Gravitational Radiation Assisted Capture

Abstract

It is shown that gravitational radiation can bind two initially unbound bodies; no third body is needed. Such captured bodies will almost always inspiral and merge due to further gravitational radiation on cosmologically negligible time scales (e.g., @ 5 years for GW150914). The capture cross-section $\sigma$ for such "capture and inspiraling" is far larger, for initial relative speed of the two objects $v_\infty\ll c$, than that $\sigma_{d}$ for "direct capture": $\sigma\propto\left(c / v_\infty\right)^{18/7}$, while $\sigma_{d}\propto\left(c / v_\infty\right)^2$. Implications of these results for black hole binary mergers, and giant black holes at galactic centers, are discussed.