Measuring binary black hole orbital-plane spin orientations

Understand

Binary black hole spins are among the key observables for gravitational wave astronomy.

• Among the spin parameters, their orientations within the orbital plane, $\phi_1$, $\phi_2$ and $\Delta \phi=\phi_1-\phi_2$, are critical for understanding the prevalence of the spin-orbit resonances and merger recoils in binary black holes.
• Unfortunately, these angles are particularly hard to measure using current detectors, LIGO and Virgo.
• Because the spin directions are not constant for precessing binaries, the traditional approach is to measure the spin components at some reference stage in the waveform evolution, typically the point at which the frequency of the detected signal reaches 20 Hz.