stellar remnants. Companion objects may reveal the presence of an IMBH in a cluster in one of several ways. Most-bound companion stars routinely suffer grazing tidal interactions with the IMBH, offering a dynamical mechanism to produce repeated flaring episodes like those seen in the IMBH candidate HLX-1. Stellar winds of companion stars provide a minimum quiescent accretion rate for IMBHs, with implications for radio searches for IMBH accretion in globular clusters. Finally, gravitational wave inspirals of compact objects are found to occur with promising frequency.