Abstract
Extreme-mass-ratio inspirals (EMRIs) are important gravitational-wave (GW) sources for future space-based detectors. The standard model consists of one stellar-mass black hole spiraling into a supermassive one, and such a process emits low-frequency (~10−3 Hz) GWs, which contain rich information about the space–time geometry around the central massive body. Here we show that the small bodies in EMRIs, in fact, could be binary black holes, which are captured by the massive black holes during earlier close encounters. About 30% of the captured binaries coalesce due to the perturbation by the massive bodies, resulting in a merger rate of 0.03 Gpc3 yr−1 in the most optimistic scenario. The coalescence generates also high-frequency (~102 Hz) GWs detectable by ground-based observatories, making these binary-EMRIs ideal targets for future multi-band GW observations.
Highlights
Extreme-mass-ratio inspirals (EMRIs) are important gravitational-wave (GW) sources for future space-based detectors
In the canonical model of an EMRI, the stellar object is captured by the supermassive black hole (SMBH) in two possible ways[3]: (i) it is scattered by other stars, a process known as relaxation, to such a small distance to the SMBH that the stellar object loses a significant amount of its orbital energy through GW radiation and becomes bound to the big BH8,9
The orbital elements of the binary EMRI” (b-EMRI) can be derived from two conservation laws: (i) because of the conservation of energy, the binding energy of a captured BH binary (BHB) is E3 = K0 − η|E0| ≃ ηGm1m2/(2a0), where the last approximation uses the previous result that η ~ 0.1
Summary
Extreme-mass-ratio inspirals (EMRIs) are important gravitational-wave (GW) sources for future space-based detectors. We show that a BH binary (BHB) could be tidally captured by a SMBH to a bound orbit.
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