4 research outputs found
Binary Encounters With Supermassive Black Holes: Zero-Eccentricity LISA Events
Current simulations of the rate at which stellar-mass compact objects merge
with supermassive black holes (called extreme mass ratio inspirals, or EMRIs)
focus on two-body capture by emission of gravitational radiation. The
gravitational wave signal of such events will likely involve a significant
eccentricity in the sensitivity range of the Laser Interferometer Space Antenna
(LISA). We show that tidal separation of stellar-mass compact object binaries
by supermassive black holes will instead produce events whose eccentricity is
nearly zero in the LISA band. Compared to two-body capture events, tidal
separations have a high cross section and result in orbits that have a large
pericenter and small apocenter. Therefore, the rate of interactions per binary
is high and the resulting systems are very unlikely to be perturbed by other
stars into nearly radial plunges. Depending on the fraction of compact objects
that are in binaries within a few parsecs of the center, the rate of
low-eccentricity LISA events could be comparable to or larger than the rate of
high-eccentricity events.Comment: Final accepted version: ApJ Letters 2005, 631, L11
Mergers of Stellar-Mass Black Holes in Nuclear Star Clusters
Mergers between stellar-mass black holes will be key sources of gravitational
radiation for ground-based detectors. However, the rates of these events are
highly uncertain, given that such systems are invisible. One formation scenario
involves mergers in field binaries, where our lack of complete understanding of
common envelopes and the distribution of supernova kicks has led to rate
estimates that range over a factor of several hundred. A different, and highly
promising, channel involves multiple encounters of binaries in globular
clusters or young star clusters. However, we currently lack solid evidence for
black holes in almost all such clusters, and their low escape speeds raise the
possibility that most are ejected because of supernova recoil. Here we propose
that a robust environment for mergers could be the nuclear star clusters found
in the centers of small galaxies. These clusters have millions of stars, black
hole relaxation times well under a Hubble time, and escape speeds that are
several times those of globulars, hence they retain most of their black holes.
We present simulations of the three-body dynamics of black holes in this
environment and estimate that, if most nuclear star clusters do not have
supermassive black holes that interfere with the mergers, at least several tens
of events per year will be detectable with Advanced LIGO.Comment: 15 pages including one figure, submitted to The Astrophysical Journa